Research ArticlePrecast Prestressed Concrete Truss-Girder for Roof Applications

Peter Samir1 and George Morcous2

1Omaha Public Power District 444 South 16th Street Mall Omaha NE 68102 USA2University of Nebraska-Lincoln 1110 South 67th Street Omaha NE 68182 USA

Correspondence should be addressed to George Morcous gmorcous2unledu

Received 24 July 2014 Revised 4 November 2014 Accepted 19 November 2014 Published 14 December 2014

Academic Editor Domenico Bruno

Copyright copy 2014 P Samir and G Morcous This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

Steel trusses are the most popular system for supporting long-span roofs in commercial buildings such as warehouses and aircrafthangars There are several advantages of steel trusses such as lightweight ease of handling and erection and geometric flexibilityHowever they have some drawbacks such as high material and maintenance cost and low fire resistance In this paper a precastconcrete truss is proposed as an alternative to steel trusses for spans up to 48m (160 ft) without intermediate supportsThe proposeddesign is easy to produce and has lower construction and maintenance costs than steel trusses The truss consists of two segmentsthat are formed using standard bridge girder forms with block-outs in the web which result in having diagonals and verticalmembers and reduces girder weight The two segments are then connected using a wet joint and post-tensioned longitudinallyto form a crowned trussThe proposed design optimizes the truss-girder member locations cross-sections andmaterial use A 9m(30 ft) long truss specimen is constructed using self-consolidated concrete to investigate the constructability and structural capacityof the proposed design A finite element analysis of the specimen is conducted to investigate stresses at truss diagonals verticalsand connections Testing results indicate the production and structural efficiency of the developed system

1 Introduction

Structural steel is typically and widely used for long-spanroof applications such as warehouses storage facilities andairplanes hangars Design considerations for roof supportsystem include cost-effectiveness speed of constructionstructural capacity aesthetic appearance fire resistance andstructural integrity during construction and in service Usingstructural steel has been the only option when it comesto long-span roofs due to ease of handling and erectiongeometric flexibility and lightweight Concrete has not been acompetitive alternative for roof applications due to the heavyweight and construction complexity of concrete componentswhich results in being less cost effective than steel Despitethe advantages of structural steel roof systems they have thefollowing disadvantages low fire resistance being prone tocorrosion high maintenance cost long lag period in steelorders and increasing prices of steel Most of these disadvan-tages can be addressed by using precast concrete componentsas they have excellent fire and corrosion resistance lowproduction and maintenance cost and short order period

However existing precast concrete roof systems are eitherlimited to 30m (100 ft) span such as hollow cores and doubletees [1] or heavy and lack aesthetic appearance such as deepinverted tees and I-girders Therefore the main objectiveof this research project is to develop a precastprestressedconcrete truss system for roof applications with spans from30m to 48m (100 ft to 160 ft) which achieves the followinggoals lightweight aesthetic appealing cost effectivenessand fabrication using existing techniques and productionpractices

Precast concrete trusses were first used in 1962 for theUSScience Pavilion (now Pavilion Science Center) in SeattleWA These trusses were nonstructural trusses and weremade for architectural purposes [2] In 1976 Rock IslandParking structure was built using Vierendeel trusses madeof horizontal and vertical members with rigid joints and nodiagonal membersThe trusses used were almost 36m (12 ft)deep and had a clear span of 97m (32 ft) which resulted in aspan-to-depth ratio of 27 All top chord bottom chord andvertical members had a cross-section of 405mm times 560mm

Hindawi Publishing CorporationJournal of StructuresVolume 2014 Article ID 524156 13 pageshttpdxdoiorg1011552014524156

2 Journal of Structures

(16 in times 22 in) [3] Vertical members were posttensioned toresist tension forces

Precastprestressed concrete trusses were introduced in1978 in the ACI Journal article titled ldquoPrestressed ConcreteTrussesrdquo [4]The article discussed two prototypes prototype-I that had a clear span of 61m (203 ft) and a depth of 06m(2 ft) for a span-to-depth ratio of 10 and prototype-II that hada clear span of 184m (608 ft) and a depth of 26m (85 ft)for a span-to-depth ratio of 7 The first prototype had onlydiagonal members with no verticals however the secondprototype had diagonal members and two verticals near thecenter of the trusses All top bottom and diagonal memberswere prestressed However the prestressing in the diagonalswas only 35 of the jacking stresses due to the large frictionlosses occurring at the hold-down devicesThe authors statedthat the members cracked at an early stage of loading due tothe fact that the diagonals were not adequately prestressedThe authors also stated that using concrete trusses wouldlower the price to almost half what it would cost if steelalternativewas used In 2007 a new systemof concrete trusseswas developed for a multilevel condominium building builtin Minneapolis MN using the so-called ldquoER-Postrdquo The ER-Post is system invented by M DeSutter of Erickson Roed ampAssociates to provide a column-free space for condominiums[5] DeSutter was able to pretension Vierendeel trusses thatwere 41m (135 ft) deep and of spanning 203m (6733 ft) fora span-to-depth ratio of 5 [6]

In 2010 a precast concrete truss-girder was designedto support the roof of a coal storage facility in SharjahUnited Arab Emirates (UAE) Designed by eConstruct USALLC the 15m (5 ft) deep truss had a span of 50m (165 ft)without intermediate supports which resulted in a span-to-depth ratio of 33 The truss consisted of two precasttruss segments each segment is 25m (825 ft) long The twosegments were connected using posttensioning tendons andcast-in-place concrete joint Several trusses were erected at10m (30 ft) spacing to create the crowned roof Figure 1 showsthe built trusses and temporary support used during erectionat the midspan to support the two truss segments untilposttensioning was applied and the cast-in-place concretejoint was hardened

According to eConstruct USA LLC the use of precastconcrete truss system along with Z-shape steel purlins andmetal roof decking resulted in about 25 saving in thebuilding cost compared to the original design using structuralsteelThis is a significant saving that motivated the authors tofurther investigate precast concrete truss systems to optimizetheir designs improve their constructability and accom-modate production practices in the United States Severalenhancements which will be discussed in Section 2 haveresulted in cost and weight reductions and consequently thepotential of using precast concrete trusses in long-span roofapplications

2 Development of the Proposed System

Theprecast concrete truss system proposed in this study is anevolution of the Sharjah truss-girder system presented earlierThe main enhancements that have been proposed to address

Figure 1 The temporary supports used before posttensioning(courtesy of eConstruct LLC)

design fabrication and construction issues include (1) chang-ing the orientation of diagonals to be compression membersmade of conventionally reinforced concrete (2) using highstrength steel threaded rods for tension members (verticals)to eliminate cracking (3) using readily available forms oftypical precastprestressed concrete bridge I-girders such asAASHTO and bulb tee with modular block-outs to simplifyproduction (4) using high performance self-consolidatingconcrete (SCC) to ensure the quality efficiency and economyof truss fabrication and (5) placing posttensioning ducts inthe bottom flange to eliminate the need for thicker websat girder ends To present these enhancements an examplebuilding has been chosen to design the proposed truss-girdersystem Figure 2 shows the plan and elevation views of theexample building layout respectively The span of the truss-girders is 48m (160 ft) with a 5 slope (crowned truss) Thebuilding length is 90m (300 ft) and consists of 11 truss-girdersthat have 9m (30 ft) spacing

21 System Analysis and Design The proposed system isdesigned in accordance with the ASCE 7ndash10 Standard ofMinimum Design Loads for Buildings and Other Structures[7] Vertical loads that are applied to the roof are dead load(119863) (roof) live load (119871

119903) and snow load (119878) Lateral loads

such as wind and earthquake loads are considered to beresisted by shear walls or column bracing systems similar tothose used in a typical warehouse structure and thereforewill not be presented in this paper A snow design load of144 kNm2 (30 psf) in addition to 072 kNm2 (15 psf) formechanical electrical and pluming (MEP) loads and metalroof decking is used for load calculations To analyze theproposed truss-girder system the AASHTO-PCI bulb teegirder (BT-72) is chosen for the truss-girder section as anexample of a readily available section to most precast bridgeproducers The structural analysis program SAP2000 is usedto model the proposed truss-girder using frame elementswith point loads applied at purlin locations Verticalmembershave moment releases at both ends to carry axial load onlyAnalysis results under factored loads show that maximumaxial forces in the top flange bottom flange and verticaland diagonal members are 7486 kN (1683 kip compression)7553 kN (1698 kip tension) 605 kN (136 kip tension) and1192 kN (268 kip compression) respectively For shippinghandling and erection phases analysis results indicate that

Journal of Structures 3

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CL

A B C D E F G H I J K

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300998400

Plan view

Con

cret

e tru

ss (t

yp)

1

2

Steelbracing(typ)

Steelbracing(typ)

Steelbracing(typ)

Figure 2 Plan and elevation views of the example building (1ft = 0302m)

forces in the top and bottom flanges are not critical Howevera tension force of 271 kN (61 kip) in the diagonal membersand a compression force of 129 kN (29 kip) in the verti-cal members are developed under factored constructionloads Midspan deflection under service live load is 160mm(63 in) which represents L305

The proposed truss system is designed using the strutand tie method according to Appendix A of the ACI 318-11 code [8] Diagonal members are designed as reinforcedconcrete struts while the vertical members are designed assteel ties The diagonal members have a specified concretecompressive strength of 55MPa (8000 psi) and 200mm times200mm(8 intimes8 in) square section reinforcedwith 4 number19 (6) Grade 420 (60) (reinforcement ratio of 275) to resistthe tension force experienced during construction when thetruss is being temporarily supported at the midspan priorto casting the wet joint and posttensioning Also number10 (3) Grade 420 (60) steel square ties are used as trans-verse reinforcement at 200mm (8 in) spacing The vertical

members are made of 38mm (15 in) diameter threaded rodswith yield strength of 724MPa (105 ksi) and ultimate strengthof 862MPa (125 ksi) [9] Despite the low tensile force carriedby the vertical members near themidspan the same threadedrods are used in all verticals to simplify fabrication and resistthe compression force induced during construction withoutbuckling [10]

The bottom and top flanges of the truss are also designedusing strut and tiemethodThe compression strut (top flange)has a specified concrete compressive strength of 55MPa(8000 psi) and is reinforced with 4 number 13 (4) Grade 420(60) The tension tie (bottom flange) has two posttensioningducts with 12ndash153mm (06 in) diameter Grade 1860 (270)low relaxation strands In addition 10ndash153mm (06 in)diameter Grade 1860 (270) low relaxation pretensionedstrands are used for truss transportation and handling Fig-ures 3 and 4 showconcrete dimensions and reinforcing detailsof the proposed truss-girder system Comparing the design ofthe proposed systemwith the one implemented in the Sharjah

4 Journal of Structures

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2

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6998400-61 4

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C L

31 2

99840099840031 2

998400998400 6998400998400-41

2

998400998400

Figure 3 Concrete truss dimensions (1ft = 0302m 1 in = 254mm)

coal storage facility presented in Figure 1 indicates that theproposed system is approximately 23 lighter in weight inaddition to being more economical to produce due to the useof standard I-girder forms conventional reinforcing detailsand self-consolidated concrete

22 Construction Sequence The proposed constructionsequence of the developed truss-girder system is as follows

(1) Truss-girders are fabricated in the precast plant intwo segments for each truss and transported to theconstruction site

Journal of Structures 5

Sect

ion

Sect

ion

Sect

ion

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ion

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85 8

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at en

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rrup

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3

5

11 2

998400998400empty

3A

8998400998400

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24998400 -1

0998400998400

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33 4

998400998400empty

PT d

uct

10998400998400

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44

bars

44

bars

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bars

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bars

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thro

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roug

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stran

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s (4

6)

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a hi

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reng

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eel r

od

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otes

at sh

eet S

K-04

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ed to

stee

l pla

te

26

strai

ght b

ars

(see

rein

forc

emen

t det

ails)

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uct3

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8998400998400

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4

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4

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998400998400

5998400998400

5998400998400

Elev

atio

n

Trus

s wei

ght =57

kips

hal

f tru

ss

Figure 4 Reinforcement details (1ft = 0302m 1 in = 254mm)

6 Journal of Structures

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Section Section Section

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2 halves of foamfor truss openings(see elevation view

for dimensions)

1

2

998400998400 plywood and(3) 2 times 4 studsfor block-out

Iowa type D girder form

3 998400-7 1

8998400998400

Figure 5 Dimensions of the concrete truss specimen (1ft = 0302m 1 in = 254mm)

(2) Each segment is erected on one column at one endand on temporary supports at the other end

(3) Roof purlins and bracings are installed to stabilize thetruss-girders

(4) Couplers are used to connect posttensioning ductsand posttensioning strands are threaded throughducts

(5) Joints between truss segments are formed reinforcedand cast in place using SCCwith similar properties tothose of the truss concrete

(6) Posttensioning is applied after joint concrete achievesadequate strength and posttensioning ducts aregrouted

(7) Temporary supports are removed and roof decking isinstalled

(8) Fire-proofing and corrosion resistance agents areapplied to steel elements if needed

3 Experimental Investigation

31 Specimen Description The purpose of the experimentalinvestigation is to evaluate the constructability and structuralperformance of the proposed truss-girder system A full-sizetruss could not be fabricated and tested due to space andbudget limitations A 9m (30 ft) long truss formed usingIowa type D bridge I-girder forms provided by Coreslab

Structures Inc Omaha NE was used instead The cross-section dimensions of Iowa type D I-girder are very closeto those of AASHTO type IV bridge girder The forms are9m (30 ft) long and 1420mm (56 in) high However toreduce the weight of the specimen a 100mm (4 in) block-out was made at the bottom of the form to have a totaldepth of 1320mm (52 in) and two foam panels 100mm(4 in) thick each were used to form for each truss openingFigure 5 shows the dimensions of the concrete truss specimenat different sections

32 Specimen Analysis and Design Two-dimensional (2D)frame analysis and three-dimensional (3D) finite elementanalysis (FEA) were carried out to determine member forcesand deformations of the specimen Comparing analysisresults of each method as shown in Table 1 indicates that thesimple 2D frame analysis results in conservative and relativelyaccurate estimates of forces and deflections compared to themore elaborate FEA The loads used in this analysis includeweight of the specimen prestressing force and midspanconcentrated load which achieve forces in the diagonal andvertical members of the specimen similar to those in the full-size truss-girder system designed in the previous section Itshould be noted that diagonalmembers of the specimen had a40∘ angle with the bottom flange in order to achieve the sameratio between diagonal and vertical forces as in the full-sizetruss system presented earlier

The analysis of the specimen indicated that a midspanpoint load of 1779 kN (400 kip) will result in the forces

Journal of Structures 7

Table 1 Comparing analysis results using 2D frame model and 3DFE model

Analysis method 2D frameanalysis

Finite elementanalysis

Max compression indiagonals (kN) 1246 1125

Max tension inverticals (kN) 574 534

Cracking load (kN) 1478 1335Correspondingdeflection (mm) 25 21

Figure 6 The FE model used to analyze truss connections

that are slightly higher than the design factored forcesin diagonal and vertical members The initial design ofthe specimen required 16ndash153mm (06 in) diameter Grade1860 (270) strands However due to the unavailability ofthis size of strands at the structural laboratory 12ndash178mm(07 in) diameter Grade 1860 (270) strands were used insteadto achieve the same prestressing force Also the analysisindicated that the cracking load is 1468 kN (330 kip) usingthe cracking stress limit at the bottom fibers and the corre-sponding deflection is 25mm (1 in) Strand jacking stress wasassumed to be 075119891pu and total prestress losses were assumedto be 20of the jacking stressThe developed FEmodel of thespecimen shown in Figure 6 consists of a simplified cross-section (ie rectangles) 8-node solid elements for concreteand frame elements for threaded rods This model was usedto conduct elastoplastic analysis using specified materialproperties to determine stresses and deformations underdifferent loading levels Figure 7 shows the stress contours onconcrete elements under ultimate load This figure indicatesthat the connection between diagonal web and the bottomflange had very high tensile stresses which are expected tocause premature cracking at those locations

The specimen was designed similar to the full-size trusswith one exception top and bottom flanges were overde-signed to ensure that the failure occurs at the verticalsdiagonals or connections The 12ndash178mm (07 in) diameterstrands were provided to achieve a flexural capacity of4371 kNsdotm (3224 kipsdotft) which is about 10 more than theapplied moment Also the top flange was reinforced with2 number 25 (8) bars as compression reinforcement toincrease the capacity of the top flange Figure 8 shows theelevation cross-sections and reinforcement details of thespecimen

E + 6

550

440

330

220

110

00

minus110

minus220

minus330

minus440

minus550

minus660

minus770

minus880

Figure 7 Stresses in the truss specimen under ultimate load atmidspan (Nm2)

33 Specimen Fabrication Fabrication of the 9m (30-ft)long specimen was done at the structural lab of the PeterKiewit Institute (PKI) in Omaha NE in five major steps(1) preparing forms and placing prestressing strands (2)cutting foam block-outs and gluing them to the steel forms(3) assembling diagonal and vertical reinforcement andinstalling them between the foam block-outs (4) casting self-consolidating concrete into the forms and (5) stripping theforms and releasing the strands

The prestressing strands were tensioned to 3176 kN(714 kip) (075 of the ultimate stress of 1860MPa (270 ksi))Foam block-outs were used to form the truss openings The100mm (4 in) thick foam panels were cut into diamondshapes and glued to form the 200mm (8 in) thick block-outs Square grooves that are 19mm times 19mm (075 in times075 in) were removed from the edges of the foam panelsto accommodate the vertical threaded rods as shown inFigure 9 To facilitate the stripping of the foam from theconcrete web plastic sheets were wrapped around the edgesof the foam All the foam block-outs were glued on the steelform after marking their locations on the form sides Bottomflange and top flange reinforcement were simple to installThe challenge was assembling and installing the diagonaland vertical reinforcement which were 4 number 19 (6)bars and number 10 (3) ties spaced at 200mm (8 in) alongthe member and 38mm (15 in) diameter threaded rodsThe rods were anchored at the top and bottom flange using200mm times 200mm times 13m (8 in times 8 in times 05 in) Grade350 (50) steel plates and structural nuts Each plate is weldedto two diagonal bars and 2 number 19 (6) straight anchorbars Initially it was planned to have the reinforcementsfor each diagonal preassembled and then connected to thethreaded rods after being installed in the form The mainproblem of this plan is that it requires very tight tolerancesin reinforcement dimensions and bent locations in additionto the difficulty of handling a very heavy assembly ofreinforcement that is not very stiff Some diagonal bars wereslightly shorter than others and do not have exactly thesame bend diameter or location To address these challengesdiagonal bars and anchor bars were cut to have only 225mm(9 in) of embedment (12 db) and transverse ties were keptloose to allow the bars to move relative to each other thenthey are tied after all reinforcement is in place to simplify thefabrication process as shown in Figure 9 Four 100 times 100 minusMW20 timesMW20 (4 times 4minusW29 timesW29)WWRwere placed atthe solid part at each endof the truss for shear reinforcements

8 Journal of Structures

A B

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30998400-0998400998400

Detail A

3 ties at8998400998400 spacings

46

SectionSection Section Section

DA B C

5998400 998400

11 2

99840099840011 2

998400998400

1998400-9998400998400

1998400-9998400998400

1998400-11998400998400

D116998400998400

36998400998400 spacing

21

2

99840099840021

2

998400998400

21

2

998400998400

61

2

998400998400

71

2

998400998400

71 2

998400998400

21

2

998400998400

1998400-11998400998400

4998400998400

6998400998400

6998400998400

8998400998400

4998400998400

4998400998400

2998400998400

11998400998400

2998400-51 2

998400998400111 2

998400998400

PL 23998400998400times6

998400998400times1

2

998400998400

w3 1

2

998400998400 studs

7998400998400 spacings

41

2

99840099840041

2

9984009984007998400998400

7998400998400

3998400998400

5998400998400

6998400998400

1998400-11998400998400

Detail A

Topreinforcements

reinforcements

36998400998400 spacings

Bottom

W2

Diagonals rebarrefer to the details below

11

2

998400998400emptythreaded rod

and a structural nut

8998400998400

8998400998400

PL 8998400998400times8

998400998400times1

2

998400998400

opening locatedat center for TRdia of openingcorresponds to

the TR dia

Plate welded to thediagonal rebar

6998400998400

1 2

998400998400

4998400-2

998400998400

11

2

998400998400emptythreaded rods w6998400998400

threads at the endsGR 120 B7 weathered

steel

Threaded rod

A-A

3998400-101 2

998400998400

3998400-2998400998400

4998400 -85

8

998400998400

2998400-6998400998400

2998400-6998400998400

8998400998400

8998400998400

R4 1

2 998400998400

59984009984008998400998400

11 2

99840099840011 2

998400998400

Diagonal steel reinforcement details

4998400-4

998400998400

Figure 8 Elevation and cross-sections of the specimen (1ft = 0302m 1 in = 254mm)

The top flange had 2 number 25 (8) bars tied using number10 (3) stirrups at 150mm (6 in) spacing After the form wasclosed the top flange reinforcement assembly was placed andlumber yokes were used to tie the forms from the top

The specimen was cast on March 11 2013 using specified55MPa (8000 psi) self-consolidating concrete The mixturewas designed using type III Portland cement with 30replacement of class C fly ash and a mixture of 10mm(38 in) crushed limestone and natural sand and gravel

The SCC had an average spread of 800mm (28 in) 11987950

less than 2 sec J-ring spread reduction less than 50mm(2 in) and visual stability index (VSI) of 10 Nine 100mm times200mm (4 in times 8 in) cylinders were taken to evaluate thecompressive strength at release testing and 28 days CastingSCC started at the middle of the specimen Two pipe cameraswere attached at the bottom of the truss one at each endto record the concrete flow around the reinforcement andstrands Another camera was recording the casting process

Journal of Structures 9

Figure 9 All reinforcements after being installed in the form

from the top The high flowability and passing ability of SCCmade it easy to fill the bottom flange with no consolidationproblems However as soon as SCC started to fill trussdiagonals and verticals two foam panels started to float asthe buoyant forces broke the bond between the steel formsand foam block-outs Several actions were taken to keep thefoam block-outs in place against the uplifting force Lumberpieces were used as spacers between the foam and top flangereinforcement and between the top reinforcement and thesteel form to prevent further movements of the block-outsThis experiment indicated that foam panels were not a goodchoice for forming

After curing the specimen with wet burlap for 3 days theconcrete strength reached 524MPa (7600 psi) forms werestripped and strands were released on March 14 2013 Steelforms were easy to strip however foam block-outs wereembedded in the concrete and did not come out easily Thepressure of the concrete on the foam made it difficult topull out of the concrete in addition to having a thin layerof concrete between the foam and the steel form in somelocations which had to be chipped out Foam block-outs hadto be cut into small pieces using an electric saw Removing theblock-outs at the corners was evenmore challenging A smallchipping hammerwas used to carefully remove the remainingfoam without damaging the concrete The movement of thefoam block-outs while casting resulted in deviations in thedimensions angles and locations of two diagonal membersand two vertical members as shown in Figure 10

Prestressing strands were released using gradual deten-sioning and specimen ends were inspected for cracking Fewcracks appeared at south and north ends that were mostlyhorizontal and extended for the full thickness of the web andfew inches in the longitudinal directionThese cracks are dueto the bursting force of prestressing and were not controlledproperly because the end zone reinforcements were notplaced as close as they should be to the bulkheads Fewshrinkage cracks occurred in the top flange at the locationswere the lumber pieces were placed to prevent the floatingof the foam These cracks were not critical for the proposedtesting as they occurred only in the top flange which is acompression member

34 Specimen Testing Two roller supports were placed onconcrete blocks and spaced 89m (295 ft) apart on center tosupport the 9m (30 ft) long truss specimen The rollers werecentered on the 150mm (6 in) wide bearing plates embeddedin the truss at both ends A steel frame with a 1780 kN(400 kip) loading jack was placed as shown in Figure 11 toload the specimen at midspan section

To clearly see and track crack propagation of trusselements during loading one side of the specimen waspainted in white while strain gages were attached to the otherside Linear variable differential transforms (LVDTs) wereused to monitor the slippage of strands during testing Adeflection gage was installed to measure midspan deflectionsduring loading The specimen was tested on March 29 2013Concrete cylinders were tested and the compressive strengthwas found to be 724MPa (10500 psi) at that time Duringloading the specimen was visually inspected at 2225 kN(50 kip) load increments and cracking was being marked upAt 2225 kN (50 kip) the deflection reached 5mm (019 in)with no visible cracking The loading continued to 445 kN(100 kip) and deflection reached 10mm (039 in) Minorhorizontal cracks were observed at the corners betweendiagonals and topbottom flanges as shown in Figure 12

At 667 kN (150 kip) the deflection reached 15mm(057 in) and cracking continued at acute-angled cornersbetween the solid web and bottom flangeThemiddle verticalstarted to crack as well at the top and bottom At 890 kN(200 kip) deflection reached 20mm (075 in) and crackingoccurred at all the acute-angled corners At 1112 kN (250 kip)deflection reached 24mm (093 in) and cracking severityhad not significantly increased except at the webbottomflange corner After 1112 kN (250 kip) the load increasedcontinuously without interruptions up to failure at 1712 kN(385 kip) Excessive cracking was observed at the bottomflange around its connection to vertical rods and diagonalsas shown in Figure 13 The failure was dramatic as the one-threaded rod was pulled out of the bottom flange causing theadjacent diagonal to snap as shown in Figure 14 This failureoccurred as one of the 6 anchor bars welded to the washerplate was completely sheared as shown in Figure 15 Despitethe high load carrying capacity that the specimen achieved itis believed that having longer anchor bars and hat bars forbottom flange confinement around the anchor bars wouldhave postponed or even eliminated this mode of failure Alsochamfering sharp edges and using curved corners wouldhave reduced stress concentrations andminimized prematurecracking at these locations

35 Analysis of Results Figure 16 plots the load-deflectionrelationship of the truss specimen This plot indicates thatthe specimen had a linear elastic behavior up to the crackingload which is determined to be 1580 kN (355 kip) using thetangents method This load is 7 higher than the 1468 kN(330 kip) predicted cracking loadThemeasured deflection atthe cracking load was found to be 34mm (133 in) which is33 higher than the 25mm (10 in) predicted deflectionThisis primarily due to the premature cracking that was observedat almost all acute-angled corners whichmight have resultedin stiffness reduction Also the deviations between specified

10 Journal of Structures

85 in

75 in

85

in

85 in

9 in 9 in 9 in 9 in

65

in11 in

North South

North 1 South 1North 2 South 2

Figure 10 Actual truss member concrete dimensions after casting (1 in = 254mm)

Figure 11 Truss specimen before testing

Figure 12 First cracks observed at 445 kN (100 kip)

Figure 13 Excessive cracking at the bottom flange before failure

Figure 14 Failure of the truss specimen

Figure 15 Shearing of anchor bars at the failure location

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 100 200 300 400 500 600

Load

(kN

)

Midspan deflection (mm)

Figure 16 Load versus deflection curve (black lines indicate end ofelastic behavior)

and actual dimensions could have influenced the behavior ofthe specimen Figure 16 also indicates that the ultimate loadwas 1713 kN (385 kip) which is 38 lower than the predictedcapacity of 1779 kN (400 kip) due to the premature pullout of

Journal of Structures 11

the vertical threaded rod as a result of inadequate anchoragein the bottom flange

Figure 17 shows themeasured slippage of 178mm (07 in)diameter prestressing strands during loading This plot indi-cates that all the recorded readings are significantly less than025mm (001 in) which is the limit for initial slippageThe highest recorded value was even less than 0025mm(0001 in) which is the measurement accuracy of the usedLVDTs indicating no slippage up to failure load This meansthat 178mm (07 in) diameter prestressing strands were fullydeveloped within the 45m (15 ft) distance (ie half of thespecimen length) which is the predicted development lengthusing ACI 318-11 It should be noted that the high valuerecorded by the south LVDT at the failure load is incorrectdue to the sudden movement of the specimen at the momentof failure

Figure 18 plots the measured strain in four vertical mem-bers of the truss specimen versus the applied load Theseverticals are 38mm (15 in) diameter threaded rods that havea yield strength of 724MPa (105 ksi) and ultimate strengthof 862MPa (125 ksi)Themaximummeasured strain reached26 (at the south rod 1 where the failure occurred) Also allmeasured strains in the four threaded rods had significantlyexceeded the yield strain of 036 Figure 19 plots the forcesin the four threaded rods versus the applied load This plotindicates that the forces in all the four threaded rods reachedthe yield force of 689 kN (155 kip) which is 14 more thanthe predicted design force of 605 kN (136 kip)

Figure 20 plots the measured strains in the four rein-forced concrete diagonal members of the truss specimenversus the applied loadThis plot indicates that the measuredstrains varied significantly among the four diagonalmembersdue to the variation in their angles and concrete dimensions(eg the width of the south diagonal 1 was 165mm (65 in)while the width of the north diagonal 2 was 280mm(11 in)) However they were all much lower than the ultimatedesign strain of concrete (03) The maximum ultimatestrain reached 01 at the south diagonal 1 where failurehappened and the minimum ultimate strain reached 0045at the north diagonal 2 Also the straight line relationshipsin all diagonal members indicate their linear elastic behaviorup to failure loadTherefore the truss failure due to crushingof the diagonals is a low probability mode of failure Figure 21plots the forces in all four diagonal members versus theapplied loadThis plot indicates that the ultimate compressiveforce varied from 1179 kN (265 kip) at the north diagonal2 to 1446 kN (325 kip) at the south diagonal 1 wherefailure occurred The forces in all diagonal members exceptthe north diagonal 2 exceeded the design force of 1192 kN(268 kip) which indicates that the design of the diagonals wasadequate

4 Conclusions and Recommendations

This research aimed to develop a precast concrete truss sys-tem for roof applications that is lightweight and aestheticallypleasing can span up to 48m (160 ft) and can be fabricated

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025

Load

(kN

)

Slippage (mm)

North strand slippageSouth strand slippage

Figure 17 Strand slippage measurements (1 kip = 445 kN 1 in =254mm)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 05 10 15 20 25 30

Load

(kN

)

Strain in TR ()

N rod 1

N rod 2

S rod 1

S rod 2

Figure 18 Tensile strains in the four threaded rods (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

mdash 100 200 300 400 500 600 700 800

Load

(kN

)

Force in TR (kN)

N rod 1

N rod 2

S rod 1

S rod 2

Figure 19 Tension forces in the four threaded rods (black lineindicates yield force 1 kip = 445 kN)

12 Journal of Structures

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025 030

Load

(kN

)

Strain in diagonals ()

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 20 Compressive strains in the diagonal members (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 200 400 600 800 1000 1200 1400 1600

Load

(kN

)

Force in diagonals (kN)

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 21 Compressive forces in the diagonal members (black lineindicates yield force 1 kip = 445 kN)

using standard precast practices in the USA Based on thepresented work three main conclusions can be made

(1) Fabrication of the proposed truss system is practicaland efficient The proposed method of fabricationwas evaluated experimentally by producing a 9m(30 ft) long truss specimen using economical andcommercially available components standard bridgegirder form foam block-outs vertical threaded rodsand conventional reinforcement The success of theproposed method is also attributed to the use of highstrength self-consolidating concrete (SCC) that fillsthe complex form encapsulates the reinforcementand provides a smooth finished surface without anymechanical consolidation

(2) Structural testing of the truss specimen that wasdesigned and detailed to resist the forces generatedin an example building indicated the adequacy ofthe proposed design method and connection details

Few recommendations were made as shown belowto further improve the performance of the proposedsystem

(3) The 2D frame models and 3D finite element modelscan be used to accurately predict the behavior of theproposed system Forces obtained from these modelscan be used to design truss members using the strut-and-tie method and in accordance to the existingbuilding code FEA can be used to accurately predictstress concentration at truss member connections

Based on the analytical and experimental investigationsseveral recommendations could be made to improve theproposed truss system

(1) Use chamfered edges and curved corners instead ofthe sharp ones to avoid stress concentrations and addnominal reinforcement to control cracking at theselocations

(2) Use adequate top flange and bottom flange confer-ment reinforcement to help anchoring diagonal andvertical reinforcement whichmust be fully developedto prevent pullout

(3) Avoid using foam block-outs due to the difficultyof gluing them to the form and stripping themUsing light gage steel pans or fiber glass panes ishighly recommended for efficient and economicalfabrication as they can have multiple reuses

(4) Diagonal bars should be tied together after beingplaced in the forms to accommodate the tolerancesespecially when bar lengths and bent diameters arenot exact This practice will allow the diagonalsto slide against each other Another suggestion forfabrication is to assemble all the reinforcements in astiff manner outside the form to precisely match theform dimensionsThen the assembly is tied togetherlifted by a crane and placed in the form in one step

(5) Self-consolidating concrete with high flowability(average spread of 800mm (28 in) plusmn 50mm (2 in))passing ability (nominal maximum size aggregate is10mm (38 in)) resistance to segregation (VSI notmore than 10) and low viscosity (119879

50lt 2 sec) is

required to simplify production and ensure properfilling of the complex truss form without mechanicalconsolidation

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The technical support of the PrecastPrestressed ConcreteInstitute (PCI) advisory committee members is acknowl-edged and the financial support of the PCI Daniel PJenny Fellowship is appreciated Assistance was provided by

Journal of Structures 13

Musa Alawneh as a scholarship partner Support was alsogiven by University of Nebraska-Lincoln (UNL) graduatesstudents and staff Kelvin Lein AfshinHatami Shaddi AssadMohamed El-Kady and Micheal Asaad

References

[1] PCI PCI Design Hnadbook PrecastPrestressed Concrete Insti-tute (PCI) Chicago Ill USA 2010

[2] PCI ldquoThe 50 most significant precast concrete projectsrdquoASCENT Magazine 2004

[3] P H Shah and H R May ldquoPrecast vierendeel trusses pro-vide unique structural facade for parking structurerdquo Pre-castPrestressed Concrete Institute Journal vol 22 no 4 pp 24ndash39 1977

[4] W T Carroll F W Beaufait and R H Bryan ldquoPrestressedconcrete trussesrdquo ACI Journal vol 75 no 8 pp 367ndash373 1978

[5] MDeSutter United States of America Patent 7275348 B2 2007[6] A Trygestad andM DeSutter ldquoBookmen stacks cobalt condos

use ER-POST for column-free spacerdquo PCI Journal vol 52 no3 pp 58ndash71 2007

[7] ASCESEI 7 Minimum Design Loads for Buildings and OtherStructures American Society of Civil Engineers (ASCE)Reston Va USA 2010

[8] ACI 318 Building Code Requirments for Structural Concrete(ACI 318-11) and Commentary American Concrete Institute(ACI) Farmington Hills Mich USA 2011

[9] ASTMA193 ldquoStandard specification for alloy-steel and stainlesssteel bolting for high temperature or high pressure service andother special purpose applicationsrdquo ASTM Standard A193M-14ASTM International West Conshohocken Pa USA 2014

[10] AISC Steel Construction Manual American Institute of SteelConstruction (AISC) Chicago Ill USA 14th edition 2012

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RotatingMachinery

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VLSI Design

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Shock and Vibration

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Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

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Electrical and Computer Engineering

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Advances inOptoElectronics

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Volume 2014

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Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

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Chemical EngineeringInternational Journal of Antennas and

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International Journal of

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Navigation and Observation

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International Journal of

2 Journal of Structures

(16 in times 22 in) [3] Vertical members were posttensioned toresist tension forces

Precastprestressed concrete trusses were introduced in1978 in the ACI Journal article titled ldquoPrestressed ConcreteTrussesrdquo [4]The article discussed two prototypes prototype-I that had a clear span of 61m (203 ft) and a depth of 06m(2 ft) for a span-to-depth ratio of 10 and prototype-II that hada clear span of 184m (608 ft) and a depth of 26m (85 ft)for a span-to-depth ratio of 7 The first prototype had onlydiagonal members with no verticals however the secondprototype had diagonal members and two verticals near thecenter of the trusses All top bottom and diagonal memberswere prestressed However the prestressing in the diagonalswas only 35 of the jacking stresses due to the large frictionlosses occurring at the hold-down devicesThe authors statedthat the members cracked at an early stage of loading due tothe fact that the diagonals were not adequately prestressedThe authors also stated that using concrete trusses wouldlower the price to almost half what it would cost if steelalternativewas used In 2007 a new systemof concrete trusseswas developed for a multilevel condominium building builtin Minneapolis MN using the so-called ldquoER-Postrdquo The ER-Post is system invented by M DeSutter of Erickson Roed ampAssociates to provide a column-free space for condominiums[5] DeSutter was able to pretension Vierendeel trusses thatwere 41m (135 ft) deep and of spanning 203m (6733 ft) fora span-to-depth ratio of 5 [6]

In 2010 a precast concrete truss-girder was designedto support the roof of a coal storage facility in SharjahUnited Arab Emirates (UAE) Designed by eConstruct USALLC the 15m (5 ft) deep truss had a span of 50m (165 ft)without intermediate supports which resulted in a span-to-depth ratio of 33 The truss consisted of two precasttruss segments each segment is 25m (825 ft) long The twosegments were connected using posttensioning tendons andcast-in-place concrete joint Several trusses were erected at10m (30 ft) spacing to create the crowned roof Figure 1 showsthe built trusses and temporary support used during erectionat the midspan to support the two truss segments untilposttensioning was applied and the cast-in-place concretejoint was hardened

According to eConstruct USA LLC the use of precastconcrete truss system along with Z-shape steel purlins andmetal roof decking resulted in about 25 saving in thebuilding cost compared to the original design using structuralsteelThis is a significant saving that motivated the authors tofurther investigate precast concrete truss systems to optimizetheir designs improve their constructability and accom-modate production practices in the United States Severalenhancements which will be discussed in Section 2 haveresulted in cost and weight reductions and consequently thepotential of using precast concrete trusses in long-span roofapplications

2 Development of the Proposed System

Theprecast concrete truss system proposed in this study is anevolution of the Sharjah truss-girder system presented earlierThe main enhancements that have been proposed to address

Figure 1 The temporary supports used before posttensioning(courtesy of eConstruct LLC)

design fabrication and construction issues include (1) chang-ing the orientation of diagonals to be compression membersmade of conventionally reinforced concrete (2) using highstrength steel threaded rods for tension members (verticals)to eliminate cracking (3) using readily available forms oftypical precastprestressed concrete bridge I-girders such asAASHTO and bulb tee with modular block-outs to simplifyproduction (4) using high performance self-consolidatingconcrete (SCC) to ensure the quality efficiency and economyof truss fabrication and (5) placing posttensioning ducts inthe bottom flange to eliminate the need for thicker websat girder ends To present these enhancements an examplebuilding has been chosen to design the proposed truss-girdersystem Figure 2 shows the plan and elevation views of theexample building layout respectively The span of the truss-girders is 48m (160 ft) with a 5 slope (crowned truss) Thebuilding length is 90m (300 ft) and consists of 11 truss-girdersthat have 9m (30 ft) spacing

21 System Analysis and Design The proposed system isdesigned in accordance with the ASCE 7ndash10 Standard ofMinimum Design Loads for Buildings and Other Structures[7] Vertical loads that are applied to the roof are dead load(119863) (roof) live load (119871

119903) and snow load (119878) Lateral loads

such as wind and earthquake loads are considered to beresisted by shear walls or column bracing systems similar tothose used in a typical warehouse structure and thereforewill not be presented in this paper A snow design load of144 kNm2 (30 psf) in addition to 072 kNm2 (15 psf) formechanical electrical and pluming (MEP) loads and metalroof decking is used for load calculations To analyze theproposed truss-girder system the AASHTO-PCI bulb teegirder (BT-72) is chosen for the truss-girder section as anexample of a readily available section to most precast bridgeproducers The structural analysis program SAP2000 is usedto model the proposed truss-girder using frame elementswith point loads applied at purlin locations Verticalmembershave moment releases at both ends to carry axial load onlyAnalysis results under factored loads show that maximumaxial forces in the top flange bottom flange and verticaland diagonal members are 7486 kN (1683 kip compression)7553 kN (1698 kip tension) 605 kN (136 kip tension) and1192 kN (268 kip compression) respectively For shippinghandling and erection phases analysis results indicate that

Journal of Structures 3

33998400 -0

998400998400

5 slope 5 slope

160998400-0998400998400

Truss elevation

CL

A B C D E F G H I J K

30998400-0998400998400 30

998400-0998400998400 30998400-0998400998400 30

998400-0998400998400 30998400-0998400998400 30

998400-0998400998400 30998400-0998400998400 30

998400-0998400998400 30998400-0998400998400 30

998400-0998400998400

80998400 -0

99840099840080998400 -0

998400998400

160998400 -0

998400998400

8998400

typ

Steel purlins(typ)

300998400

Plan view

Con

cret

e tru

ss (t

yp)

1

2

Steelbracing(typ)

Steelbracing(typ)

Steelbracing(typ)

Figure 2 Plan and elevation views of the example building (1ft = 0302m)

forces in the top and bottom flanges are not critical Howevera tension force of 271 kN (61 kip) in the diagonal membersand a compression force of 129 kN (29 kip) in the verti-cal members are developed under factored constructionloads Midspan deflection under service live load is 160mm(63 in) which represents L305

The proposed truss system is designed using the strutand tie method according to Appendix A of the ACI 318-11 code [8] Diagonal members are designed as reinforcedconcrete struts while the vertical members are designed assteel ties The diagonal members have a specified concretecompressive strength of 55MPa (8000 psi) and 200mm times200mm(8 intimes8 in) square section reinforcedwith 4 number19 (6) Grade 420 (60) (reinforcement ratio of 275) to resistthe tension force experienced during construction when thetruss is being temporarily supported at the midspan priorto casting the wet joint and posttensioning Also number10 (3) Grade 420 (60) steel square ties are used as trans-verse reinforcement at 200mm (8 in) spacing The vertical

members are made of 38mm (15 in) diameter threaded rodswith yield strength of 724MPa (105 ksi) and ultimate strengthof 862MPa (125 ksi) [9] Despite the low tensile force carriedby the vertical members near themidspan the same threadedrods are used in all verticals to simplify fabrication and resistthe compression force induced during construction withoutbuckling [10]

The bottom and top flanges of the truss are also designedusing strut and tiemethodThe compression strut (top flange)has a specified concrete compressive strength of 55MPa(8000 psi) and is reinforced with 4 number 13 (4) Grade 420(60) The tension tie (bottom flange) has two posttensioningducts with 12ndash153mm (06 in) diameter Grade 1860 (270)low relaxation strands In addition 10ndash153mm (06 in)diameter Grade 1860 (270) low relaxation pretensionedstrands are used for truss transportation and handling Fig-ures 3 and 4 showconcrete dimensions and reinforcing detailsof the proposed truss-girder system Comparing the design ofthe proposed systemwith the one implemented in the Sharjah

4 Journal of Structures

1998400-71 4

998400998400

1998400-71 4

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

998400998400

81998400-2

998400998400

24998400-10998400998400

24998400-10998400998400

3998400-8

998400998400

2998400-4

998400998400

Det

ail-A

Plan

G

D

1998400-0

998400998400

332∘

7998400-0998400998400

6998400-6 1

4

998400998400

8998400998400

F

BC

A

5998400-0

998400998400

4998400-0

998400998400

1998400-8

998400998400

1998400-10 1

2

998400998400

6998400-0998400998400

49984009984004998400998400

49984009984004998400998400

1998400998400

8998400998400

8998400998400

10998400998400

1998400998400

1998400998400

Sect

ion

Sect

ion

Sect

ion

Sect

ion

Sect

ion

Sect

ion

G

AB

CD

G

6998400-0998400998400

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Figure 3 Concrete truss dimensions (1ft = 0302m 1 in = 254mm)

coal storage facility presented in Figure 1 indicates that theproposed system is approximately 23 lighter in weight inaddition to being more economical to produce due to the useof standard I-girder forms conventional reinforcing detailsand self-consolidated concrete

22 Construction Sequence The proposed constructionsequence of the developed truss-girder system is as follows

(1) Truss-girders are fabricated in the precast plant intwo segments for each truss and transported to theconstruction site

Journal of Structures 5

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6 Journal of Structures

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for dimensions)

1

2

998400998400 plywood and(3) 2 times 4 studsfor block-out

Iowa type D girder form

3 998400-7 1

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Figure 5 Dimensions of the concrete truss specimen (1ft = 0302m 1 in = 254mm)

(2) Each segment is erected on one column at one endand on temporary supports at the other end

(3) Roof purlins and bracings are installed to stabilize thetruss-girders

(4) Couplers are used to connect posttensioning ductsand posttensioning strands are threaded throughducts

(5) Joints between truss segments are formed reinforcedand cast in place using SCCwith similar properties tothose of the truss concrete

(6) Posttensioning is applied after joint concrete achievesadequate strength and posttensioning ducts aregrouted

(7) Temporary supports are removed and roof decking isinstalled

(8) Fire-proofing and corrosion resistance agents areapplied to steel elements if needed

3 Experimental Investigation

31 Specimen Description The purpose of the experimentalinvestigation is to evaluate the constructability and structuralperformance of the proposed truss-girder system A full-sizetruss could not be fabricated and tested due to space andbudget limitations A 9m (30 ft) long truss formed usingIowa type D bridge I-girder forms provided by Coreslab

Structures Inc Omaha NE was used instead The cross-section dimensions of Iowa type D I-girder are very closeto those of AASHTO type IV bridge girder The forms are9m (30 ft) long and 1420mm (56 in) high However toreduce the weight of the specimen a 100mm (4 in) block-out was made at the bottom of the form to have a totaldepth of 1320mm (52 in) and two foam panels 100mm(4 in) thick each were used to form for each truss openingFigure 5 shows the dimensions of the concrete truss specimenat different sections

32 Specimen Analysis and Design Two-dimensional (2D)frame analysis and three-dimensional (3D) finite elementanalysis (FEA) were carried out to determine member forcesand deformations of the specimen Comparing analysisresults of each method as shown in Table 1 indicates that thesimple 2D frame analysis results in conservative and relativelyaccurate estimates of forces and deflections compared to themore elaborate FEA The loads used in this analysis includeweight of the specimen prestressing force and midspanconcentrated load which achieve forces in the diagonal andvertical members of the specimen similar to those in the full-size truss-girder system designed in the previous section Itshould be noted that diagonalmembers of the specimen had a40∘ angle with the bottom flange in order to achieve the sameratio between diagonal and vertical forces as in the full-sizetruss system presented earlier

The analysis of the specimen indicated that a midspanpoint load of 1779 kN (400 kip) will result in the forces

Journal of Structures 7

Table 1 Comparing analysis results using 2D frame model and 3DFE model

Analysis method 2D frameanalysis

Finite elementanalysis

Max compression indiagonals (kN) 1246 1125

Max tension inverticals (kN) 574 534

Cracking load (kN) 1478 1335Correspondingdeflection (mm) 25 21

Figure 6 The FE model used to analyze truss connections

that are slightly higher than the design factored forcesin diagonal and vertical members The initial design ofthe specimen required 16ndash153mm (06 in) diameter Grade1860 (270) strands However due to the unavailability ofthis size of strands at the structural laboratory 12ndash178mm(07 in) diameter Grade 1860 (270) strands were used insteadto achieve the same prestressing force Also the analysisindicated that the cracking load is 1468 kN (330 kip) usingthe cracking stress limit at the bottom fibers and the corre-sponding deflection is 25mm (1 in) Strand jacking stress wasassumed to be 075119891pu and total prestress losses were assumedto be 20of the jacking stressThe developed FEmodel of thespecimen shown in Figure 6 consists of a simplified cross-section (ie rectangles) 8-node solid elements for concreteand frame elements for threaded rods This model was usedto conduct elastoplastic analysis using specified materialproperties to determine stresses and deformations underdifferent loading levels Figure 7 shows the stress contours onconcrete elements under ultimate load This figure indicatesthat the connection between diagonal web and the bottomflange had very high tensile stresses which are expected tocause premature cracking at those locations

The specimen was designed similar to the full-size trusswith one exception top and bottom flanges were overde-signed to ensure that the failure occurs at the verticalsdiagonals or connections The 12ndash178mm (07 in) diameterstrands were provided to achieve a flexural capacity of4371 kNsdotm (3224 kipsdotft) which is about 10 more than theapplied moment Also the top flange was reinforced with2 number 25 (8) bars as compression reinforcement toincrease the capacity of the top flange Figure 8 shows theelevation cross-sections and reinforcement details of thespecimen

E + 6

550

440

330

220

110

00

minus110

minus220

minus330

minus440

minus550

minus660

minus770

minus880

Figure 7 Stresses in the truss specimen under ultimate load atmidspan (Nm2)

33 Specimen Fabrication Fabrication of the 9m (30-ft)long specimen was done at the structural lab of the PeterKiewit Institute (PKI) in Omaha NE in five major steps(1) preparing forms and placing prestressing strands (2)cutting foam block-outs and gluing them to the steel forms(3) assembling diagonal and vertical reinforcement andinstalling them between the foam block-outs (4) casting self-consolidating concrete into the forms and (5) stripping theforms and releasing the strands

The prestressing strands were tensioned to 3176 kN(714 kip) (075 of the ultimate stress of 1860MPa (270 ksi))Foam block-outs were used to form the truss openings The100mm (4 in) thick foam panels were cut into diamondshapes and glued to form the 200mm (8 in) thick block-outs Square grooves that are 19mm times 19mm (075 in times075 in) were removed from the edges of the foam panelsto accommodate the vertical threaded rods as shown inFigure 9 To facilitate the stripping of the foam from theconcrete web plastic sheets were wrapped around the edgesof the foam All the foam block-outs were glued on the steelform after marking their locations on the form sides Bottomflange and top flange reinforcement were simple to installThe challenge was assembling and installing the diagonaland vertical reinforcement which were 4 number 19 (6)bars and number 10 (3) ties spaced at 200mm (8 in) alongthe member and 38mm (15 in) diameter threaded rodsThe rods were anchored at the top and bottom flange using200mm times 200mm times 13m (8 in times 8 in times 05 in) Grade350 (50) steel plates and structural nuts Each plate is weldedto two diagonal bars and 2 number 19 (6) straight anchorbars Initially it was planned to have the reinforcementsfor each diagonal preassembled and then connected to thethreaded rods after being installed in the form The mainproblem of this plan is that it requires very tight tolerancesin reinforcement dimensions and bent locations in additionto the difficulty of handling a very heavy assembly ofreinforcement that is not very stiff Some diagonal bars wereslightly shorter than others and do not have exactly thesame bend diameter or location To address these challengesdiagonal bars and anchor bars were cut to have only 225mm(9 in) of embedment (12 db) and transverse ties were keptloose to allow the bars to move relative to each other thenthey are tied after all reinforcement is in place to simplify thefabrication process as shown in Figure 9 Four 100 times 100 minusMW20 timesMW20 (4 times 4minusW29 timesW29)WWRwere placed atthe solid part at each endof the truss for shear reinforcements

8 Journal of Structures

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Figure 8 Elevation and cross-sections of the specimen (1ft = 0302m 1 in = 254mm)

The top flange had 2 number 25 (8) bars tied using number10 (3) stirrups at 150mm (6 in) spacing After the form wasclosed the top flange reinforcement assembly was placed andlumber yokes were used to tie the forms from the top

The specimen was cast on March 11 2013 using specified55MPa (8000 psi) self-consolidating concrete The mixturewas designed using type III Portland cement with 30replacement of class C fly ash and a mixture of 10mm(38 in) crushed limestone and natural sand and gravel

The SCC had an average spread of 800mm (28 in) 11987950

less than 2 sec J-ring spread reduction less than 50mm(2 in) and visual stability index (VSI) of 10 Nine 100mm times200mm (4 in times 8 in) cylinders were taken to evaluate thecompressive strength at release testing and 28 days CastingSCC started at the middle of the specimen Two pipe cameraswere attached at the bottom of the truss one at each endto record the concrete flow around the reinforcement andstrands Another camera was recording the casting process

Journal of Structures 9

Figure 9 All reinforcements after being installed in the form

from the top The high flowability and passing ability of SCCmade it easy to fill the bottom flange with no consolidationproblems However as soon as SCC started to fill trussdiagonals and verticals two foam panels started to float asthe buoyant forces broke the bond between the steel formsand foam block-outs Several actions were taken to keep thefoam block-outs in place against the uplifting force Lumberpieces were used as spacers between the foam and top flangereinforcement and between the top reinforcement and thesteel form to prevent further movements of the block-outsThis experiment indicated that foam panels were not a goodchoice for forming

After curing the specimen with wet burlap for 3 days theconcrete strength reached 524MPa (7600 psi) forms werestripped and strands were released on March 14 2013 Steelforms were easy to strip however foam block-outs wereembedded in the concrete and did not come out easily Thepressure of the concrete on the foam made it difficult topull out of the concrete in addition to having a thin layerof concrete between the foam and the steel form in somelocations which had to be chipped out Foam block-outs hadto be cut into small pieces using an electric saw Removing theblock-outs at the corners was evenmore challenging A smallchipping hammerwas used to carefully remove the remainingfoam without damaging the concrete The movement of thefoam block-outs while casting resulted in deviations in thedimensions angles and locations of two diagonal membersand two vertical members as shown in Figure 10

Prestressing strands were released using gradual deten-sioning and specimen ends were inspected for cracking Fewcracks appeared at south and north ends that were mostlyhorizontal and extended for the full thickness of the web andfew inches in the longitudinal directionThese cracks are dueto the bursting force of prestressing and were not controlledproperly because the end zone reinforcements were notplaced as close as they should be to the bulkheads Fewshrinkage cracks occurred in the top flange at the locationswere the lumber pieces were placed to prevent the floatingof the foam These cracks were not critical for the proposedtesting as they occurred only in the top flange which is acompression member

34 Specimen Testing Two roller supports were placed onconcrete blocks and spaced 89m (295 ft) apart on center tosupport the 9m (30 ft) long truss specimen The rollers werecentered on the 150mm (6 in) wide bearing plates embeddedin the truss at both ends A steel frame with a 1780 kN(400 kip) loading jack was placed as shown in Figure 11 toload the specimen at midspan section

To clearly see and track crack propagation of trusselements during loading one side of the specimen waspainted in white while strain gages were attached to the otherside Linear variable differential transforms (LVDTs) wereused to monitor the slippage of strands during testing Adeflection gage was installed to measure midspan deflectionsduring loading The specimen was tested on March 29 2013Concrete cylinders were tested and the compressive strengthwas found to be 724MPa (10500 psi) at that time Duringloading the specimen was visually inspected at 2225 kN(50 kip) load increments and cracking was being marked upAt 2225 kN (50 kip) the deflection reached 5mm (019 in)with no visible cracking The loading continued to 445 kN(100 kip) and deflection reached 10mm (039 in) Minorhorizontal cracks were observed at the corners betweendiagonals and topbottom flanges as shown in Figure 12

At 667 kN (150 kip) the deflection reached 15mm(057 in) and cracking continued at acute-angled cornersbetween the solid web and bottom flangeThemiddle verticalstarted to crack as well at the top and bottom At 890 kN(200 kip) deflection reached 20mm (075 in) and crackingoccurred at all the acute-angled corners At 1112 kN (250 kip)deflection reached 24mm (093 in) and cracking severityhad not significantly increased except at the webbottomflange corner After 1112 kN (250 kip) the load increasedcontinuously without interruptions up to failure at 1712 kN(385 kip) Excessive cracking was observed at the bottomflange around its connection to vertical rods and diagonalsas shown in Figure 13 The failure was dramatic as the one-threaded rod was pulled out of the bottom flange causing theadjacent diagonal to snap as shown in Figure 14 This failureoccurred as one of the 6 anchor bars welded to the washerplate was completely sheared as shown in Figure 15 Despitethe high load carrying capacity that the specimen achieved itis believed that having longer anchor bars and hat bars forbottom flange confinement around the anchor bars wouldhave postponed or even eliminated this mode of failure Alsochamfering sharp edges and using curved corners wouldhave reduced stress concentrations andminimized prematurecracking at these locations

35 Analysis of Results Figure 16 plots the load-deflectionrelationship of the truss specimen This plot indicates thatthe specimen had a linear elastic behavior up to the crackingload which is determined to be 1580 kN (355 kip) using thetangents method This load is 7 higher than the 1468 kN(330 kip) predicted cracking loadThemeasured deflection atthe cracking load was found to be 34mm (133 in) which is33 higher than the 25mm (10 in) predicted deflectionThisis primarily due to the premature cracking that was observedat almost all acute-angled corners whichmight have resultedin stiffness reduction Also the deviations between specified

10 Journal of Structures

85 in

75 in

85

in

85 in

9 in 9 in 9 in 9 in

65

in11 in

North South

North 1 South 1North 2 South 2

Figure 10 Actual truss member concrete dimensions after casting (1 in = 254mm)

Figure 11 Truss specimen before testing

Figure 12 First cracks observed at 445 kN (100 kip)

Figure 13 Excessive cracking at the bottom flange before failure

Figure 14 Failure of the truss specimen

Figure 15 Shearing of anchor bars at the failure location

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Figure 16 Load versus deflection curve (black lines indicate end ofelastic behavior)

and actual dimensions could have influenced the behavior ofthe specimen Figure 16 also indicates that the ultimate loadwas 1713 kN (385 kip) which is 38 lower than the predictedcapacity of 1779 kN (400 kip) due to the premature pullout of

Journal of Structures 11

the vertical threaded rod as a result of inadequate anchoragein the bottom flange

Figure 17 shows themeasured slippage of 178mm (07 in)diameter prestressing strands during loading This plot indi-cates that all the recorded readings are significantly less than025mm (001 in) which is the limit for initial slippageThe highest recorded value was even less than 0025mm(0001 in) which is the measurement accuracy of the usedLVDTs indicating no slippage up to failure load This meansthat 178mm (07 in) diameter prestressing strands were fullydeveloped within the 45m (15 ft) distance (ie half of thespecimen length) which is the predicted development lengthusing ACI 318-11 It should be noted that the high valuerecorded by the south LVDT at the failure load is incorrectdue to the sudden movement of the specimen at the momentof failure

Figure 18 plots the measured strain in four vertical mem-bers of the truss specimen versus the applied load Theseverticals are 38mm (15 in) diameter threaded rods that havea yield strength of 724MPa (105 ksi) and ultimate strengthof 862MPa (125 ksi)Themaximummeasured strain reached26 (at the south rod 1 where the failure occurred) Also allmeasured strains in the four threaded rods had significantlyexceeded the yield strain of 036 Figure 19 plots the forcesin the four threaded rods versus the applied load This plotindicates that the forces in all the four threaded rods reachedthe yield force of 689 kN (155 kip) which is 14 more thanthe predicted design force of 605 kN (136 kip)

Figure 20 plots the measured strains in the four rein-forced concrete diagonal members of the truss specimenversus the applied loadThis plot indicates that the measuredstrains varied significantly among the four diagonalmembersdue to the variation in their angles and concrete dimensions(eg the width of the south diagonal 1 was 165mm (65 in)while the width of the north diagonal 2 was 280mm(11 in)) However they were all much lower than the ultimatedesign strain of concrete (03) The maximum ultimatestrain reached 01 at the south diagonal 1 where failurehappened and the minimum ultimate strain reached 0045at the north diagonal 2 Also the straight line relationshipsin all diagonal members indicate their linear elastic behaviorup to failure loadTherefore the truss failure due to crushingof the diagonals is a low probability mode of failure Figure 21plots the forces in all four diagonal members versus theapplied loadThis plot indicates that the ultimate compressiveforce varied from 1179 kN (265 kip) at the north diagonal2 to 1446 kN (325 kip) at the south diagonal 1 wherefailure occurred The forces in all diagonal members exceptthe north diagonal 2 exceeded the design force of 1192 kN(268 kip) which indicates that the design of the diagonals wasadequate

4 Conclusions and Recommendations

This research aimed to develop a precast concrete truss sys-tem for roof applications that is lightweight and aestheticallypleasing can span up to 48m (160 ft) and can be fabricated

0

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Figure 17 Strand slippage measurements (1 kip = 445 kN 1 in =254mm)

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Figure 18 Tensile strains in the four threaded rods (1 kip =445 kN)

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Figure 19 Tension forces in the four threaded rods (black lineindicates yield force 1 kip = 445 kN)

12 Journal of Structures

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Figure 20 Compressive strains in the diagonal members (1 kip =445 kN)

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800

1000

1200

1400

1600

1800

2000

0 200 400 600 800 1000 1200 1400 1600

Load

(kN

)

Force in diagonals (kN)

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 21 Compressive forces in the diagonal members (black lineindicates yield force 1 kip = 445 kN)

using standard precast practices in the USA Based on thepresented work three main conclusions can be made

(1) Fabrication of the proposed truss system is practicaland efficient The proposed method of fabricationwas evaluated experimentally by producing a 9m(30 ft) long truss specimen using economical andcommercially available components standard bridgegirder form foam block-outs vertical threaded rodsand conventional reinforcement The success of theproposed method is also attributed to the use of highstrength self-consolidating concrete (SCC) that fillsthe complex form encapsulates the reinforcementand provides a smooth finished surface without anymechanical consolidation

(2) Structural testing of the truss specimen that wasdesigned and detailed to resist the forces generatedin an example building indicated the adequacy ofthe proposed design method and connection details

Few recommendations were made as shown belowto further improve the performance of the proposedsystem

(3) The 2D frame models and 3D finite element modelscan be used to accurately predict the behavior of theproposed system Forces obtained from these modelscan be used to design truss members using the strut-and-tie method and in accordance to the existingbuilding code FEA can be used to accurately predictstress concentration at truss member connections

Based on the analytical and experimental investigationsseveral recommendations could be made to improve theproposed truss system

(1) Use chamfered edges and curved corners instead ofthe sharp ones to avoid stress concentrations and addnominal reinforcement to control cracking at theselocations

(2) Use adequate top flange and bottom flange confer-ment reinforcement to help anchoring diagonal andvertical reinforcement whichmust be fully developedto prevent pullout

(3) Avoid using foam block-outs due to the difficultyof gluing them to the form and stripping themUsing light gage steel pans or fiber glass panes ishighly recommended for efficient and economicalfabrication as they can have multiple reuses

(4) Diagonal bars should be tied together after beingplaced in the forms to accommodate the tolerancesespecially when bar lengths and bent diameters arenot exact This practice will allow the diagonalsto slide against each other Another suggestion forfabrication is to assemble all the reinforcements in astiff manner outside the form to precisely match theform dimensionsThen the assembly is tied togetherlifted by a crane and placed in the form in one step

(5) Self-consolidating concrete with high flowability(average spread of 800mm (28 in) plusmn 50mm (2 in))passing ability (nominal maximum size aggregate is10mm (38 in)) resistance to segregation (VSI notmore than 10) and low viscosity (119879

50lt 2 sec) is

required to simplify production and ensure properfilling of the complex truss form without mechanicalconsolidation

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The technical support of the PrecastPrestressed ConcreteInstitute (PCI) advisory committee members is acknowl-edged and the financial support of the PCI Daniel PJenny Fellowship is appreciated Assistance was provided by

Journal of Structures 13

Musa Alawneh as a scholarship partner Support was alsogiven by University of Nebraska-Lincoln (UNL) graduatesstudents and staff Kelvin Lein AfshinHatami Shaddi AssadMohamed El-Kady and Micheal Asaad

References

[1] PCI PCI Design Hnadbook PrecastPrestressed Concrete Insti-tute (PCI) Chicago Ill USA 2010

[2] PCI ldquoThe 50 most significant precast concrete projectsrdquoASCENT Magazine 2004

[3] P H Shah and H R May ldquoPrecast vierendeel trusses pro-vide unique structural facade for parking structurerdquo Pre-castPrestressed Concrete Institute Journal vol 22 no 4 pp 24ndash39 1977

[4] W T Carroll F W Beaufait and R H Bryan ldquoPrestressedconcrete trussesrdquo ACI Journal vol 75 no 8 pp 367ndash373 1978

[5] MDeSutter United States of America Patent 7275348 B2 2007[6] A Trygestad andM DeSutter ldquoBookmen stacks cobalt condos

use ER-POST for column-free spacerdquo PCI Journal vol 52 no3 pp 58ndash71 2007

[7] ASCESEI 7 Minimum Design Loads for Buildings and OtherStructures American Society of Civil Engineers (ASCE)Reston Va USA 2010

[8] ACI 318 Building Code Requirments for Structural Concrete(ACI 318-11) and Commentary American Concrete Institute(ACI) Farmington Hills Mich USA 2011

[9] ASTMA193 ldquoStandard specification for alloy-steel and stainlesssteel bolting for high temperature or high pressure service andother special purpose applicationsrdquo ASTM Standard A193M-14ASTM International West Conshohocken Pa USA 2014

[10] AISC Steel Construction Manual American Institute of SteelConstruction (AISC) Chicago Ill USA 14th edition 2012

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International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Submit your manuscripts athttpwwwhindawicom

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Shock and Vibration

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Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

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Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

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Volume 2014

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Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

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Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

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Navigation and Observation

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DistributedSensor Networks

International Journal of

Journal of Structures 3

33998400 -0

998400998400

5 slope 5 slope

160998400-0998400998400

Truss elevation

CL

A B C D E F G H I J K

30998400-0998400998400 30

998400-0998400998400 30998400-0998400998400 30

998400-0998400998400 30998400-0998400998400 30

998400-0998400998400 30998400-0998400998400 30

998400-0998400998400 30998400-0998400998400 30

998400-0998400998400

80998400 -0

99840099840080998400 -0

998400998400

160998400 -0

998400998400

8998400

typ

Steel purlins(typ)

300998400

Plan view

Con

cret

e tru

ss (t

yp)

1

2

Steelbracing(typ)

Steelbracing(typ)

Steelbracing(typ)

Figure 2 Plan and elevation views of the example building (1ft = 0302m)

forces in the top and bottom flanges are not critical Howevera tension force of 271 kN (61 kip) in the diagonal membersand a compression force of 129 kN (29 kip) in the verti-cal members are developed under factored constructionloads Midspan deflection under service live load is 160mm(63 in) which represents L305

The proposed truss system is designed using the strutand tie method according to Appendix A of the ACI 318-11 code [8] Diagonal members are designed as reinforcedconcrete struts while the vertical members are designed assteel ties The diagonal members have a specified concretecompressive strength of 55MPa (8000 psi) and 200mm times200mm(8 intimes8 in) square section reinforcedwith 4 number19 (6) Grade 420 (60) (reinforcement ratio of 275) to resistthe tension force experienced during construction when thetruss is being temporarily supported at the midspan priorto casting the wet joint and posttensioning Also number10 (3) Grade 420 (60) steel square ties are used as trans-verse reinforcement at 200mm (8 in) spacing The vertical

members are made of 38mm (15 in) diameter threaded rodswith yield strength of 724MPa (105 ksi) and ultimate strengthof 862MPa (125 ksi) [9] Despite the low tensile force carriedby the vertical members near themidspan the same threadedrods are used in all verticals to simplify fabrication and resistthe compression force induced during construction withoutbuckling [10]

The bottom and top flanges of the truss are also designedusing strut and tiemethodThe compression strut (top flange)has a specified concrete compressive strength of 55MPa(8000 psi) and is reinforced with 4 number 13 (4) Grade 420(60) The tension tie (bottom flange) has two posttensioningducts with 12ndash153mm (06 in) diameter Grade 1860 (270)low relaxation strands In addition 10ndash153mm (06 in)diameter Grade 1860 (270) low relaxation pretensionedstrands are used for truss transportation and handling Fig-ures 3 and 4 showconcrete dimensions and reinforcing detailsof the proposed truss-girder system Comparing the design ofthe proposed systemwith the one implemented in the Sharjah

4 Journal of Structures

1998400-71 4

998400998400

1998400-71 4

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

998400998400

81998400-2

998400998400

24998400-10998400998400

24998400-10998400998400

3998400-8

998400998400

2998400-4

998400998400

Det

ail-A

Plan

G

D

1998400-0

998400998400

332∘

7998400-0998400998400

6998400-6 1

4

998400998400

8998400998400

F

BC

A

5998400-0

998400998400

4998400-0

998400998400

1998400-8

998400998400

1998400-10 1

2

998400998400

6998400-0998400998400

49984009984004998400998400

49984009984004998400998400

1998400998400

8998400998400

8998400998400

10998400998400

1998400998400

1998400998400

Sect

ion

Sect

ion

Sect

ion

Sect

ion

Sect

ion

Sect

ion

G

AB

CD

G

6998400-0998400998400

6998400-0998400998400

6998400-0998400998400

6998400-0998400998400

1998400-0998400998400

1998400-0998400998400

4998400-0998400998400

4998400-0998400998400

1998400-0998400998400

1998400-0998400998400

Har

dwar

e spi

ral

85 8

998400998400ha

rdw

are a

ncho

r

1998400-0998400998400

2998400-4

9984009984002998400-4

998400998400

2998400-4

9984009984002998400-4

998400998400

1998400-6 1

4

998400998400

33 4

998400998400empty

PT d

uct

1998400-6

9984009984008998400998400

8998400998400

8998400998400

1998400998400

1998400-6

9984009984001998400-6

9984009984001998400-6

998400998400

1998400-6

9984009984001998400-6

9984009984003998400-8

9984009984003998400-8

9984009984003998400-8

9984009984003998400-8

998400998400

7998400998400

7998400998400

7998400998400

7998400998400

1998400998400

6998400998400-41

2

998400998400

2998400998400

2998400998400

2998400998400

2998400998400

41

2

998400998400

41

2

998400998400

79998400-6998400998400

81998400-2

998400998400

5

slope

Elev

atio

n

6998400-61 4

998400998400

C L

C L

31 2

99840099840031 2

998400998400 6998400998400-41

2

998400998400

Figure 3 Concrete truss dimensions (1ft = 0302m 1 in = 254mm)

coal storage facility presented in Figure 1 indicates that theproposed system is approximately 23 lighter in weight inaddition to being more economical to produce due to the useof standard I-girder forms conventional reinforcing detailsand self-consolidated concrete

22 Construction Sequence The proposed constructionsequence of the developed truss-girder system is as follows

(1) Truss-girders are fabricated in the precast plant intwo segments for each truss and transported to theconstruction site

Journal of Structures 5

Sect

ion

Sect

ion

Sect

ion

Sect

ion

Sect

ion

Sect

ion

Sect

ion

12

33A

4

56

6998400-0998400998400

Har

dwar

e spi

ral

85 8

998400998400ha

rdw

are a

ncho

r

2998400-4

998400998400

1998400-2

998400998400

3998400-8

998400998400

4tie

s at

6998400998400

spac

ings

3tie

s at

12998400998400

spac

ings

W2

W2

Varia

ble

Botto

mre

info

rcem

ent

Botto

mre

info

rcem

ent

at en

ds

4998400998400

4sti

rrup

6998400998400

for2

9984006998400998400

12

6998400-0998400998400

2998400-6

998400998400

5998400-0

998400998400

8998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

998400998400

3

5

11 2

998400998400empty

3A

8998400998400

81998400-2

998400998400

24998400 -1

0998400998400

41998400-0

998400998400

6

33 4

998400998400empty

PT d

uct

10998400998400

4998400998400

4998400998400

4998400998400

8998400998400

8998400998400

8998400998400

8998400998400

44

bars

44

bars

44

bars

44

bars

D11

6998400998400

26

thro

ugho

ut2

6th

roug

hout

312998400998400

spac

ing

312998400998400

spac

ing

312998400998400

spac

ing

10

stran

ds1 2

998400998400di

a10

stran

ds1 2

998400998400di

a

Dia

gona

l bar

s (4

6)

11 2

998400998400di

a hi

gh st

reng

th st

eel r

od

see n

otes

at sh

eet S

K-04

Nut

weld

ed to

stee

l pla

te

26

strai

ght b

ars

(see

rein

forc

emen

t det

ails)

PT d

uct3

75998400998400

dia

(15

-06998400998400

)

10

stran

ds1 2

998400998400di

a

3tie

s at

8998400998400

spac

ings

3tie

s at

8998400998400

spac

ings

46

46

51

4

998400998400

11 2

99840099840011 2

998400998400

11

2

99840099840011

2

998400998400

41

4

998400998400

2998400998400 2

998400998400

5998400998400

5998400998400

Elev

atio

n

Trus

s wei

ght =57

kips

hal

f tru

ss

Figure 4 Reinforcement details (1ft = 0302m 1 in = 254mm)

6 Journal of Structures

4998400-5998400998400

3998400-2 3

8

998400998400

2998400-51 2

998400998400

30998400-0998400998400

3

4

4

400∘

21

Elevation

3998400-3 5

8

998400998400

Section

Section Section Section

4

1 2 3

8998400998400

8998400998400

8998400998400

8998400998400

4998400998400

2998400998400

8998400998400

4998400998400

4998400998400

1998400998400

1998400998400

1998400998400

1998400998400

1998400-9998400998400 1

998400-9998400998400 1998400-9998400998400

4998400-4

998400998400

4998400-4

998400998400

4998400-8

998400998400

61

2

998400998400 61

2

998400998400

2998400-71 2

998400998400

2998400-71 2

998400998400

2998400-51 2

9984009984001998400-31 2

998400998400

71 2

998400998400

71 2

998400998400

71

2

99840099840071

2

998400998400

71

2

998400998400

1998400-11998400998400 1

998400-11998400998400 1998400-11998400998400

6998400998400

6998400998400

11998400998400

4998400998400

6998400998400 6998400998400

11998400998400

111 2

9984009984002998400-51 2

998400998400

2 halves of foamfor truss openings(see elevation view

for dimensions)

1

2

998400998400 plywood and(3) 2 times 4 studsfor block-out

Iowa type D girder form

3 998400-7 1

8998400998400

Figure 5 Dimensions of the concrete truss specimen (1ft = 0302m 1 in = 254mm)

(2) Each segment is erected on one column at one endand on temporary supports at the other end

(3) Roof purlins and bracings are installed to stabilize thetruss-girders

(4) Couplers are used to connect posttensioning ductsand posttensioning strands are threaded throughducts

(5) Joints between truss segments are formed reinforcedand cast in place using SCCwith similar properties tothose of the truss concrete

(6) Posttensioning is applied after joint concrete achievesadequate strength and posttensioning ducts aregrouted

(7) Temporary supports are removed and roof decking isinstalled

(8) Fire-proofing and corrosion resistance agents areapplied to steel elements if needed

3 Experimental Investigation

31 Specimen Description The purpose of the experimentalinvestigation is to evaluate the constructability and structuralperformance of the proposed truss-girder system A full-sizetruss could not be fabricated and tested due to space andbudget limitations A 9m (30 ft) long truss formed usingIowa type D bridge I-girder forms provided by Coreslab

Structures Inc Omaha NE was used instead The cross-section dimensions of Iowa type D I-girder are very closeto those of AASHTO type IV bridge girder The forms are9m (30 ft) long and 1420mm (56 in) high However toreduce the weight of the specimen a 100mm (4 in) block-out was made at the bottom of the form to have a totaldepth of 1320mm (52 in) and two foam panels 100mm(4 in) thick each were used to form for each truss openingFigure 5 shows the dimensions of the concrete truss specimenat different sections

32 Specimen Analysis and Design Two-dimensional (2D)frame analysis and three-dimensional (3D) finite elementanalysis (FEA) were carried out to determine member forcesand deformations of the specimen Comparing analysisresults of each method as shown in Table 1 indicates that thesimple 2D frame analysis results in conservative and relativelyaccurate estimates of forces and deflections compared to themore elaborate FEA The loads used in this analysis includeweight of the specimen prestressing force and midspanconcentrated load which achieve forces in the diagonal andvertical members of the specimen similar to those in the full-size truss-girder system designed in the previous section Itshould be noted that diagonalmembers of the specimen had a40∘ angle with the bottom flange in order to achieve the sameratio between diagonal and vertical forces as in the full-sizetruss system presented earlier

The analysis of the specimen indicated that a midspanpoint load of 1779 kN (400 kip) will result in the forces

Journal of Structures 7

Table 1 Comparing analysis results using 2D frame model and 3DFE model

Analysis method 2D frameanalysis

Finite elementanalysis

Max compression indiagonals (kN) 1246 1125

Max tension inverticals (kN) 574 534

Cracking load (kN) 1478 1335Correspondingdeflection (mm) 25 21

Figure 6 The FE model used to analyze truss connections

that are slightly higher than the design factored forcesin diagonal and vertical members The initial design ofthe specimen required 16ndash153mm (06 in) diameter Grade1860 (270) strands However due to the unavailability ofthis size of strands at the structural laboratory 12ndash178mm(07 in) diameter Grade 1860 (270) strands were used insteadto achieve the same prestressing force Also the analysisindicated that the cracking load is 1468 kN (330 kip) usingthe cracking stress limit at the bottom fibers and the corre-sponding deflection is 25mm (1 in) Strand jacking stress wasassumed to be 075119891pu and total prestress losses were assumedto be 20of the jacking stressThe developed FEmodel of thespecimen shown in Figure 6 consists of a simplified cross-section (ie rectangles) 8-node solid elements for concreteand frame elements for threaded rods This model was usedto conduct elastoplastic analysis using specified materialproperties to determine stresses and deformations underdifferent loading levels Figure 7 shows the stress contours onconcrete elements under ultimate load This figure indicatesthat the connection between diagonal web and the bottomflange had very high tensile stresses which are expected tocause premature cracking at those locations

The specimen was designed similar to the full-size trusswith one exception top and bottom flanges were overde-signed to ensure that the failure occurs at the verticalsdiagonals or connections The 12ndash178mm (07 in) diameterstrands were provided to achieve a flexural capacity of4371 kNsdotm (3224 kipsdotft) which is about 10 more than theapplied moment Also the top flange was reinforced with2 number 25 (8) bars as compression reinforcement toincrease the capacity of the top flange Figure 8 shows theelevation cross-sections and reinforcement details of thespecimen

E + 6

550

440

330

220

110

00

minus110

minus220

minus330

minus440

minus550

minus660

minus770

minus880

Figure 7 Stresses in the truss specimen under ultimate load atmidspan (Nm2)

33 Specimen Fabrication Fabrication of the 9m (30-ft)long specimen was done at the structural lab of the PeterKiewit Institute (PKI) in Omaha NE in five major steps(1) preparing forms and placing prestressing strands (2)cutting foam block-outs and gluing them to the steel forms(3) assembling diagonal and vertical reinforcement andinstalling them between the foam block-outs (4) casting self-consolidating concrete into the forms and (5) stripping theforms and releasing the strands

The prestressing strands were tensioned to 3176 kN(714 kip) (075 of the ultimate stress of 1860MPa (270 ksi))Foam block-outs were used to form the truss openings The100mm (4 in) thick foam panels were cut into diamondshapes and glued to form the 200mm (8 in) thick block-outs Square grooves that are 19mm times 19mm (075 in times075 in) were removed from the edges of the foam panelsto accommodate the vertical threaded rods as shown inFigure 9 To facilitate the stripping of the foam from theconcrete web plastic sheets were wrapped around the edgesof the foam All the foam block-outs were glued on the steelform after marking their locations on the form sides Bottomflange and top flange reinforcement were simple to installThe challenge was assembling and installing the diagonaland vertical reinforcement which were 4 number 19 (6)bars and number 10 (3) ties spaced at 200mm (8 in) alongthe member and 38mm (15 in) diameter threaded rodsThe rods were anchored at the top and bottom flange using200mm times 200mm times 13m (8 in times 8 in times 05 in) Grade350 (50) steel plates and structural nuts Each plate is weldedto two diagonal bars and 2 number 19 (6) straight anchorbars Initially it was planned to have the reinforcementsfor each diagonal preassembled and then connected to thethreaded rods after being installed in the form The mainproblem of this plan is that it requires very tight tolerancesin reinforcement dimensions and bent locations in additionto the difficulty of handling a very heavy assembly ofreinforcement that is not very stiff Some diagonal bars wereslightly shorter than others and do not have exactly thesame bend diameter or location To address these challengesdiagonal bars and anchor bars were cut to have only 225mm(9 in) of embedment (12 db) and transverse ties were keptloose to allow the bars to move relative to each other thenthey are tied after all reinforcement is in place to simplify thefabrication process as shown in Figure 9 Four 100 times 100 minusMW20 timesMW20 (4 times 4minusW29 timesW29)WWRwere placed atthe solid part at each endof the truss for shear reinforcements

8 Journal of Structures

A B

DD

C

Detail A

3998400-3 5

8

998400998400

1998400-0998400998400

30998400-0998400998400

Detail A

3 ties at8998400998400 spacings

46

SectionSection Section Section

DA B C

5998400 998400

11 2

99840099840011 2

998400998400

1998400-9998400998400

1998400-9998400998400

1998400-11998400998400

D116998400998400

36998400998400 spacing

21

2

99840099840021

2

998400998400

21

2

998400998400

61

2

998400998400

71

2

998400998400

71 2

998400998400

21

2

998400998400

1998400-11998400998400

4998400998400

6998400998400

6998400998400

8998400998400

4998400998400

4998400998400

2998400998400

11998400998400

2998400-51 2

998400998400111 2

998400998400

PL 23998400998400times6

998400998400times1

2

998400998400

w3 1

2

998400998400 studs

7998400998400 spacings

41

2

99840099840041

2

9984009984007998400998400

7998400998400

3998400998400

5998400998400

6998400998400

1998400-11998400998400

Detail A

Topreinforcements

reinforcements

36998400998400 spacings

Bottom

W2

Diagonals rebarrefer to the details below

11

2

998400998400emptythreaded rod

and a structural nut

8998400998400

8998400998400

PL 8998400998400times8

998400998400times1

2

998400998400

opening locatedat center for TRdia of openingcorresponds to

the TR dia

Plate welded to thediagonal rebar

6998400998400

1 2

998400998400

4998400-2

998400998400

11

2

998400998400emptythreaded rods w6998400998400

threads at the endsGR 120 B7 weathered

steel

Threaded rod

A-A

3998400-101 2

998400998400

3998400-2998400998400

4998400 -85

8

998400998400

2998400-6998400998400

2998400-6998400998400

8998400998400

8998400998400

R4 1

2 998400998400

59984009984008998400998400

11 2

99840099840011 2

998400998400

Diagonal steel reinforcement details

4998400-4

998400998400

Figure 8 Elevation and cross-sections of the specimen (1ft = 0302m 1 in = 254mm)

The top flange had 2 number 25 (8) bars tied using number10 (3) stirrups at 150mm (6 in) spacing After the form wasclosed the top flange reinforcement assembly was placed andlumber yokes were used to tie the forms from the top

The specimen was cast on March 11 2013 using specified55MPa (8000 psi) self-consolidating concrete The mixturewas designed using type III Portland cement with 30replacement of class C fly ash and a mixture of 10mm(38 in) crushed limestone and natural sand and gravel

The SCC had an average spread of 800mm (28 in) 11987950

less than 2 sec J-ring spread reduction less than 50mm(2 in) and visual stability index (VSI) of 10 Nine 100mm times200mm (4 in times 8 in) cylinders were taken to evaluate thecompressive strength at release testing and 28 days CastingSCC started at the middle of the specimen Two pipe cameraswere attached at the bottom of the truss one at each endto record the concrete flow around the reinforcement andstrands Another camera was recording the casting process

Journal of Structures 9

Figure 9 All reinforcements after being installed in the form

from the top The high flowability and passing ability of SCCmade it easy to fill the bottom flange with no consolidationproblems However as soon as SCC started to fill trussdiagonals and verticals two foam panels started to float asthe buoyant forces broke the bond between the steel formsand foam block-outs Several actions were taken to keep thefoam block-outs in place against the uplifting force Lumberpieces were used as spacers between the foam and top flangereinforcement and between the top reinforcement and thesteel form to prevent further movements of the block-outsThis experiment indicated that foam panels were not a goodchoice for forming

After curing the specimen with wet burlap for 3 days theconcrete strength reached 524MPa (7600 psi) forms werestripped and strands were released on March 14 2013 Steelforms were easy to strip however foam block-outs wereembedded in the concrete and did not come out easily Thepressure of the concrete on the foam made it difficult topull out of the concrete in addition to having a thin layerof concrete between the foam and the steel form in somelocations which had to be chipped out Foam block-outs hadto be cut into small pieces using an electric saw Removing theblock-outs at the corners was evenmore challenging A smallchipping hammerwas used to carefully remove the remainingfoam without damaging the concrete The movement of thefoam block-outs while casting resulted in deviations in thedimensions angles and locations of two diagonal membersand two vertical members as shown in Figure 10

Prestressing strands were released using gradual deten-sioning and specimen ends were inspected for cracking Fewcracks appeared at south and north ends that were mostlyhorizontal and extended for the full thickness of the web andfew inches in the longitudinal directionThese cracks are dueto the bursting force of prestressing and were not controlledproperly because the end zone reinforcements were notplaced as close as they should be to the bulkheads Fewshrinkage cracks occurred in the top flange at the locationswere the lumber pieces were placed to prevent the floatingof the foam These cracks were not critical for the proposedtesting as they occurred only in the top flange which is acompression member

34 Specimen Testing Two roller supports were placed onconcrete blocks and spaced 89m (295 ft) apart on center tosupport the 9m (30 ft) long truss specimen The rollers werecentered on the 150mm (6 in) wide bearing plates embeddedin the truss at both ends A steel frame with a 1780 kN(400 kip) loading jack was placed as shown in Figure 11 toload the specimen at midspan section

To clearly see and track crack propagation of trusselements during loading one side of the specimen waspainted in white while strain gages were attached to the otherside Linear variable differential transforms (LVDTs) wereused to monitor the slippage of strands during testing Adeflection gage was installed to measure midspan deflectionsduring loading The specimen was tested on March 29 2013Concrete cylinders were tested and the compressive strengthwas found to be 724MPa (10500 psi) at that time Duringloading the specimen was visually inspected at 2225 kN(50 kip) load increments and cracking was being marked upAt 2225 kN (50 kip) the deflection reached 5mm (019 in)with no visible cracking The loading continued to 445 kN(100 kip) and deflection reached 10mm (039 in) Minorhorizontal cracks were observed at the corners betweendiagonals and topbottom flanges as shown in Figure 12

At 667 kN (150 kip) the deflection reached 15mm(057 in) and cracking continued at acute-angled cornersbetween the solid web and bottom flangeThemiddle verticalstarted to crack as well at the top and bottom At 890 kN(200 kip) deflection reached 20mm (075 in) and crackingoccurred at all the acute-angled corners At 1112 kN (250 kip)deflection reached 24mm (093 in) and cracking severityhad not significantly increased except at the webbottomflange corner After 1112 kN (250 kip) the load increasedcontinuously without interruptions up to failure at 1712 kN(385 kip) Excessive cracking was observed at the bottomflange around its connection to vertical rods and diagonalsas shown in Figure 13 The failure was dramatic as the one-threaded rod was pulled out of the bottom flange causing theadjacent diagonal to snap as shown in Figure 14 This failureoccurred as one of the 6 anchor bars welded to the washerplate was completely sheared as shown in Figure 15 Despitethe high load carrying capacity that the specimen achieved itis believed that having longer anchor bars and hat bars forbottom flange confinement around the anchor bars wouldhave postponed or even eliminated this mode of failure Alsochamfering sharp edges and using curved corners wouldhave reduced stress concentrations andminimized prematurecracking at these locations

35 Analysis of Results Figure 16 plots the load-deflectionrelationship of the truss specimen This plot indicates thatthe specimen had a linear elastic behavior up to the crackingload which is determined to be 1580 kN (355 kip) using thetangents method This load is 7 higher than the 1468 kN(330 kip) predicted cracking loadThemeasured deflection atthe cracking load was found to be 34mm (133 in) which is33 higher than the 25mm (10 in) predicted deflectionThisis primarily due to the premature cracking that was observedat almost all acute-angled corners whichmight have resultedin stiffness reduction Also the deviations between specified

10 Journal of Structures

85 in

75 in

85

in

85 in

9 in 9 in 9 in 9 in

65

in11 in

North South

North 1 South 1North 2 South 2

Figure 10 Actual truss member concrete dimensions after casting (1 in = 254mm)

Figure 11 Truss specimen before testing

Figure 12 First cracks observed at 445 kN (100 kip)

Figure 13 Excessive cracking at the bottom flange before failure

Figure 14 Failure of the truss specimen

Figure 15 Shearing of anchor bars at the failure location

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 100 200 300 400 500 600

Load

(kN

)

Midspan deflection (mm)

Figure 16 Load versus deflection curve (black lines indicate end ofelastic behavior)

and actual dimensions could have influenced the behavior ofthe specimen Figure 16 also indicates that the ultimate loadwas 1713 kN (385 kip) which is 38 lower than the predictedcapacity of 1779 kN (400 kip) due to the premature pullout of

Journal of Structures 11

the vertical threaded rod as a result of inadequate anchoragein the bottom flange

Figure 17 shows themeasured slippage of 178mm (07 in)diameter prestressing strands during loading This plot indi-cates that all the recorded readings are significantly less than025mm (001 in) which is the limit for initial slippageThe highest recorded value was even less than 0025mm(0001 in) which is the measurement accuracy of the usedLVDTs indicating no slippage up to failure load This meansthat 178mm (07 in) diameter prestressing strands were fullydeveloped within the 45m (15 ft) distance (ie half of thespecimen length) which is the predicted development lengthusing ACI 318-11 It should be noted that the high valuerecorded by the south LVDT at the failure load is incorrectdue to the sudden movement of the specimen at the momentof failure

Figure 18 plots the measured strain in four vertical mem-bers of the truss specimen versus the applied load Theseverticals are 38mm (15 in) diameter threaded rods that havea yield strength of 724MPa (105 ksi) and ultimate strengthof 862MPa (125 ksi)Themaximummeasured strain reached26 (at the south rod 1 where the failure occurred) Also allmeasured strains in the four threaded rods had significantlyexceeded the yield strain of 036 Figure 19 plots the forcesin the four threaded rods versus the applied load This plotindicates that the forces in all the four threaded rods reachedthe yield force of 689 kN (155 kip) which is 14 more thanthe predicted design force of 605 kN (136 kip)

Figure 20 plots the measured strains in the four rein-forced concrete diagonal members of the truss specimenversus the applied loadThis plot indicates that the measuredstrains varied significantly among the four diagonalmembersdue to the variation in their angles and concrete dimensions(eg the width of the south diagonal 1 was 165mm (65 in)while the width of the north diagonal 2 was 280mm(11 in)) However they were all much lower than the ultimatedesign strain of concrete (03) The maximum ultimatestrain reached 01 at the south diagonal 1 where failurehappened and the minimum ultimate strain reached 0045at the north diagonal 2 Also the straight line relationshipsin all diagonal members indicate their linear elastic behaviorup to failure loadTherefore the truss failure due to crushingof the diagonals is a low probability mode of failure Figure 21plots the forces in all four diagonal members versus theapplied loadThis plot indicates that the ultimate compressiveforce varied from 1179 kN (265 kip) at the north diagonal2 to 1446 kN (325 kip) at the south diagonal 1 wherefailure occurred The forces in all diagonal members exceptthe north diagonal 2 exceeded the design force of 1192 kN(268 kip) which indicates that the design of the diagonals wasadequate

4 Conclusions and Recommendations

This research aimed to develop a precast concrete truss sys-tem for roof applications that is lightweight and aestheticallypleasing can span up to 48m (160 ft) and can be fabricated

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025

Load

(kN

)

Slippage (mm)

North strand slippageSouth strand slippage

Figure 17 Strand slippage measurements (1 kip = 445 kN 1 in =254mm)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 05 10 15 20 25 30

Load

(kN

)

Strain in TR ()

N rod 1

N rod 2

S rod 1

S rod 2

Figure 18 Tensile strains in the four threaded rods (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

mdash 100 200 300 400 500 600 700 800

Load

(kN

)

Force in TR (kN)

N rod 1

N rod 2

S rod 1

S rod 2

Figure 19 Tension forces in the four threaded rods (black lineindicates yield force 1 kip = 445 kN)

12 Journal of Structures

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025 030

Load

(kN

)

Strain in diagonals ()

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 20 Compressive strains in the diagonal members (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 200 400 600 800 1000 1200 1400 1600

Load

(kN

)

Force in diagonals (kN)

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 21 Compressive forces in the diagonal members (black lineindicates yield force 1 kip = 445 kN)

using standard precast practices in the USA Based on thepresented work three main conclusions can be made

(1) Fabrication of the proposed truss system is practicaland efficient The proposed method of fabricationwas evaluated experimentally by producing a 9m(30 ft) long truss specimen using economical andcommercially available components standard bridgegirder form foam block-outs vertical threaded rodsand conventional reinforcement The success of theproposed method is also attributed to the use of highstrength self-consolidating concrete (SCC) that fillsthe complex form encapsulates the reinforcementand provides a smooth finished surface without anymechanical consolidation

(2) Structural testing of the truss specimen that wasdesigned and detailed to resist the forces generatedin an example building indicated the adequacy ofthe proposed design method and connection details

Few recommendations were made as shown belowto further improve the performance of the proposedsystem

(3) The 2D frame models and 3D finite element modelscan be used to accurately predict the behavior of theproposed system Forces obtained from these modelscan be used to design truss members using the strut-and-tie method and in accordance to the existingbuilding code FEA can be used to accurately predictstress concentration at truss member connections

Based on the analytical and experimental investigationsseveral recommendations could be made to improve theproposed truss system

(1) Use chamfered edges and curved corners instead ofthe sharp ones to avoid stress concentrations and addnominal reinforcement to control cracking at theselocations

(2) Use adequate top flange and bottom flange confer-ment reinforcement to help anchoring diagonal andvertical reinforcement whichmust be fully developedto prevent pullout

(3) Avoid using foam block-outs due to the difficultyof gluing them to the form and stripping themUsing light gage steel pans or fiber glass panes ishighly recommended for efficient and economicalfabrication as they can have multiple reuses

(4) Diagonal bars should be tied together after beingplaced in the forms to accommodate the tolerancesespecially when bar lengths and bent diameters arenot exact This practice will allow the diagonalsto slide against each other Another suggestion forfabrication is to assemble all the reinforcements in astiff manner outside the form to precisely match theform dimensionsThen the assembly is tied togetherlifted by a crane and placed in the form in one step

(5) Self-consolidating concrete with high flowability(average spread of 800mm (28 in) plusmn 50mm (2 in))passing ability (nominal maximum size aggregate is10mm (38 in)) resistance to segregation (VSI notmore than 10) and low viscosity (119879

50lt 2 sec) is

required to simplify production and ensure properfilling of the complex truss form without mechanicalconsolidation

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The technical support of the PrecastPrestressed ConcreteInstitute (PCI) advisory committee members is acknowl-edged and the financial support of the PCI Daniel PJenny Fellowship is appreciated Assistance was provided by

Journal of Structures 13

Musa Alawneh as a scholarship partner Support was alsogiven by University of Nebraska-Lincoln (UNL) graduatesstudents and staff Kelvin Lein AfshinHatami Shaddi AssadMohamed El-Kady and Micheal Asaad

References

[1] PCI PCI Design Hnadbook PrecastPrestressed Concrete Insti-tute (PCI) Chicago Ill USA 2010

[2] PCI ldquoThe 50 most significant precast concrete projectsrdquoASCENT Magazine 2004

[3] P H Shah and H R May ldquoPrecast vierendeel trusses pro-vide unique structural facade for parking structurerdquo Pre-castPrestressed Concrete Institute Journal vol 22 no 4 pp 24ndash39 1977

[4] W T Carroll F W Beaufait and R H Bryan ldquoPrestressedconcrete trussesrdquo ACI Journal vol 75 no 8 pp 367ndash373 1978

[5] MDeSutter United States of America Patent 7275348 B2 2007[6] A Trygestad andM DeSutter ldquoBookmen stacks cobalt condos

use ER-POST for column-free spacerdquo PCI Journal vol 52 no3 pp 58ndash71 2007

[7] ASCESEI 7 Minimum Design Loads for Buildings and OtherStructures American Society of Civil Engineers (ASCE)Reston Va USA 2010

[8] ACI 318 Building Code Requirments for Structural Concrete(ACI 318-11) and Commentary American Concrete Institute(ACI) Farmington Hills Mich USA 2011

[9] ASTMA193 ldquoStandard specification for alloy-steel and stainlesssteel bolting for high temperature or high pressure service andother special purpose applicationsrdquo ASTM Standard A193M-14ASTM International West Conshohocken Pa USA 2014

[10] AISC Steel Construction Manual American Institute of SteelConstruction (AISC) Chicago Ill USA 14th edition 2012

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Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

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Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

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Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

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Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

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Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

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Navigation and Observation

International Journal of

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DistributedSensor Networks

International Journal of

4 Journal of Structures

1998400-71 4

998400998400

1998400-71 4

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

998400998400

8998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

998400998400

81998400-2

998400998400

24998400-10998400998400

24998400-10998400998400

3998400-8

998400998400

2998400-4

998400998400

Det

ail-A

Plan

G

D

1998400-0

998400998400

332∘

7998400-0998400998400

6998400-6 1

4

998400998400

8998400998400

F

BC

A

5998400-0

998400998400

4998400-0

998400998400

1998400-8

998400998400

1998400-10 1

2

998400998400

6998400-0998400998400

49984009984004998400998400

49984009984004998400998400

1998400998400

8998400998400

8998400998400

10998400998400

1998400998400

1998400998400

Sect

ion

Sect

ion

Sect

ion

Sect

ion

Sect

ion

Sect

ion

G

AB

CD

G

6998400-0998400998400

6998400-0998400998400

6998400-0998400998400

6998400-0998400998400

1998400-0998400998400

1998400-0998400998400

4998400-0998400998400

4998400-0998400998400

1998400-0998400998400

1998400-0998400998400

Har

dwar

e spi

ral

85 8

998400998400ha

rdw

are a

ncho

r

1998400-0998400998400

2998400-4

9984009984002998400-4

998400998400

2998400-4

9984009984002998400-4

998400998400

1998400-6 1

4

998400998400

33 4

998400998400empty

PT d

uct

1998400-6

9984009984008998400998400

8998400998400

8998400998400

1998400998400

1998400-6

9984009984001998400-6

9984009984001998400-6

998400998400

1998400-6

9984009984001998400-6

9984009984003998400-8

9984009984003998400-8

9984009984003998400-8

9984009984003998400-8

998400998400

7998400998400

7998400998400

7998400998400

7998400998400

1998400998400

6998400998400-41

2

998400998400

2998400998400

2998400998400

2998400998400

2998400998400

41

2

998400998400

41

2

998400998400

79998400-6998400998400

81998400-2

998400998400

5

slope

Elev

atio

n

6998400-61 4

998400998400

C L

C L

31 2

99840099840031 2

998400998400 6998400998400-41

2

998400998400

Figure 3 Concrete truss dimensions (1ft = 0302m 1 in = 254mm)

coal storage facility presented in Figure 1 indicates that theproposed system is approximately 23 lighter in weight inaddition to being more economical to produce due to the useof standard I-girder forms conventional reinforcing detailsand self-consolidated concrete

22 Construction Sequence The proposed constructionsequence of the developed truss-girder system is as follows

(1) Truss-girders are fabricated in the precast plant intwo segments for each truss and transported to theconstruction site

Journal of Structures 5

Sect

ion

Sect

ion

Sect

ion

Sect

ion

Sect

ion

Sect

ion

Sect

ion

12

33A

4

56

6998400-0998400998400

Har

dwar

e spi

ral

85 8

998400998400ha

rdw

are a

ncho

r

2998400-4

998400998400

1998400-2

998400998400

3998400-8

998400998400

4tie

s at

6998400998400

spac

ings

3tie

s at

12998400998400

spac

ings

W2

W2

Varia

ble

Botto

mre

info

rcem

ent

Botto

mre

info

rcem

ent

at en

ds

4998400998400

4sti

rrup

6998400998400

for2

9984006998400998400

12

6998400-0998400998400

2998400-6

998400998400

5998400-0

998400998400

8998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

998400998400

3

5

11 2

998400998400empty

3A

8998400998400

81998400-2

998400998400

24998400 -1

0998400998400

41998400-0

998400998400

6

33 4

998400998400empty

PT d

uct

10998400998400

4998400998400

4998400998400

4998400998400

8998400998400

8998400998400

8998400998400

8998400998400

44

bars

44

bars

44

bars

44

bars

D11

6998400998400

26

thro

ugho

ut2

6th

roug

hout

312998400998400

spac

ing

312998400998400

spac

ing

312998400998400

spac

ing

10

stran

ds1 2

998400998400di

a10

stran

ds1 2

998400998400di

a

Dia

gona

l bar

s (4

6)

11 2

998400998400di

a hi

gh st

reng

th st

eel r

od

see n

otes

at sh

eet S

K-04

Nut

weld

ed to

stee

l pla

te

26

strai

ght b

ars

(see

rein

forc

emen

t det

ails)

PT d

uct3

75998400998400

dia

(15

-06998400998400

)

10

stran

ds1 2

998400998400di

a

3tie

s at

8998400998400

spac

ings

3tie

s at

8998400998400

spac

ings

46

46

51

4

998400998400

11 2

99840099840011 2

998400998400

11

2

99840099840011

2

998400998400

41

4

998400998400

2998400998400 2

998400998400

5998400998400

5998400998400

Elev

atio

n

Trus

s wei

ght =57

kips

hal

f tru

ss

Figure 4 Reinforcement details (1ft = 0302m 1 in = 254mm)

6 Journal of Structures

4998400-5998400998400

3998400-2 3

8

998400998400

2998400-51 2

998400998400

30998400-0998400998400

3

4

4

400∘

21

Elevation

3998400-3 5

8

998400998400

Section

Section Section Section

4

1 2 3

8998400998400

8998400998400

8998400998400

8998400998400

4998400998400

2998400998400

8998400998400

4998400998400

4998400998400

1998400998400

1998400998400

1998400998400

1998400998400

1998400-9998400998400 1

998400-9998400998400 1998400-9998400998400

4998400-4

998400998400

4998400-4

998400998400

4998400-8

998400998400

61

2

998400998400 61

2

998400998400

2998400-71 2

998400998400

2998400-71 2

998400998400

2998400-51 2

9984009984001998400-31 2

998400998400

71 2

998400998400

71 2

998400998400

71

2

99840099840071

2

998400998400

71

2

998400998400

1998400-11998400998400 1

998400-11998400998400 1998400-11998400998400

6998400998400

6998400998400

11998400998400

4998400998400

6998400998400 6998400998400

11998400998400

111 2

9984009984002998400-51 2

998400998400

2 halves of foamfor truss openings(see elevation view

for dimensions)

1

2

998400998400 plywood and(3) 2 times 4 studsfor block-out

Iowa type D girder form

3 998400-7 1

8998400998400

Figure 5 Dimensions of the concrete truss specimen (1ft = 0302m 1 in = 254mm)

(2) Each segment is erected on one column at one endand on temporary supports at the other end

(3) Roof purlins and bracings are installed to stabilize thetruss-girders

(4) Couplers are used to connect posttensioning ductsand posttensioning strands are threaded throughducts

(5) Joints between truss segments are formed reinforcedand cast in place using SCCwith similar properties tothose of the truss concrete

(6) Posttensioning is applied after joint concrete achievesadequate strength and posttensioning ducts aregrouted

(7) Temporary supports are removed and roof decking isinstalled

(8) Fire-proofing and corrosion resistance agents areapplied to steel elements if needed

3 Experimental Investigation

31 Specimen Description The purpose of the experimentalinvestigation is to evaluate the constructability and structuralperformance of the proposed truss-girder system A full-sizetruss could not be fabricated and tested due to space andbudget limitations A 9m (30 ft) long truss formed usingIowa type D bridge I-girder forms provided by Coreslab

Structures Inc Omaha NE was used instead The cross-section dimensions of Iowa type D I-girder are very closeto those of AASHTO type IV bridge girder The forms are9m (30 ft) long and 1420mm (56 in) high However toreduce the weight of the specimen a 100mm (4 in) block-out was made at the bottom of the form to have a totaldepth of 1320mm (52 in) and two foam panels 100mm(4 in) thick each were used to form for each truss openingFigure 5 shows the dimensions of the concrete truss specimenat different sections

32 Specimen Analysis and Design Two-dimensional (2D)frame analysis and three-dimensional (3D) finite elementanalysis (FEA) were carried out to determine member forcesand deformations of the specimen Comparing analysisresults of each method as shown in Table 1 indicates that thesimple 2D frame analysis results in conservative and relativelyaccurate estimates of forces and deflections compared to themore elaborate FEA The loads used in this analysis includeweight of the specimen prestressing force and midspanconcentrated load which achieve forces in the diagonal andvertical members of the specimen similar to those in the full-size truss-girder system designed in the previous section Itshould be noted that diagonalmembers of the specimen had a40∘ angle with the bottom flange in order to achieve the sameratio between diagonal and vertical forces as in the full-sizetruss system presented earlier

The analysis of the specimen indicated that a midspanpoint load of 1779 kN (400 kip) will result in the forces

Journal of Structures 7

Table 1 Comparing analysis results using 2D frame model and 3DFE model

Analysis method 2D frameanalysis

Finite elementanalysis

Max compression indiagonals (kN) 1246 1125

Max tension inverticals (kN) 574 534

Cracking load (kN) 1478 1335Correspondingdeflection (mm) 25 21

Figure 6 The FE model used to analyze truss connections

that are slightly higher than the design factored forcesin diagonal and vertical members The initial design ofthe specimen required 16ndash153mm (06 in) diameter Grade1860 (270) strands However due to the unavailability ofthis size of strands at the structural laboratory 12ndash178mm(07 in) diameter Grade 1860 (270) strands were used insteadto achieve the same prestressing force Also the analysisindicated that the cracking load is 1468 kN (330 kip) usingthe cracking stress limit at the bottom fibers and the corre-sponding deflection is 25mm (1 in) Strand jacking stress wasassumed to be 075119891pu and total prestress losses were assumedto be 20of the jacking stressThe developed FEmodel of thespecimen shown in Figure 6 consists of a simplified cross-section (ie rectangles) 8-node solid elements for concreteand frame elements for threaded rods This model was usedto conduct elastoplastic analysis using specified materialproperties to determine stresses and deformations underdifferent loading levels Figure 7 shows the stress contours onconcrete elements under ultimate load This figure indicatesthat the connection between diagonal web and the bottomflange had very high tensile stresses which are expected tocause premature cracking at those locations

The specimen was designed similar to the full-size trusswith one exception top and bottom flanges were overde-signed to ensure that the failure occurs at the verticalsdiagonals or connections The 12ndash178mm (07 in) diameterstrands were provided to achieve a flexural capacity of4371 kNsdotm (3224 kipsdotft) which is about 10 more than theapplied moment Also the top flange was reinforced with2 number 25 (8) bars as compression reinforcement toincrease the capacity of the top flange Figure 8 shows theelevation cross-sections and reinforcement details of thespecimen

E + 6

550

440

330

220

110

00

minus110

minus220

minus330

minus440

minus550

minus660

minus770

minus880

Figure 7 Stresses in the truss specimen under ultimate load atmidspan (Nm2)

33 Specimen Fabrication Fabrication of the 9m (30-ft)long specimen was done at the structural lab of the PeterKiewit Institute (PKI) in Omaha NE in five major steps(1) preparing forms and placing prestressing strands (2)cutting foam block-outs and gluing them to the steel forms(3) assembling diagonal and vertical reinforcement andinstalling them between the foam block-outs (4) casting self-consolidating concrete into the forms and (5) stripping theforms and releasing the strands

The prestressing strands were tensioned to 3176 kN(714 kip) (075 of the ultimate stress of 1860MPa (270 ksi))Foam block-outs were used to form the truss openings The100mm (4 in) thick foam panels were cut into diamondshapes and glued to form the 200mm (8 in) thick block-outs Square grooves that are 19mm times 19mm (075 in times075 in) were removed from the edges of the foam panelsto accommodate the vertical threaded rods as shown inFigure 9 To facilitate the stripping of the foam from theconcrete web plastic sheets were wrapped around the edgesof the foam All the foam block-outs were glued on the steelform after marking their locations on the form sides Bottomflange and top flange reinforcement were simple to installThe challenge was assembling and installing the diagonaland vertical reinforcement which were 4 number 19 (6)bars and number 10 (3) ties spaced at 200mm (8 in) alongthe member and 38mm (15 in) diameter threaded rodsThe rods were anchored at the top and bottom flange using200mm times 200mm times 13m (8 in times 8 in times 05 in) Grade350 (50) steel plates and structural nuts Each plate is weldedto two diagonal bars and 2 number 19 (6) straight anchorbars Initially it was planned to have the reinforcementsfor each diagonal preassembled and then connected to thethreaded rods after being installed in the form The mainproblem of this plan is that it requires very tight tolerancesin reinforcement dimensions and bent locations in additionto the difficulty of handling a very heavy assembly ofreinforcement that is not very stiff Some diagonal bars wereslightly shorter than others and do not have exactly thesame bend diameter or location To address these challengesdiagonal bars and anchor bars were cut to have only 225mm(9 in) of embedment (12 db) and transverse ties were keptloose to allow the bars to move relative to each other thenthey are tied after all reinforcement is in place to simplify thefabrication process as shown in Figure 9 Four 100 times 100 minusMW20 timesMW20 (4 times 4minusW29 timesW29)WWRwere placed atthe solid part at each endof the truss for shear reinforcements

8 Journal of Structures

A B

DD

C

Detail A

3998400-3 5

8

998400998400

1998400-0998400998400

30998400-0998400998400

Detail A

3 ties at8998400998400 spacings

46

SectionSection Section Section

DA B C

5998400 998400

11 2

99840099840011 2

998400998400

1998400-9998400998400

1998400-9998400998400

1998400-11998400998400

D116998400998400

36998400998400 spacing

21

2

99840099840021

2

998400998400

21

2

998400998400

61

2

998400998400

71

2

998400998400

71 2

998400998400

21

2

998400998400

1998400-11998400998400

4998400998400

6998400998400

6998400998400

8998400998400

4998400998400

4998400998400

2998400998400

11998400998400

2998400-51 2

998400998400111 2

998400998400

PL 23998400998400times6

998400998400times1

2

998400998400

w3 1

2

998400998400 studs

7998400998400 spacings

41

2

99840099840041

2

9984009984007998400998400

7998400998400

3998400998400

5998400998400

6998400998400

1998400-11998400998400

Detail A

Topreinforcements

reinforcements

36998400998400 spacings

Bottom

W2

Diagonals rebarrefer to the details below

11

2

998400998400emptythreaded rod

and a structural nut

8998400998400

8998400998400

PL 8998400998400times8

998400998400times1

2

998400998400

opening locatedat center for TRdia of openingcorresponds to

the TR dia

Plate welded to thediagonal rebar

6998400998400

1 2

998400998400

4998400-2

998400998400

11

2

998400998400emptythreaded rods w6998400998400

threads at the endsGR 120 B7 weathered

steel

Threaded rod

A-A

3998400-101 2

998400998400

3998400-2998400998400

4998400 -85

8

998400998400

2998400-6998400998400

2998400-6998400998400

8998400998400

8998400998400

R4 1

2 998400998400

59984009984008998400998400

11 2

99840099840011 2

998400998400

Diagonal steel reinforcement details

4998400-4

998400998400

Figure 8 Elevation and cross-sections of the specimen (1ft = 0302m 1 in = 254mm)

The top flange had 2 number 25 (8) bars tied using number10 (3) stirrups at 150mm (6 in) spacing After the form wasclosed the top flange reinforcement assembly was placed andlumber yokes were used to tie the forms from the top

The specimen was cast on March 11 2013 using specified55MPa (8000 psi) self-consolidating concrete The mixturewas designed using type III Portland cement with 30replacement of class C fly ash and a mixture of 10mm(38 in) crushed limestone and natural sand and gravel

The SCC had an average spread of 800mm (28 in) 11987950

less than 2 sec J-ring spread reduction less than 50mm(2 in) and visual stability index (VSI) of 10 Nine 100mm times200mm (4 in times 8 in) cylinders were taken to evaluate thecompressive strength at release testing and 28 days CastingSCC started at the middle of the specimen Two pipe cameraswere attached at the bottom of the truss one at each endto record the concrete flow around the reinforcement andstrands Another camera was recording the casting process

Journal of Structures 9

Figure 9 All reinforcements after being installed in the form

from the top The high flowability and passing ability of SCCmade it easy to fill the bottom flange with no consolidationproblems However as soon as SCC started to fill trussdiagonals and verticals two foam panels started to float asthe buoyant forces broke the bond between the steel formsand foam block-outs Several actions were taken to keep thefoam block-outs in place against the uplifting force Lumberpieces were used as spacers between the foam and top flangereinforcement and between the top reinforcement and thesteel form to prevent further movements of the block-outsThis experiment indicated that foam panels were not a goodchoice for forming

After curing the specimen with wet burlap for 3 days theconcrete strength reached 524MPa (7600 psi) forms werestripped and strands were released on March 14 2013 Steelforms were easy to strip however foam block-outs wereembedded in the concrete and did not come out easily Thepressure of the concrete on the foam made it difficult topull out of the concrete in addition to having a thin layerof concrete between the foam and the steel form in somelocations which had to be chipped out Foam block-outs hadto be cut into small pieces using an electric saw Removing theblock-outs at the corners was evenmore challenging A smallchipping hammerwas used to carefully remove the remainingfoam without damaging the concrete The movement of thefoam block-outs while casting resulted in deviations in thedimensions angles and locations of two diagonal membersand two vertical members as shown in Figure 10

Prestressing strands were released using gradual deten-sioning and specimen ends were inspected for cracking Fewcracks appeared at south and north ends that were mostlyhorizontal and extended for the full thickness of the web andfew inches in the longitudinal directionThese cracks are dueto the bursting force of prestressing and were not controlledproperly because the end zone reinforcements were notplaced as close as they should be to the bulkheads Fewshrinkage cracks occurred in the top flange at the locationswere the lumber pieces were placed to prevent the floatingof the foam These cracks were not critical for the proposedtesting as they occurred only in the top flange which is acompression member

34 Specimen Testing Two roller supports were placed onconcrete blocks and spaced 89m (295 ft) apart on center tosupport the 9m (30 ft) long truss specimen The rollers werecentered on the 150mm (6 in) wide bearing plates embeddedin the truss at both ends A steel frame with a 1780 kN(400 kip) loading jack was placed as shown in Figure 11 toload the specimen at midspan section

To clearly see and track crack propagation of trusselements during loading one side of the specimen waspainted in white while strain gages were attached to the otherside Linear variable differential transforms (LVDTs) wereused to monitor the slippage of strands during testing Adeflection gage was installed to measure midspan deflectionsduring loading The specimen was tested on March 29 2013Concrete cylinders were tested and the compressive strengthwas found to be 724MPa (10500 psi) at that time Duringloading the specimen was visually inspected at 2225 kN(50 kip) load increments and cracking was being marked upAt 2225 kN (50 kip) the deflection reached 5mm (019 in)with no visible cracking The loading continued to 445 kN(100 kip) and deflection reached 10mm (039 in) Minorhorizontal cracks were observed at the corners betweendiagonals and topbottom flanges as shown in Figure 12

At 667 kN (150 kip) the deflection reached 15mm(057 in) and cracking continued at acute-angled cornersbetween the solid web and bottom flangeThemiddle verticalstarted to crack as well at the top and bottom At 890 kN(200 kip) deflection reached 20mm (075 in) and crackingoccurred at all the acute-angled corners At 1112 kN (250 kip)deflection reached 24mm (093 in) and cracking severityhad not significantly increased except at the webbottomflange corner After 1112 kN (250 kip) the load increasedcontinuously without interruptions up to failure at 1712 kN(385 kip) Excessive cracking was observed at the bottomflange around its connection to vertical rods and diagonalsas shown in Figure 13 The failure was dramatic as the one-threaded rod was pulled out of the bottom flange causing theadjacent diagonal to snap as shown in Figure 14 This failureoccurred as one of the 6 anchor bars welded to the washerplate was completely sheared as shown in Figure 15 Despitethe high load carrying capacity that the specimen achieved itis believed that having longer anchor bars and hat bars forbottom flange confinement around the anchor bars wouldhave postponed or even eliminated this mode of failure Alsochamfering sharp edges and using curved corners wouldhave reduced stress concentrations andminimized prematurecracking at these locations

35 Analysis of Results Figure 16 plots the load-deflectionrelationship of the truss specimen This plot indicates thatthe specimen had a linear elastic behavior up to the crackingload which is determined to be 1580 kN (355 kip) using thetangents method This load is 7 higher than the 1468 kN(330 kip) predicted cracking loadThemeasured deflection atthe cracking load was found to be 34mm (133 in) which is33 higher than the 25mm (10 in) predicted deflectionThisis primarily due to the premature cracking that was observedat almost all acute-angled corners whichmight have resultedin stiffness reduction Also the deviations between specified

10 Journal of Structures

85 in

75 in

85

in

85 in

9 in 9 in 9 in 9 in

65

in11 in

North South

North 1 South 1North 2 South 2

Figure 10 Actual truss member concrete dimensions after casting (1 in = 254mm)

Figure 11 Truss specimen before testing

Figure 12 First cracks observed at 445 kN (100 kip)

Figure 13 Excessive cracking at the bottom flange before failure

Figure 14 Failure of the truss specimen

Figure 15 Shearing of anchor bars at the failure location

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 100 200 300 400 500 600

Load

(kN

)

Midspan deflection (mm)

Figure 16 Load versus deflection curve (black lines indicate end ofelastic behavior)

and actual dimensions could have influenced the behavior ofthe specimen Figure 16 also indicates that the ultimate loadwas 1713 kN (385 kip) which is 38 lower than the predictedcapacity of 1779 kN (400 kip) due to the premature pullout of

Journal of Structures 11

the vertical threaded rod as a result of inadequate anchoragein the bottom flange

Figure 17 shows themeasured slippage of 178mm (07 in)diameter prestressing strands during loading This plot indi-cates that all the recorded readings are significantly less than025mm (001 in) which is the limit for initial slippageThe highest recorded value was even less than 0025mm(0001 in) which is the measurement accuracy of the usedLVDTs indicating no slippage up to failure load This meansthat 178mm (07 in) diameter prestressing strands were fullydeveloped within the 45m (15 ft) distance (ie half of thespecimen length) which is the predicted development lengthusing ACI 318-11 It should be noted that the high valuerecorded by the south LVDT at the failure load is incorrectdue to the sudden movement of the specimen at the momentof failure

Figure 18 plots the measured strain in four vertical mem-bers of the truss specimen versus the applied load Theseverticals are 38mm (15 in) diameter threaded rods that havea yield strength of 724MPa (105 ksi) and ultimate strengthof 862MPa (125 ksi)Themaximummeasured strain reached26 (at the south rod 1 where the failure occurred) Also allmeasured strains in the four threaded rods had significantlyexceeded the yield strain of 036 Figure 19 plots the forcesin the four threaded rods versus the applied load This plotindicates that the forces in all the four threaded rods reachedthe yield force of 689 kN (155 kip) which is 14 more thanthe predicted design force of 605 kN (136 kip)

Figure 20 plots the measured strains in the four rein-forced concrete diagonal members of the truss specimenversus the applied loadThis plot indicates that the measuredstrains varied significantly among the four diagonalmembersdue to the variation in their angles and concrete dimensions(eg the width of the south diagonal 1 was 165mm (65 in)while the width of the north diagonal 2 was 280mm(11 in)) However they were all much lower than the ultimatedesign strain of concrete (03) The maximum ultimatestrain reached 01 at the south diagonal 1 where failurehappened and the minimum ultimate strain reached 0045at the north diagonal 2 Also the straight line relationshipsin all diagonal members indicate their linear elastic behaviorup to failure loadTherefore the truss failure due to crushingof the diagonals is a low probability mode of failure Figure 21plots the forces in all four diagonal members versus theapplied loadThis plot indicates that the ultimate compressiveforce varied from 1179 kN (265 kip) at the north diagonal2 to 1446 kN (325 kip) at the south diagonal 1 wherefailure occurred The forces in all diagonal members exceptthe north diagonal 2 exceeded the design force of 1192 kN(268 kip) which indicates that the design of the diagonals wasadequate

4 Conclusions and Recommendations

This research aimed to develop a precast concrete truss sys-tem for roof applications that is lightweight and aestheticallypleasing can span up to 48m (160 ft) and can be fabricated

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025

Load

(kN

)

Slippage (mm)

North strand slippageSouth strand slippage

Figure 17 Strand slippage measurements (1 kip = 445 kN 1 in =254mm)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 05 10 15 20 25 30

Load

(kN

)

Strain in TR ()

N rod 1

N rod 2

S rod 1

S rod 2

Figure 18 Tensile strains in the four threaded rods (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

mdash 100 200 300 400 500 600 700 800

Load

(kN

)

Force in TR (kN)

N rod 1

N rod 2

S rod 1

S rod 2

Figure 19 Tension forces in the four threaded rods (black lineindicates yield force 1 kip = 445 kN)

12 Journal of Structures

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025 030

Load

(kN

)

Strain in diagonals ()

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 20 Compressive strains in the diagonal members (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 200 400 600 800 1000 1200 1400 1600

Load

(kN

)

Force in diagonals (kN)

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 21 Compressive forces in the diagonal members (black lineindicates yield force 1 kip = 445 kN)

using standard precast practices in the USA Based on thepresented work three main conclusions can be made

(1) Fabrication of the proposed truss system is practicaland efficient The proposed method of fabricationwas evaluated experimentally by producing a 9m(30 ft) long truss specimen using economical andcommercially available components standard bridgegirder form foam block-outs vertical threaded rodsand conventional reinforcement The success of theproposed method is also attributed to the use of highstrength self-consolidating concrete (SCC) that fillsthe complex form encapsulates the reinforcementand provides a smooth finished surface without anymechanical consolidation

(2) Structural testing of the truss specimen that wasdesigned and detailed to resist the forces generatedin an example building indicated the adequacy ofthe proposed design method and connection details

Few recommendations were made as shown belowto further improve the performance of the proposedsystem

(3) The 2D frame models and 3D finite element modelscan be used to accurately predict the behavior of theproposed system Forces obtained from these modelscan be used to design truss members using the strut-and-tie method and in accordance to the existingbuilding code FEA can be used to accurately predictstress concentration at truss member connections

Based on the analytical and experimental investigationsseveral recommendations could be made to improve theproposed truss system

(1) Use chamfered edges and curved corners instead ofthe sharp ones to avoid stress concentrations and addnominal reinforcement to control cracking at theselocations

(2) Use adequate top flange and bottom flange confer-ment reinforcement to help anchoring diagonal andvertical reinforcement whichmust be fully developedto prevent pullout

(3) Avoid using foam block-outs due to the difficultyof gluing them to the form and stripping themUsing light gage steel pans or fiber glass panes ishighly recommended for efficient and economicalfabrication as they can have multiple reuses

(4) Diagonal bars should be tied together after beingplaced in the forms to accommodate the tolerancesespecially when bar lengths and bent diameters arenot exact This practice will allow the diagonalsto slide against each other Another suggestion forfabrication is to assemble all the reinforcements in astiff manner outside the form to precisely match theform dimensionsThen the assembly is tied togetherlifted by a crane and placed in the form in one step

(5) Self-consolidating concrete with high flowability(average spread of 800mm (28 in) plusmn 50mm (2 in))passing ability (nominal maximum size aggregate is10mm (38 in)) resistance to segregation (VSI notmore than 10) and low viscosity (119879

50lt 2 sec) is

required to simplify production and ensure properfilling of the complex truss form without mechanicalconsolidation

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The technical support of the PrecastPrestressed ConcreteInstitute (PCI) advisory committee members is acknowl-edged and the financial support of the PCI Daniel PJenny Fellowship is appreciated Assistance was provided by

Journal of Structures 13

Musa Alawneh as a scholarship partner Support was alsogiven by University of Nebraska-Lincoln (UNL) graduatesstudents and staff Kelvin Lein AfshinHatami Shaddi AssadMohamed El-Kady and Micheal Asaad

References

[1] PCI PCI Design Hnadbook PrecastPrestressed Concrete Insti-tute (PCI) Chicago Ill USA 2010

[2] PCI ldquoThe 50 most significant precast concrete projectsrdquoASCENT Magazine 2004

[3] P H Shah and H R May ldquoPrecast vierendeel trusses pro-vide unique structural facade for parking structurerdquo Pre-castPrestressed Concrete Institute Journal vol 22 no 4 pp 24ndash39 1977

[4] W T Carroll F W Beaufait and R H Bryan ldquoPrestressedconcrete trussesrdquo ACI Journal vol 75 no 8 pp 367ndash373 1978

[5] MDeSutter United States of America Patent 7275348 B2 2007[6] A Trygestad andM DeSutter ldquoBookmen stacks cobalt condos

use ER-POST for column-free spacerdquo PCI Journal vol 52 no3 pp 58ndash71 2007

[7] ASCESEI 7 Minimum Design Loads for Buildings and OtherStructures American Society of Civil Engineers (ASCE)Reston Va USA 2010

[8] ACI 318 Building Code Requirments for Structural Concrete(ACI 318-11) and Commentary American Concrete Institute(ACI) Farmington Hills Mich USA 2011

[9] ASTMA193 ldquoStandard specification for alloy-steel and stainlesssteel bolting for high temperature or high pressure service andother special purpose applicationsrdquo ASTM Standard A193M-14ASTM International West Conshohocken Pa USA 2014

[10] AISC Steel Construction Manual American Institute of SteelConstruction (AISC) Chicago Ill USA 14th edition 2012

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Navigation and Observation

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DistributedSensor Networks

International Journal of

Journal of Structures 5

Sect

ion

Sect

ion

Sect

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Sect

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Sect

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12

33A

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6998400-0998400998400

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e spi

ral

85 8

998400998400ha

rdw

are a

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r

2998400-4

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s at

6998400998400

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ings

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s at

12998400998400

spac

ings

W2

W2

Varia

ble

Botto

mre

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rcem

ent

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mre

info

rcem

ent

at en

ds

4998400998400

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rrup

6998400998400

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9984006998400998400

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6998400-0998400998400

2998400-6

998400998400

5998400-0

998400998400

8998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

9984009984008998400-0

998400998400

3

5

11 2

998400998400empty

3A

8998400998400

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998400998400

24998400 -1

0998400998400

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33 4

998400998400empty

PT d

uct

10998400998400

4998400998400

4998400998400

4998400998400

8998400998400

8998400998400

8998400998400

8998400998400

44

bars

44

bars

44

bars

44

bars

D11

6998400998400

26

thro

ugho

ut2

6th

roug

hout

312998400998400

spac

ing

312998400998400

spac

ing

312998400998400

spac

ing

10

stran

ds1 2

998400998400di

a10

stran

ds1 2

998400998400di

a

Dia

gona

l bar

s (4

6)

11 2

998400998400di

a hi

gh st

reng

th st

eel r

od

see n

otes

at sh

eet S

K-04

Nut

weld

ed to

stee

l pla

te

26

strai

ght b

ars

(see

rein

forc

emen

t det

ails)

PT d

uct3

75998400998400

dia

(15

-06998400998400

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10

stran

ds1 2

998400998400di

a

3tie

s at

8998400998400

spac

ings

3tie

s at

8998400998400

spac

ings

46

46

51

4

998400998400

11 2

99840099840011 2

998400998400

11

2

99840099840011

2

998400998400

41

4

998400998400

2998400998400 2

998400998400

5998400998400

5998400998400

Elev

atio

n

Trus

s wei

ght =57

kips

hal

f tru

ss

Figure 4 Reinforcement details (1ft = 0302m 1 in = 254mm)

6 Journal of Structures

4998400-5998400998400

3998400-2 3

8

998400998400

2998400-51 2

998400998400

30998400-0998400998400

3

4

4

400∘

21

Elevation

3998400-3 5

8

998400998400

Section

Section Section Section

4

1 2 3

8998400998400

8998400998400

8998400998400

8998400998400

4998400998400

2998400998400

8998400998400

4998400998400

4998400998400

1998400998400

1998400998400

1998400998400

1998400998400

1998400-9998400998400 1

998400-9998400998400 1998400-9998400998400

4998400-4

998400998400

4998400-4

998400998400

4998400-8

998400998400

61

2

998400998400 61

2

998400998400

2998400-71 2

998400998400

2998400-71 2

998400998400

2998400-51 2

9984009984001998400-31 2

998400998400

71 2

998400998400

71 2

998400998400

71

2

99840099840071

2

998400998400

71

2

998400998400

1998400-11998400998400 1

998400-11998400998400 1998400-11998400998400

6998400998400

6998400998400

11998400998400

4998400998400

6998400998400 6998400998400

11998400998400

111 2

9984009984002998400-51 2

998400998400

2 halves of foamfor truss openings(see elevation view

for dimensions)

1

2

998400998400 plywood and(3) 2 times 4 studsfor block-out

Iowa type D girder form

3 998400-7 1

8998400998400

Figure 5 Dimensions of the concrete truss specimen (1ft = 0302m 1 in = 254mm)

(2) Each segment is erected on one column at one endand on temporary supports at the other end

(3) Roof purlins and bracings are installed to stabilize thetruss-girders

(4) Couplers are used to connect posttensioning ductsand posttensioning strands are threaded throughducts

(5) Joints between truss segments are formed reinforcedand cast in place using SCCwith similar properties tothose of the truss concrete

(6) Posttensioning is applied after joint concrete achievesadequate strength and posttensioning ducts aregrouted

(7) Temporary supports are removed and roof decking isinstalled

(8) Fire-proofing and corrosion resistance agents areapplied to steel elements if needed

3 Experimental Investigation

31 Specimen Description The purpose of the experimentalinvestigation is to evaluate the constructability and structuralperformance of the proposed truss-girder system A full-sizetruss could not be fabricated and tested due to space andbudget limitations A 9m (30 ft) long truss formed usingIowa type D bridge I-girder forms provided by Coreslab

Structures Inc Omaha NE was used instead The cross-section dimensions of Iowa type D I-girder are very closeto those of AASHTO type IV bridge girder The forms are9m (30 ft) long and 1420mm (56 in) high However toreduce the weight of the specimen a 100mm (4 in) block-out was made at the bottom of the form to have a totaldepth of 1320mm (52 in) and two foam panels 100mm(4 in) thick each were used to form for each truss openingFigure 5 shows the dimensions of the concrete truss specimenat different sections

32 Specimen Analysis and Design Two-dimensional (2D)frame analysis and three-dimensional (3D) finite elementanalysis (FEA) were carried out to determine member forcesand deformations of the specimen Comparing analysisresults of each method as shown in Table 1 indicates that thesimple 2D frame analysis results in conservative and relativelyaccurate estimates of forces and deflections compared to themore elaborate FEA The loads used in this analysis includeweight of the specimen prestressing force and midspanconcentrated load which achieve forces in the diagonal andvertical members of the specimen similar to those in the full-size truss-girder system designed in the previous section Itshould be noted that diagonalmembers of the specimen had a40∘ angle with the bottom flange in order to achieve the sameratio between diagonal and vertical forces as in the full-sizetruss system presented earlier

The analysis of the specimen indicated that a midspanpoint load of 1779 kN (400 kip) will result in the forces

Journal of Structures 7

Table 1 Comparing analysis results using 2D frame model and 3DFE model

Analysis method 2D frameanalysis

Finite elementanalysis

Max compression indiagonals (kN) 1246 1125

Max tension inverticals (kN) 574 534

Cracking load (kN) 1478 1335Correspondingdeflection (mm) 25 21

Figure 6 The FE model used to analyze truss connections

that are slightly higher than the design factored forcesin diagonal and vertical members The initial design ofthe specimen required 16ndash153mm (06 in) diameter Grade1860 (270) strands However due to the unavailability ofthis size of strands at the structural laboratory 12ndash178mm(07 in) diameter Grade 1860 (270) strands were used insteadto achieve the same prestressing force Also the analysisindicated that the cracking load is 1468 kN (330 kip) usingthe cracking stress limit at the bottom fibers and the corre-sponding deflection is 25mm (1 in) Strand jacking stress wasassumed to be 075119891pu and total prestress losses were assumedto be 20of the jacking stressThe developed FEmodel of thespecimen shown in Figure 6 consists of a simplified cross-section (ie rectangles) 8-node solid elements for concreteand frame elements for threaded rods This model was usedto conduct elastoplastic analysis using specified materialproperties to determine stresses and deformations underdifferent loading levels Figure 7 shows the stress contours onconcrete elements under ultimate load This figure indicatesthat the connection between diagonal web and the bottomflange had very high tensile stresses which are expected tocause premature cracking at those locations

The specimen was designed similar to the full-size trusswith one exception top and bottom flanges were overde-signed to ensure that the failure occurs at the verticalsdiagonals or connections The 12ndash178mm (07 in) diameterstrands were provided to achieve a flexural capacity of4371 kNsdotm (3224 kipsdotft) which is about 10 more than theapplied moment Also the top flange was reinforced with2 number 25 (8) bars as compression reinforcement toincrease the capacity of the top flange Figure 8 shows theelevation cross-sections and reinforcement details of thespecimen

E + 6

550

440

330

220

110

00

minus110

minus220

minus330

minus440

minus550

minus660

minus770

minus880

Figure 7 Stresses in the truss specimen under ultimate load atmidspan (Nm2)

33 Specimen Fabrication Fabrication of the 9m (30-ft)long specimen was done at the structural lab of the PeterKiewit Institute (PKI) in Omaha NE in five major steps(1) preparing forms and placing prestressing strands (2)cutting foam block-outs and gluing them to the steel forms(3) assembling diagonal and vertical reinforcement andinstalling them between the foam block-outs (4) casting self-consolidating concrete into the forms and (5) stripping theforms and releasing the strands

The prestressing strands were tensioned to 3176 kN(714 kip) (075 of the ultimate stress of 1860MPa (270 ksi))Foam block-outs were used to form the truss openings The100mm (4 in) thick foam panels were cut into diamondshapes and glued to form the 200mm (8 in) thick block-outs Square grooves that are 19mm times 19mm (075 in times075 in) were removed from the edges of the foam panelsto accommodate the vertical threaded rods as shown inFigure 9 To facilitate the stripping of the foam from theconcrete web plastic sheets were wrapped around the edgesof the foam All the foam block-outs were glued on the steelform after marking their locations on the form sides Bottomflange and top flange reinforcement were simple to installThe challenge was assembling and installing the diagonaland vertical reinforcement which were 4 number 19 (6)bars and number 10 (3) ties spaced at 200mm (8 in) alongthe member and 38mm (15 in) diameter threaded rodsThe rods were anchored at the top and bottom flange using200mm times 200mm times 13m (8 in times 8 in times 05 in) Grade350 (50) steel plates and structural nuts Each plate is weldedto two diagonal bars and 2 number 19 (6) straight anchorbars Initially it was planned to have the reinforcementsfor each diagonal preassembled and then connected to thethreaded rods after being installed in the form The mainproblem of this plan is that it requires very tight tolerancesin reinforcement dimensions and bent locations in additionto the difficulty of handling a very heavy assembly ofreinforcement that is not very stiff Some diagonal bars wereslightly shorter than others and do not have exactly thesame bend diameter or location To address these challengesdiagonal bars and anchor bars were cut to have only 225mm(9 in) of embedment (12 db) and transverse ties were keptloose to allow the bars to move relative to each other thenthey are tied after all reinforcement is in place to simplify thefabrication process as shown in Figure 9 Four 100 times 100 minusMW20 timesMW20 (4 times 4minusW29 timesW29)WWRwere placed atthe solid part at each endof the truss for shear reinforcements

8 Journal of Structures

A B

DD

C

Detail A

3998400-3 5

8

998400998400

1998400-0998400998400

30998400-0998400998400

Detail A

3 ties at8998400998400 spacings

46

SectionSection Section Section

DA B C

5998400 998400

11 2

99840099840011 2

998400998400

1998400-9998400998400

1998400-9998400998400

1998400-11998400998400

D116998400998400

36998400998400 spacing

21

2

99840099840021

2

998400998400

21

2

998400998400

61

2

998400998400

71

2

998400998400

71 2

998400998400

21

2

998400998400

1998400-11998400998400

4998400998400

6998400998400

6998400998400

8998400998400

4998400998400

4998400998400

2998400998400

11998400998400

2998400-51 2

998400998400111 2

998400998400

PL 23998400998400times6

998400998400times1

2

998400998400

w3 1

2

998400998400 studs

7998400998400 spacings

41

2

99840099840041

2

9984009984007998400998400

7998400998400

3998400998400

5998400998400

6998400998400

1998400-11998400998400

Detail A

Topreinforcements

reinforcements

36998400998400 spacings

Bottom

W2

Diagonals rebarrefer to the details below

11

2

998400998400emptythreaded rod

and a structural nut

8998400998400

8998400998400

PL 8998400998400times8

998400998400times1

2

998400998400

opening locatedat center for TRdia of openingcorresponds to

the TR dia

Plate welded to thediagonal rebar

6998400998400

1 2

998400998400

4998400-2

998400998400

11

2

998400998400emptythreaded rods w6998400998400

threads at the endsGR 120 B7 weathered

steel

Threaded rod

A-A

3998400-101 2

998400998400

3998400-2998400998400

4998400 -85

8

998400998400

2998400-6998400998400

2998400-6998400998400

8998400998400

8998400998400

R4 1

2 998400998400

59984009984008998400998400

11 2

99840099840011 2

998400998400

Diagonal steel reinforcement details

4998400-4

998400998400

Figure 8 Elevation and cross-sections of the specimen (1ft = 0302m 1 in = 254mm)

The top flange had 2 number 25 (8) bars tied using number10 (3) stirrups at 150mm (6 in) spacing After the form wasclosed the top flange reinforcement assembly was placed andlumber yokes were used to tie the forms from the top

The specimen was cast on March 11 2013 using specified55MPa (8000 psi) self-consolidating concrete The mixturewas designed using type III Portland cement with 30replacement of class C fly ash and a mixture of 10mm(38 in) crushed limestone and natural sand and gravel

The SCC had an average spread of 800mm (28 in) 11987950

less than 2 sec J-ring spread reduction less than 50mm(2 in) and visual stability index (VSI) of 10 Nine 100mm times200mm (4 in times 8 in) cylinders were taken to evaluate thecompressive strength at release testing and 28 days CastingSCC started at the middle of the specimen Two pipe cameraswere attached at the bottom of the truss one at each endto record the concrete flow around the reinforcement andstrands Another camera was recording the casting process

Journal of Structures 9

Figure 9 All reinforcements after being installed in the form

from the top The high flowability and passing ability of SCCmade it easy to fill the bottom flange with no consolidationproblems However as soon as SCC started to fill trussdiagonals and verticals two foam panels started to float asthe buoyant forces broke the bond between the steel formsand foam block-outs Several actions were taken to keep thefoam block-outs in place against the uplifting force Lumberpieces were used as spacers between the foam and top flangereinforcement and between the top reinforcement and thesteel form to prevent further movements of the block-outsThis experiment indicated that foam panels were not a goodchoice for forming

After curing the specimen with wet burlap for 3 days theconcrete strength reached 524MPa (7600 psi) forms werestripped and strands were released on March 14 2013 Steelforms were easy to strip however foam block-outs wereembedded in the concrete and did not come out easily Thepressure of the concrete on the foam made it difficult topull out of the concrete in addition to having a thin layerof concrete between the foam and the steel form in somelocations which had to be chipped out Foam block-outs hadto be cut into small pieces using an electric saw Removing theblock-outs at the corners was evenmore challenging A smallchipping hammerwas used to carefully remove the remainingfoam without damaging the concrete The movement of thefoam block-outs while casting resulted in deviations in thedimensions angles and locations of two diagonal membersand two vertical members as shown in Figure 10

Prestressing strands were released using gradual deten-sioning and specimen ends were inspected for cracking Fewcracks appeared at south and north ends that were mostlyhorizontal and extended for the full thickness of the web andfew inches in the longitudinal directionThese cracks are dueto the bursting force of prestressing and were not controlledproperly because the end zone reinforcements were notplaced as close as they should be to the bulkheads Fewshrinkage cracks occurred in the top flange at the locationswere the lumber pieces were placed to prevent the floatingof the foam These cracks were not critical for the proposedtesting as they occurred only in the top flange which is acompression member

34 Specimen Testing Two roller supports were placed onconcrete blocks and spaced 89m (295 ft) apart on center tosupport the 9m (30 ft) long truss specimen The rollers werecentered on the 150mm (6 in) wide bearing plates embeddedin the truss at both ends A steel frame with a 1780 kN(400 kip) loading jack was placed as shown in Figure 11 toload the specimen at midspan section

To clearly see and track crack propagation of trusselements during loading one side of the specimen waspainted in white while strain gages were attached to the otherside Linear variable differential transforms (LVDTs) wereused to monitor the slippage of strands during testing Adeflection gage was installed to measure midspan deflectionsduring loading The specimen was tested on March 29 2013Concrete cylinders were tested and the compressive strengthwas found to be 724MPa (10500 psi) at that time Duringloading the specimen was visually inspected at 2225 kN(50 kip) load increments and cracking was being marked upAt 2225 kN (50 kip) the deflection reached 5mm (019 in)with no visible cracking The loading continued to 445 kN(100 kip) and deflection reached 10mm (039 in) Minorhorizontal cracks were observed at the corners betweendiagonals and topbottom flanges as shown in Figure 12

At 667 kN (150 kip) the deflection reached 15mm(057 in) and cracking continued at acute-angled cornersbetween the solid web and bottom flangeThemiddle verticalstarted to crack as well at the top and bottom At 890 kN(200 kip) deflection reached 20mm (075 in) and crackingoccurred at all the acute-angled corners At 1112 kN (250 kip)deflection reached 24mm (093 in) and cracking severityhad not significantly increased except at the webbottomflange corner After 1112 kN (250 kip) the load increasedcontinuously without interruptions up to failure at 1712 kN(385 kip) Excessive cracking was observed at the bottomflange around its connection to vertical rods and diagonalsas shown in Figure 13 The failure was dramatic as the one-threaded rod was pulled out of the bottom flange causing theadjacent diagonal to snap as shown in Figure 14 This failureoccurred as one of the 6 anchor bars welded to the washerplate was completely sheared as shown in Figure 15 Despitethe high load carrying capacity that the specimen achieved itis believed that having longer anchor bars and hat bars forbottom flange confinement around the anchor bars wouldhave postponed or even eliminated this mode of failure Alsochamfering sharp edges and using curved corners wouldhave reduced stress concentrations andminimized prematurecracking at these locations

35 Analysis of Results Figure 16 plots the load-deflectionrelationship of the truss specimen This plot indicates thatthe specimen had a linear elastic behavior up to the crackingload which is determined to be 1580 kN (355 kip) using thetangents method This load is 7 higher than the 1468 kN(330 kip) predicted cracking loadThemeasured deflection atthe cracking load was found to be 34mm (133 in) which is33 higher than the 25mm (10 in) predicted deflectionThisis primarily due to the premature cracking that was observedat almost all acute-angled corners whichmight have resultedin stiffness reduction Also the deviations between specified

10 Journal of Structures

85 in

75 in

85

in

85 in

9 in 9 in 9 in 9 in

65

in11 in

North South

North 1 South 1North 2 South 2

Figure 10 Actual truss member concrete dimensions after casting (1 in = 254mm)

Figure 11 Truss specimen before testing

Figure 12 First cracks observed at 445 kN (100 kip)

Figure 13 Excessive cracking at the bottom flange before failure

Figure 14 Failure of the truss specimen

Figure 15 Shearing of anchor bars at the failure location

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 100 200 300 400 500 600

Load

(kN

)

Midspan deflection (mm)

Figure 16 Load versus deflection curve (black lines indicate end ofelastic behavior)

and actual dimensions could have influenced the behavior ofthe specimen Figure 16 also indicates that the ultimate loadwas 1713 kN (385 kip) which is 38 lower than the predictedcapacity of 1779 kN (400 kip) due to the premature pullout of

Journal of Structures 11

the vertical threaded rod as a result of inadequate anchoragein the bottom flange

Figure 17 shows themeasured slippage of 178mm (07 in)diameter prestressing strands during loading This plot indi-cates that all the recorded readings are significantly less than025mm (001 in) which is the limit for initial slippageThe highest recorded value was even less than 0025mm(0001 in) which is the measurement accuracy of the usedLVDTs indicating no slippage up to failure load This meansthat 178mm (07 in) diameter prestressing strands were fullydeveloped within the 45m (15 ft) distance (ie half of thespecimen length) which is the predicted development lengthusing ACI 318-11 It should be noted that the high valuerecorded by the south LVDT at the failure load is incorrectdue to the sudden movement of the specimen at the momentof failure

Figure 18 plots the measured strain in four vertical mem-bers of the truss specimen versus the applied load Theseverticals are 38mm (15 in) diameter threaded rods that havea yield strength of 724MPa (105 ksi) and ultimate strengthof 862MPa (125 ksi)Themaximummeasured strain reached26 (at the south rod 1 where the failure occurred) Also allmeasured strains in the four threaded rods had significantlyexceeded the yield strain of 036 Figure 19 plots the forcesin the four threaded rods versus the applied load This plotindicates that the forces in all the four threaded rods reachedthe yield force of 689 kN (155 kip) which is 14 more thanthe predicted design force of 605 kN (136 kip)

Figure 20 plots the measured strains in the four rein-forced concrete diagonal members of the truss specimenversus the applied loadThis plot indicates that the measuredstrains varied significantly among the four diagonalmembersdue to the variation in their angles and concrete dimensions(eg the width of the south diagonal 1 was 165mm (65 in)while the width of the north diagonal 2 was 280mm(11 in)) However they were all much lower than the ultimatedesign strain of concrete (03) The maximum ultimatestrain reached 01 at the south diagonal 1 where failurehappened and the minimum ultimate strain reached 0045at the north diagonal 2 Also the straight line relationshipsin all diagonal members indicate their linear elastic behaviorup to failure loadTherefore the truss failure due to crushingof the diagonals is a low probability mode of failure Figure 21plots the forces in all four diagonal members versus theapplied loadThis plot indicates that the ultimate compressiveforce varied from 1179 kN (265 kip) at the north diagonal2 to 1446 kN (325 kip) at the south diagonal 1 wherefailure occurred The forces in all diagonal members exceptthe north diagonal 2 exceeded the design force of 1192 kN(268 kip) which indicates that the design of the diagonals wasadequate

4 Conclusions and Recommendations

This research aimed to develop a precast concrete truss sys-tem for roof applications that is lightweight and aestheticallypleasing can span up to 48m (160 ft) and can be fabricated

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025

Load

(kN

)

Slippage (mm)

North strand slippageSouth strand slippage

Figure 17 Strand slippage measurements (1 kip = 445 kN 1 in =254mm)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 05 10 15 20 25 30

Load

(kN

)

Strain in TR ()

N rod 1

N rod 2

S rod 1

S rod 2

Figure 18 Tensile strains in the four threaded rods (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

mdash 100 200 300 400 500 600 700 800

Load

(kN

)

Force in TR (kN)

N rod 1

N rod 2

S rod 1

S rod 2

Figure 19 Tension forces in the four threaded rods (black lineindicates yield force 1 kip = 445 kN)

12 Journal of Structures

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025 030

Load

(kN

)

Strain in diagonals ()

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 20 Compressive strains in the diagonal members (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 200 400 600 800 1000 1200 1400 1600

Load

(kN

)

Force in diagonals (kN)

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 21 Compressive forces in the diagonal members (black lineindicates yield force 1 kip = 445 kN)

using standard precast practices in the USA Based on thepresented work three main conclusions can be made

(1) Fabrication of the proposed truss system is practicaland efficient The proposed method of fabricationwas evaluated experimentally by producing a 9m(30 ft) long truss specimen using economical andcommercially available components standard bridgegirder form foam block-outs vertical threaded rodsand conventional reinforcement The success of theproposed method is also attributed to the use of highstrength self-consolidating concrete (SCC) that fillsthe complex form encapsulates the reinforcementand provides a smooth finished surface without anymechanical consolidation

(2) Structural testing of the truss specimen that wasdesigned and detailed to resist the forces generatedin an example building indicated the adequacy ofthe proposed design method and connection details

Few recommendations were made as shown belowto further improve the performance of the proposedsystem

(3) The 2D frame models and 3D finite element modelscan be used to accurately predict the behavior of theproposed system Forces obtained from these modelscan be used to design truss members using the strut-and-tie method and in accordance to the existingbuilding code FEA can be used to accurately predictstress concentration at truss member connections

Based on the analytical and experimental investigationsseveral recommendations could be made to improve theproposed truss system

(1) Use chamfered edges and curved corners instead ofthe sharp ones to avoid stress concentrations and addnominal reinforcement to control cracking at theselocations

(2) Use adequate top flange and bottom flange confer-ment reinforcement to help anchoring diagonal andvertical reinforcement whichmust be fully developedto prevent pullout

(3) Avoid using foam block-outs due to the difficultyof gluing them to the form and stripping themUsing light gage steel pans or fiber glass panes ishighly recommended for efficient and economicalfabrication as they can have multiple reuses

(4) Diagonal bars should be tied together after beingplaced in the forms to accommodate the tolerancesespecially when bar lengths and bent diameters arenot exact This practice will allow the diagonalsto slide against each other Another suggestion forfabrication is to assemble all the reinforcements in astiff manner outside the form to precisely match theform dimensionsThen the assembly is tied togetherlifted by a crane and placed in the form in one step

(5) Self-consolidating concrete with high flowability(average spread of 800mm (28 in) plusmn 50mm (2 in))passing ability (nominal maximum size aggregate is10mm (38 in)) resistance to segregation (VSI notmore than 10) and low viscosity (119879

50lt 2 sec) is

required to simplify production and ensure properfilling of the complex truss form without mechanicalconsolidation

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The technical support of the PrecastPrestressed ConcreteInstitute (PCI) advisory committee members is acknowl-edged and the financial support of the PCI Daniel PJenny Fellowship is appreciated Assistance was provided by

Journal of Structures 13

Musa Alawneh as a scholarship partner Support was alsogiven by University of Nebraska-Lincoln (UNL) graduatesstudents and staff Kelvin Lein AfshinHatami Shaddi AssadMohamed El-Kady and Micheal Asaad

References

[1] PCI PCI Design Hnadbook PrecastPrestressed Concrete Insti-tute (PCI) Chicago Ill USA 2010

[2] PCI ldquoThe 50 most significant precast concrete projectsrdquoASCENT Magazine 2004

[3] P H Shah and H R May ldquoPrecast vierendeel trusses pro-vide unique structural facade for parking structurerdquo Pre-castPrestressed Concrete Institute Journal vol 22 no 4 pp 24ndash39 1977

[4] W T Carroll F W Beaufait and R H Bryan ldquoPrestressedconcrete trussesrdquo ACI Journal vol 75 no 8 pp 367ndash373 1978

[5] MDeSutter United States of America Patent 7275348 B2 2007[6] A Trygestad andM DeSutter ldquoBookmen stacks cobalt condos

use ER-POST for column-free spacerdquo PCI Journal vol 52 no3 pp 58ndash71 2007

[7] ASCESEI 7 Minimum Design Loads for Buildings and OtherStructures American Society of Civil Engineers (ASCE)Reston Va USA 2010

[8] ACI 318 Building Code Requirments for Structural Concrete(ACI 318-11) and Commentary American Concrete Institute(ACI) Farmington Hills Mich USA 2011

[9] ASTMA193 ldquoStandard specification for alloy-steel and stainlesssteel bolting for high temperature or high pressure service andother special purpose applicationsrdquo ASTM Standard A193M-14ASTM International West Conshohocken Pa USA 2014

[10] AISC Steel Construction Manual American Institute of SteelConstruction (AISC) Chicago Ill USA 14th edition 2012

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DistributedSensor Networks

International Journal of

6 Journal of Structures

4998400-5998400998400

3998400-2 3

8

998400998400

2998400-51 2

998400998400

30998400-0998400998400

3

4

4

400∘

21

Elevation

3998400-3 5

8

998400998400

Section

Section Section Section

4

1 2 3

8998400998400

8998400998400

8998400998400

8998400998400

4998400998400

2998400998400

8998400998400

4998400998400

4998400998400

1998400998400

1998400998400

1998400998400

1998400998400

1998400-9998400998400 1

998400-9998400998400 1998400-9998400998400

4998400-4

998400998400

4998400-4

998400998400

4998400-8

998400998400

61

2

998400998400 61

2

998400998400

2998400-71 2

998400998400

2998400-71 2

998400998400

2998400-51 2

9984009984001998400-31 2

998400998400

71 2

998400998400

71 2

998400998400

71

2

99840099840071

2

998400998400

71

2

998400998400

1998400-11998400998400 1

998400-11998400998400 1998400-11998400998400

6998400998400

6998400998400

11998400998400

4998400998400

6998400998400 6998400998400

11998400998400

111 2

9984009984002998400-51 2

998400998400

2 halves of foamfor truss openings(see elevation view

for dimensions)

1

2

998400998400 plywood and(3) 2 times 4 studsfor block-out

Iowa type D girder form

3 998400-7 1

8998400998400

Figure 5 Dimensions of the concrete truss specimen (1ft = 0302m 1 in = 254mm)

(2) Each segment is erected on one column at one endand on temporary supports at the other end

(3) Roof purlins and bracings are installed to stabilize thetruss-girders

(4) Couplers are used to connect posttensioning ductsand posttensioning strands are threaded throughducts

(5) Joints between truss segments are formed reinforcedand cast in place using SCCwith similar properties tothose of the truss concrete

(6) Posttensioning is applied after joint concrete achievesadequate strength and posttensioning ducts aregrouted

(7) Temporary supports are removed and roof decking isinstalled

(8) Fire-proofing and corrosion resistance agents areapplied to steel elements if needed

3 Experimental Investigation

31 Specimen Description The purpose of the experimentalinvestigation is to evaluate the constructability and structuralperformance of the proposed truss-girder system A full-sizetruss could not be fabricated and tested due to space andbudget limitations A 9m (30 ft) long truss formed usingIowa type D bridge I-girder forms provided by Coreslab

Structures Inc Omaha NE was used instead The cross-section dimensions of Iowa type D I-girder are very closeto those of AASHTO type IV bridge girder The forms are9m (30 ft) long and 1420mm (56 in) high However toreduce the weight of the specimen a 100mm (4 in) block-out was made at the bottom of the form to have a totaldepth of 1320mm (52 in) and two foam panels 100mm(4 in) thick each were used to form for each truss openingFigure 5 shows the dimensions of the concrete truss specimenat different sections

32 Specimen Analysis and Design Two-dimensional (2D)frame analysis and three-dimensional (3D) finite elementanalysis (FEA) were carried out to determine member forcesand deformations of the specimen Comparing analysisresults of each method as shown in Table 1 indicates that thesimple 2D frame analysis results in conservative and relativelyaccurate estimates of forces and deflections compared to themore elaborate FEA The loads used in this analysis includeweight of the specimen prestressing force and midspanconcentrated load which achieve forces in the diagonal andvertical members of the specimen similar to those in the full-size truss-girder system designed in the previous section Itshould be noted that diagonalmembers of the specimen had a40∘ angle with the bottom flange in order to achieve the sameratio between diagonal and vertical forces as in the full-sizetruss system presented earlier

The analysis of the specimen indicated that a midspanpoint load of 1779 kN (400 kip) will result in the forces

Journal of Structures 7

Table 1 Comparing analysis results using 2D frame model and 3DFE model

Analysis method 2D frameanalysis

Finite elementanalysis

Max compression indiagonals (kN) 1246 1125

Max tension inverticals (kN) 574 534

Cracking load (kN) 1478 1335Correspondingdeflection (mm) 25 21

Figure 6 The FE model used to analyze truss connections

that are slightly higher than the design factored forcesin diagonal and vertical members The initial design ofthe specimen required 16ndash153mm (06 in) diameter Grade1860 (270) strands However due to the unavailability ofthis size of strands at the structural laboratory 12ndash178mm(07 in) diameter Grade 1860 (270) strands were used insteadto achieve the same prestressing force Also the analysisindicated that the cracking load is 1468 kN (330 kip) usingthe cracking stress limit at the bottom fibers and the corre-sponding deflection is 25mm (1 in) Strand jacking stress wasassumed to be 075119891pu and total prestress losses were assumedto be 20of the jacking stressThe developed FEmodel of thespecimen shown in Figure 6 consists of a simplified cross-section (ie rectangles) 8-node solid elements for concreteand frame elements for threaded rods This model was usedto conduct elastoplastic analysis using specified materialproperties to determine stresses and deformations underdifferent loading levels Figure 7 shows the stress contours onconcrete elements under ultimate load This figure indicatesthat the connection between diagonal web and the bottomflange had very high tensile stresses which are expected tocause premature cracking at those locations

The specimen was designed similar to the full-size trusswith one exception top and bottom flanges were overde-signed to ensure that the failure occurs at the verticalsdiagonals or connections The 12ndash178mm (07 in) diameterstrands were provided to achieve a flexural capacity of4371 kNsdotm (3224 kipsdotft) which is about 10 more than theapplied moment Also the top flange was reinforced with2 number 25 (8) bars as compression reinforcement toincrease the capacity of the top flange Figure 8 shows theelevation cross-sections and reinforcement details of thespecimen

E + 6

550

440

330

220

110

00

minus110

minus220

minus330

minus440

minus550

minus660

minus770

minus880

Figure 7 Stresses in the truss specimen under ultimate load atmidspan (Nm2)

33 Specimen Fabrication Fabrication of the 9m (30-ft)long specimen was done at the structural lab of the PeterKiewit Institute (PKI) in Omaha NE in five major steps(1) preparing forms and placing prestressing strands (2)cutting foam block-outs and gluing them to the steel forms(3) assembling diagonal and vertical reinforcement andinstalling them between the foam block-outs (4) casting self-consolidating concrete into the forms and (5) stripping theforms and releasing the strands

The prestressing strands were tensioned to 3176 kN(714 kip) (075 of the ultimate stress of 1860MPa (270 ksi))Foam block-outs were used to form the truss openings The100mm (4 in) thick foam panels were cut into diamondshapes and glued to form the 200mm (8 in) thick block-outs Square grooves that are 19mm times 19mm (075 in times075 in) were removed from the edges of the foam panelsto accommodate the vertical threaded rods as shown inFigure 9 To facilitate the stripping of the foam from theconcrete web plastic sheets were wrapped around the edgesof the foam All the foam block-outs were glued on the steelform after marking their locations on the form sides Bottomflange and top flange reinforcement were simple to installThe challenge was assembling and installing the diagonaland vertical reinforcement which were 4 number 19 (6)bars and number 10 (3) ties spaced at 200mm (8 in) alongthe member and 38mm (15 in) diameter threaded rodsThe rods were anchored at the top and bottom flange using200mm times 200mm times 13m (8 in times 8 in times 05 in) Grade350 (50) steel plates and structural nuts Each plate is weldedto two diagonal bars and 2 number 19 (6) straight anchorbars Initially it was planned to have the reinforcementsfor each diagonal preassembled and then connected to thethreaded rods after being installed in the form The mainproblem of this plan is that it requires very tight tolerancesin reinforcement dimensions and bent locations in additionto the difficulty of handling a very heavy assembly ofreinforcement that is not very stiff Some diagonal bars wereslightly shorter than others and do not have exactly thesame bend diameter or location To address these challengesdiagonal bars and anchor bars were cut to have only 225mm(9 in) of embedment (12 db) and transverse ties were keptloose to allow the bars to move relative to each other thenthey are tied after all reinforcement is in place to simplify thefabrication process as shown in Figure 9 Four 100 times 100 minusMW20 timesMW20 (4 times 4minusW29 timesW29)WWRwere placed atthe solid part at each endof the truss for shear reinforcements

8 Journal of Structures

A B

DD

C

Detail A

3998400-3 5

8

998400998400

1998400-0998400998400

30998400-0998400998400

Detail A

3 ties at8998400998400 spacings

46

SectionSection Section Section

DA B C

5998400 998400

11 2

99840099840011 2

998400998400

1998400-9998400998400

1998400-9998400998400

1998400-11998400998400

D116998400998400

36998400998400 spacing

21

2

99840099840021

2

998400998400

21

2

998400998400

61

2

998400998400

71

2

998400998400

71 2

998400998400

21

2

998400998400

1998400-11998400998400

4998400998400

6998400998400

6998400998400

8998400998400

4998400998400

4998400998400

2998400998400

11998400998400

2998400-51 2

998400998400111 2

998400998400

PL 23998400998400times6

998400998400times1

2

998400998400

w3 1

2

998400998400 studs

7998400998400 spacings

41

2

99840099840041

2

9984009984007998400998400

7998400998400

3998400998400

5998400998400

6998400998400

1998400-11998400998400

Detail A

Topreinforcements

reinforcements

36998400998400 spacings

Bottom

W2

Diagonals rebarrefer to the details below

11

2

998400998400emptythreaded rod

and a structural nut

8998400998400

8998400998400

PL 8998400998400times8

998400998400times1

2

998400998400

opening locatedat center for TRdia of openingcorresponds to

the TR dia

Plate welded to thediagonal rebar

6998400998400

1 2

998400998400

4998400-2

998400998400

11

2

998400998400emptythreaded rods w6998400998400

threads at the endsGR 120 B7 weathered

steel

Threaded rod

A-A

3998400-101 2

998400998400

3998400-2998400998400

4998400 -85

8

998400998400

2998400-6998400998400

2998400-6998400998400

8998400998400

8998400998400

R4 1

2 998400998400

59984009984008998400998400

11 2

99840099840011 2

998400998400

Diagonal steel reinforcement details

4998400-4

998400998400

Figure 8 Elevation and cross-sections of the specimen (1ft = 0302m 1 in = 254mm)

The top flange had 2 number 25 (8) bars tied using number10 (3) stirrups at 150mm (6 in) spacing After the form wasclosed the top flange reinforcement assembly was placed andlumber yokes were used to tie the forms from the top

The specimen was cast on March 11 2013 using specified55MPa (8000 psi) self-consolidating concrete The mixturewas designed using type III Portland cement with 30replacement of class C fly ash and a mixture of 10mm(38 in) crushed limestone and natural sand and gravel

The SCC had an average spread of 800mm (28 in) 11987950

less than 2 sec J-ring spread reduction less than 50mm(2 in) and visual stability index (VSI) of 10 Nine 100mm times200mm (4 in times 8 in) cylinders were taken to evaluate thecompressive strength at release testing and 28 days CastingSCC started at the middle of the specimen Two pipe cameraswere attached at the bottom of the truss one at each endto record the concrete flow around the reinforcement andstrands Another camera was recording the casting process

Journal of Structures 9

Figure 9 All reinforcements after being installed in the form

from the top The high flowability and passing ability of SCCmade it easy to fill the bottom flange with no consolidationproblems However as soon as SCC started to fill trussdiagonals and verticals two foam panels started to float asthe buoyant forces broke the bond between the steel formsand foam block-outs Several actions were taken to keep thefoam block-outs in place against the uplifting force Lumberpieces were used as spacers between the foam and top flangereinforcement and between the top reinforcement and thesteel form to prevent further movements of the block-outsThis experiment indicated that foam panels were not a goodchoice for forming

After curing the specimen with wet burlap for 3 days theconcrete strength reached 524MPa (7600 psi) forms werestripped and strands were released on March 14 2013 Steelforms were easy to strip however foam block-outs wereembedded in the concrete and did not come out easily Thepressure of the concrete on the foam made it difficult topull out of the concrete in addition to having a thin layerof concrete between the foam and the steel form in somelocations which had to be chipped out Foam block-outs hadto be cut into small pieces using an electric saw Removing theblock-outs at the corners was evenmore challenging A smallchipping hammerwas used to carefully remove the remainingfoam without damaging the concrete The movement of thefoam block-outs while casting resulted in deviations in thedimensions angles and locations of two diagonal membersand two vertical members as shown in Figure 10

Prestressing strands were released using gradual deten-sioning and specimen ends were inspected for cracking Fewcracks appeared at south and north ends that were mostlyhorizontal and extended for the full thickness of the web andfew inches in the longitudinal directionThese cracks are dueto the bursting force of prestressing and were not controlledproperly because the end zone reinforcements were notplaced as close as they should be to the bulkheads Fewshrinkage cracks occurred in the top flange at the locationswere the lumber pieces were placed to prevent the floatingof the foam These cracks were not critical for the proposedtesting as they occurred only in the top flange which is acompression member

34 Specimen Testing Two roller supports were placed onconcrete blocks and spaced 89m (295 ft) apart on center tosupport the 9m (30 ft) long truss specimen The rollers werecentered on the 150mm (6 in) wide bearing plates embeddedin the truss at both ends A steel frame with a 1780 kN(400 kip) loading jack was placed as shown in Figure 11 toload the specimen at midspan section

To clearly see and track crack propagation of trusselements during loading one side of the specimen waspainted in white while strain gages were attached to the otherside Linear variable differential transforms (LVDTs) wereused to monitor the slippage of strands during testing Adeflection gage was installed to measure midspan deflectionsduring loading The specimen was tested on March 29 2013Concrete cylinders were tested and the compressive strengthwas found to be 724MPa (10500 psi) at that time Duringloading the specimen was visually inspected at 2225 kN(50 kip) load increments and cracking was being marked upAt 2225 kN (50 kip) the deflection reached 5mm (019 in)with no visible cracking The loading continued to 445 kN(100 kip) and deflection reached 10mm (039 in) Minorhorizontal cracks were observed at the corners betweendiagonals and topbottom flanges as shown in Figure 12

At 667 kN (150 kip) the deflection reached 15mm(057 in) and cracking continued at acute-angled cornersbetween the solid web and bottom flangeThemiddle verticalstarted to crack as well at the top and bottom At 890 kN(200 kip) deflection reached 20mm (075 in) and crackingoccurred at all the acute-angled corners At 1112 kN (250 kip)deflection reached 24mm (093 in) and cracking severityhad not significantly increased except at the webbottomflange corner After 1112 kN (250 kip) the load increasedcontinuously without interruptions up to failure at 1712 kN(385 kip) Excessive cracking was observed at the bottomflange around its connection to vertical rods and diagonalsas shown in Figure 13 The failure was dramatic as the one-threaded rod was pulled out of the bottom flange causing theadjacent diagonal to snap as shown in Figure 14 This failureoccurred as one of the 6 anchor bars welded to the washerplate was completely sheared as shown in Figure 15 Despitethe high load carrying capacity that the specimen achieved itis believed that having longer anchor bars and hat bars forbottom flange confinement around the anchor bars wouldhave postponed or even eliminated this mode of failure Alsochamfering sharp edges and using curved corners wouldhave reduced stress concentrations andminimized prematurecracking at these locations

35 Analysis of Results Figure 16 plots the load-deflectionrelationship of the truss specimen This plot indicates thatthe specimen had a linear elastic behavior up to the crackingload which is determined to be 1580 kN (355 kip) using thetangents method This load is 7 higher than the 1468 kN(330 kip) predicted cracking loadThemeasured deflection atthe cracking load was found to be 34mm (133 in) which is33 higher than the 25mm (10 in) predicted deflectionThisis primarily due to the premature cracking that was observedat almost all acute-angled corners whichmight have resultedin stiffness reduction Also the deviations between specified

10 Journal of Structures

85 in

75 in

85

in

85 in

9 in 9 in 9 in 9 in

65

in11 in

North South

North 1 South 1North 2 South 2

Figure 10 Actual truss member concrete dimensions after casting (1 in = 254mm)

Figure 11 Truss specimen before testing

Figure 12 First cracks observed at 445 kN (100 kip)

Figure 13 Excessive cracking at the bottom flange before failure

Figure 14 Failure of the truss specimen

Figure 15 Shearing of anchor bars at the failure location

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 100 200 300 400 500 600

Load

(kN

)

Midspan deflection (mm)

Figure 16 Load versus deflection curve (black lines indicate end ofelastic behavior)

and actual dimensions could have influenced the behavior ofthe specimen Figure 16 also indicates that the ultimate loadwas 1713 kN (385 kip) which is 38 lower than the predictedcapacity of 1779 kN (400 kip) due to the premature pullout of

Journal of Structures 11

the vertical threaded rod as a result of inadequate anchoragein the bottom flange

Figure 17 shows themeasured slippage of 178mm (07 in)diameter prestressing strands during loading This plot indi-cates that all the recorded readings are significantly less than025mm (001 in) which is the limit for initial slippageThe highest recorded value was even less than 0025mm(0001 in) which is the measurement accuracy of the usedLVDTs indicating no slippage up to failure load This meansthat 178mm (07 in) diameter prestressing strands were fullydeveloped within the 45m (15 ft) distance (ie half of thespecimen length) which is the predicted development lengthusing ACI 318-11 It should be noted that the high valuerecorded by the south LVDT at the failure load is incorrectdue to the sudden movement of the specimen at the momentof failure

Figure 18 plots the measured strain in four vertical mem-bers of the truss specimen versus the applied load Theseverticals are 38mm (15 in) diameter threaded rods that havea yield strength of 724MPa (105 ksi) and ultimate strengthof 862MPa (125 ksi)Themaximummeasured strain reached26 (at the south rod 1 where the failure occurred) Also allmeasured strains in the four threaded rods had significantlyexceeded the yield strain of 036 Figure 19 plots the forcesin the four threaded rods versus the applied load This plotindicates that the forces in all the four threaded rods reachedthe yield force of 689 kN (155 kip) which is 14 more thanthe predicted design force of 605 kN (136 kip)

Figure 20 plots the measured strains in the four rein-forced concrete diagonal members of the truss specimenversus the applied loadThis plot indicates that the measuredstrains varied significantly among the four diagonalmembersdue to the variation in their angles and concrete dimensions(eg the width of the south diagonal 1 was 165mm (65 in)while the width of the north diagonal 2 was 280mm(11 in)) However they were all much lower than the ultimatedesign strain of concrete (03) The maximum ultimatestrain reached 01 at the south diagonal 1 where failurehappened and the minimum ultimate strain reached 0045at the north diagonal 2 Also the straight line relationshipsin all diagonal members indicate their linear elastic behaviorup to failure loadTherefore the truss failure due to crushingof the diagonals is a low probability mode of failure Figure 21plots the forces in all four diagonal members versus theapplied loadThis plot indicates that the ultimate compressiveforce varied from 1179 kN (265 kip) at the north diagonal2 to 1446 kN (325 kip) at the south diagonal 1 wherefailure occurred The forces in all diagonal members exceptthe north diagonal 2 exceeded the design force of 1192 kN(268 kip) which indicates that the design of the diagonals wasadequate

4 Conclusions and Recommendations

This research aimed to develop a precast concrete truss sys-tem for roof applications that is lightweight and aestheticallypleasing can span up to 48m (160 ft) and can be fabricated

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025

Load

(kN

)

Slippage (mm)

North strand slippageSouth strand slippage

Figure 17 Strand slippage measurements (1 kip = 445 kN 1 in =254mm)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 05 10 15 20 25 30

Load

(kN

)

Strain in TR ()

N rod 1

N rod 2

S rod 1

S rod 2

Figure 18 Tensile strains in the four threaded rods (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

mdash 100 200 300 400 500 600 700 800

Load

(kN

)

Force in TR (kN)

N rod 1

N rod 2

S rod 1

S rod 2

Figure 19 Tension forces in the four threaded rods (black lineindicates yield force 1 kip = 445 kN)

12 Journal of Structures

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025 030

Load

(kN

)

Strain in diagonals ()

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 20 Compressive strains in the diagonal members (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 200 400 600 800 1000 1200 1400 1600

Load

(kN

)

Force in diagonals (kN)

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 21 Compressive forces in the diagonal members (black lineindicates yield force 1 kip = 445 kN)

using standard precast practices in the USA Based on thepresented work three main conclusions can be made

(1) Fabrication of the proposed truss system is practicaland efficient The proposed method of fabricationwas evaluated experimentally by producing a 9m(30 ft) long truss specimen using economical andcommercially available components standard bridgegirder form foam block-outs vertical threaded rodsand conventional reinforcement The success of theproposed method is also attributed to the use of highstrength self-consolidating concrete (SCC) that fillsthe complex form encapsulates the reinforcementand provides a smooth finished surface without anymechanical consolidation

(2) Structural testing of the truss specimen that wasdesigned and detailed to resist the forces generatedin an example building indicated the adequacy ofthe proposed design method and connection details

Few recommendations were made as shown belowto further improve the performance of the proposedsystem

(3) The 2D frame models and 3D finite element modelscan be used to accurately predict the behavior of theproposed system Forces obtained from these modelscan be used to design truss members using the strut-and-tie method and in accordance to the existingbuilding code FEA can be used to accurately predictstress concentration at truss member connections

Based on the analytical and experimental investigationsseveral recommendations could be made to improve theproposed truss system

(1) Use chamfered edges and curved corners instead ofthe sharp ones to avoid stress concentrations and addnominal reinforcement to control cracking at theselocations

(2) Use adequate top flange and bottom flange confer-ment reinforcement to help anchoring diagonal andvertical reinforcement whichmust be fully developedto prevent pullout

(3) Avoid using foam block-outs due to the difficultyof gluing them to the form and stripping themUsing light gage steel pans or fiber glass panes ishighly recommended for efficient and economicalfabrication as they can have multiple reuses

(4) Diagonal bars should be tied together after beingplaced in the forms to accommodate the tolerancesespecially when bar lengths and bent diameters arenot exact This practice will allow the diagonalsto slide against each other Another suggestion forfabrication is to assemble all the reinforcements in astiff manner outside the form to precisely match theform dimensionsThen the assembly is tied togetherlifted by a crane and placed in the form in one step

(5) Self-consolidating concrete with high flowability(average spread of 800mm (28 in) plusmn 50mm (2 in))passing ability (nominal maximum size aggregate is10mm (38 in)) resistance to segregation (VSI notmore than 10) and low viscosity (119879

50lt 2 sec) is

required to simplify production and ensure properfilling of the complex truss form without mechanicalconsolidation

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The technical support of the PrecastPrestressed ConcreteInstitute (PCI) advisory committee members is acknowl-edged and the financial support of the PCI Daniel PJenny Fellowship is appreciated Assistance was provided by

Journal of Structures 13

Musa Alawneh as a scholarship partner Support was alsogiven by University of Nebraska-Lincoln (UNL) graduatesstudents and staff Kelvin Lein AfshinHatami Shaddi AssadMohamed El-Kady and Micheal Asaad

References

[1] PCI PCI Design Hnadbook PrecastPrestressed Concrete Insti-tute (PCI) Chicago Ill USA 2010

[2] PCI ldquoThe 50 most significant precast concrete projectsrdquoASCENT Magazine 2004

[3] P H Shah and H R May ldquoPrecast vierendeel trusses pro-vide unique structural facade for parking structurerdquo Pre-castPrestressed Concrete Institute Journal vol 22 no 4 pp 24ndash39 1977

[4] W T Carroll F W Beaufait and R H Bryan ldquoPrestressedconcrete trussesrdquo ACI Journal vol 75 no 8 pp 367ndash373 1978

[5] MDeSutter United States of America Patent 7275348 B2 2007[6] A Trygestad andM DeSutter ldquoBookmen stacks cobalt condos

use ER-POST for column-free spacerdquo PCI Journal vol 52 no3 pp 58ndash71 2007

[7] ASCESEI 7 Minimum Design Loads for Buildings and OtherStructures American Society of Civil Engineers (ASCE)Reston Va USA 2010

[8] ACI 318 Building Code Requirments for Structural Concrete(ACI 318-11) and Commentary American Concrete Institute(ACI) Farmington Hills Mich USA 2011

[9] ASTMA193 ldquoStandard specification for alloy-steel and stainlesssteel bolting for high temperature or high pressure service andother special purpose applicationsrdquo ASTM Standard A193M-14ASTM International West Conshohocken Pa USA 2014

[10] AISC Steel Construction Manual American Institute of SteelConstruction (AISC) Chicago Ill USA 14th edition 2012

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Journal of Structures 7

Table 1 Comparing analysis results using 2D frame model and 3DFE model

Analysis method 2D frameanalysis

Finite elementanalysis

Max compression indiagonals (kN) 1246 1125

Max tension inverticals (kN) 574 534

Cracking load (kN) 1478 1335Correspondingdeflection (mm) 25 21

Figure 6 The FE model used to analyze truss connections

that are slightly higher than the design factored forcesin diagonal and vertical members The initial design ofthe specimen required 16ndash153mm (06 in) diameter Grade1860 (270) strands However due to the unavailability ofthis size of strands at the structural laboratory 12ndash178mm(07 in) diameter Grade 1860 (270) strands were used insteadto achieve the same prestressing force Also the analysisindicated that the cracking load is 1468 kN (330 kip) usingthe cracking stress limit at the bottom fibers and the corre-sponding deflection is 25mm (1 in) Strand jacking stress wasassumed to be 075119891pu and total prestress losses were assumedto be 20of the jacking stressThe developed FEmodel of thespecimen shown in Figure 6 consists of a simplified cross-section (ie rectangles) 8-node solid elements for concreteand frame elements for threaded rods This model was usedto conduct elastoplastic analysis using specified materialproperties to determine stresses and deformations underdifferent loading levels Figure 7 shows the stress contours onconcrete elements under ultimate load This figure indicatesthat the connection between diagonal web and the bottomflange had very high tensile stresses which are expected tocause premature cracking at those locations

The specimen was designed similar to the full-size trusswith one exception top and bottom flanges were overde-signed to ensure that the failure occurs at the verticalsdiagonals or connections The 12ndash178mm (07 in) diameterstrands were provided to achieve a flexural capacity of4371 kNsdotm (3224 kipsdotft) which is about 10 more than theapplied moment Also the top flange was reinforced with2 number 25 (8) bars as compression reinforcement toincrease the capacity of the top flange Figure 8 shows theelevation cross-sections and reinforcement details of thespecimen

E + 6

550

440

330

220

110

00

minus110

minus220

minus330

minus440

minus550

minus660

minus770

minus880

Figure 7 Stresses in the truss specimen under ultimate load atmidspan (Nm2)

33 Specimen Fabrication Fabrication of the 9m (30-ft)long specimen was done at the structural lab of the PeterKiewit Institute (PKI) in Omaha NE in five major steps(1) preparing forms and placing prestressing strands (2)cutting foam block-outs and gluing them to the steel forms(3) assembling diagonal and vertical reinforcement andinstalling them between the foam block-outs (4) casting self-consolidating concrete into the forms and (5) stripping theforms and releasing the strands

The prestressing strands were tensioned to 3176 kN(714 kip) (075 of the ultimate stress of 1860MPa (270 ksi))Foam block-outs were used to form the truss openings The100mm (4 in) thick foam panels were cut into diamondshapes and glued to form the 200mm (8 in) thick block-outs Square grooves that are 19mm times 19mm (075 in times075 in) were removed from the edges of the foam panelsto accommodate the vertical threaded rods as shown inFigure 9 To facilitate the stripping of the foam from theconcrete web plastic sheets were wrapped around the edgesof the foam All the foam block-outs were glued on the steelform after marking their locations on the form sides Bottomflange and top flange reinforcement were simple to installThe challenge was assembling and installing the diagonaland vertical reinforcement which were 4 number 19 (6)bars and number 10 (3) ties spaced at 200mm (8 in) alongthe member and 38mm (15 in) diameter threaded rodsThe rods were anchored at the top and bottom flange using200mm times 200mm times 13m (8 in times 8 in times 05 in) Grade350 (50) steel plates and structural nuts Each plate is weldedto two diagonal bars and 2 number 19 (6) straight anchorbars Initially it was planned to have the reinforcementsfor each diagonal preassembled and then connected to thethreaded rods after being installed in the form The mainproblem of this plan is that it requires very tight tolerancesin reinforcement dimensions and bent locations in additionto the difficulty of handling a very heavy assembly ofreinforcement that is not very stiff Some diagonal bars wereslightly shorter than others and do not have exactly thesame bend diameter or location To address these challengesdiagonal bars and anchor bars were cut to have only 225mm(9 in) of embedment (12 db) and transverse ties were keptloose to allow the bars to move relative to each other thenthey are tied after all reinforcement is in place to simplify thefabrication process as shown in Figure 9 Four 100 times 100 minusMW20 timesMW20 (4 times 4minusW29 timesW29)WWRwere placed atthe solid part at each endof the truss for shear reinforcements

8 Journal of Structures

A B

DD

C

Detail A

3998400-3 5

8

998400998400

1998400-0998400998400

30998400-0998400998400

Detail A

3 ties at8998400998400 spacings

46

SectionSection Section Section

DA B C

5998400 998400

11 2

99840099840011 2

998400998400

1998400-9998400998400

1998400-9998400998400

1998400-11998400998400

D116998400998400

36998400998400 spacing

21

2

99840099840021

2

998400998400

21

2

998400998400

61

2

998400998400

71

2

998400998400

71 2

998400998400

21

2

998400998400

1998400-11998400998400

4998400998400

6998400998400

6998400998400

8998400998400

4998400998400

4998400998400

2998400998400

11998400998400

2998400-51 2

998400998400111 2

998400998400

PL 23998400998400times6

998400998400times1

2

998400998400

w3 1

2

998400998400 studs

7998400998400 spacings

41

2

99840099840041

2

9984009984007998400998400

7998400998400

3998400998400

5998400998400

6998400998400

1998400-11998400998400

Detail A

Topreinforcements

reinforcements

36998400998400 spacings

Bottom

W2

Diagonals rebarrefer to the details below

11

2

998400998400emptythreaded rod

and a structural nut

8998400998400

8998400998400

PL 8998400998400times8

998400998400times1

2

998400998400

opening locatedat center for TRdia of openingcorresponds to

the TR dia

Plate welded to thediagonal rebar

6998400998400

1 2

998400998400

4998400-2

998400998400

11

2

998400998400emptythreaded rods w6998400998400

threads at the endsGR 120 B7 weathered

steel

Threaded rod

A-A

3998400-101 2

998400998400

3998400-2998400998400

4998400 -85

8

998400998400

2998400-6998400998400

2998400-6998400998400

8998400998400

8998400998400

R4 1

2 998400998400

59984009984008998400998400

11 2

99840099840011 2

998400998400

Diagonal steel reinforcement details

4998400-4

998400998400

Figure 8 Elevation and cross-sections of the specimen (1ft = 0302m 1 in = 254mm)

The top flange had 2 number 25 (8) bars tied using number10 (3) stirrups at 150mm (6 in) spacing After the form wasclosed the top flange reinforcement assembly was placed andlumber yokes were used to tie the forms from the top

The specimen was cast on March 11 2013 using specified55MPa (8000 psi) self-consolidating concrete The mixturewas designed using type III Portland cement with 30replacement of class C fly ash and a mixture of 10mm(38 in) crushed limestone and natural sand and gravel

The SCC had an average spread of 800mm (28 in) 11987950

less than 2 sec J-ring spread reduction less than 50mm(2 in) and visual stability index (VSI) of 10 Nine 100mm times200mm (4 in times 8 in) cylinders were taken to evaluate thecompressive strength at release testing and 28 days CastingSCC started at the middle of the specimen Two pipe cameraswere attached at the bottom of the truss one at each endto record the concrete flow around the reinforcement andstrands Another camera was recording the casting process

Journal of Structures 9

Figure 9 All reinforcements after being installed in the form

from the top The high flowability and passing ability of SCCmade it easy to fill the bottom flange with no consolidationproblems However as soon as SCC started to fill trussdiagonals and verticals two foam panels started to float asthe buoyant forces broke the bond between the steel formsand foam block-outs Several actions were taken to keep thefoam block-outs in place against the uplifting force Lumberpieces were used as spacers between the foam and top flangereinforcement and between the top reinforcement and thesteel form to prevent further movements of the block-outsThis experiment indicated that foam panels were not a goodchoice for forming

After curing the specimen with wet burlap for 3 days theconcrete strength reached 524MPa (7600 psi) forms werestripped and strands were released on March 14 2013 Steelforms were easy to strip however foam block-outs wereembedded in the concrete and did not come out easily Thepressure of the concrete on the foam made it difficult topull out of the concrete in addition to having a thin layerof concrete between the foam and the steel form in somelocations which had to be chipped out Foam block-outs hadto be cut into small pieces using an electric saw Removing theblock-outs at the corners was evenmore challenging A smallchipping hammerwas used to carefully remove the remainingfoam without damaging the concrete The movement of thefoam block-outs while casting resulted in deviations in thedimensions angles and locations of two diagonal membersand two vertical members as shown in Figure 10

Prestressing strands were released using gradual deten-sioning and specimen ends were inspected for cracking Fewcracks appeared at south and north ends that were mostlyhorizontal and extended for the full thickness of the web andfew inches in the longitudinal directionThese cracks are dueto the bursting force of prestressing and were not controlledproperly because the end zone reinforcements were notplaced as close as they should be to the bulkheads Fewshrinkage cracks occurred in the top flange at the locationswere the lumber pieces were placed to prevent the floatingof the foam These cracks were not critical for the proposedtesting as they occurred only in the top flange which is acompression member

34 Specimen Testing Two roller supports were placed onconcrete blocks and spaced 89m (295 ft) apart on center tosupport the 9m (30 ft) long truss specimen The rollers werecentered on the 150mm (6 in) wide bearing plates embeddedin the truss at both ends A steel frame with a 1780 kN(400 kip) loading jack was placed as shown in Figure 11 toload the specimen at midspan section

To clearly see and track crack propagation of trusselements during loading one side of the specimen waspainted in white while strain gages were attached to the otherside Linear variable differential transforms (LVDTs) wereused to monitor the slippage of strands during testing Adeflection gage was installed to measure midspan deflectionsduring loading The specimen was tested on March 29 2013Concrete cylinders were tested and the compressive strengthwas found to be 724MPa (10500 psi) at that time Duringloading the specimen was visually inspected at 2225 kN(50 kip) load increments and cracking was being marked upAt 2225 kN (50 kip) the deflection reached 5mm (019 in)with no visible cracking The loading continued to 445 kN(100 kip) and deflection reached 10mm (039 in) Minorhorizontal cracks were observed at the corners betweendiagonals and topbottom flanges as shown in Figure 12

At 667 kN (150 kip) the deflection reached 15mm(057 in) and cracking continued at acute-angled cornersbetween the solid web and bottom flangeThemiddle verticalstarted to crack as well at the top and bottom At 890 kN(200 kip) deflection reached 20mm (075 in) and crackingoccurred at all the acute-angled corners At 1112 kN (250 kip)deflection reached 24mm (093 in) and cracking severityhad not significantly increased except at the webbottomflange corner After 1112 kN (250 kip) the load increasedcontinuously without interruptions up to failure at 1712 kN(385 kip) Excessive cracking was observed at the bottomflange around its connection to vertical rods and diagonalsas shown in Figure 13 The failure was dramatic as the one-threaded rod was pulled out of the bottom flange causing theadjacent diagonal to snap as shown in Figure 14 This failureoccurred as one of the 6 anchor bars welded to the washerplate was completely sheared as shown in Figure 15 Despitethe high load carrying capacity that the specimen achieved itis believed that having longer anchor bars and hat bars forbottom flange confinement around the anchor bars wouldhave postponed or even eliminated this mode of failure Alsochamfering sharp edges and using curved corners wouldhave reduced stress concentrations andminimized prematurecracking at these locations

35 Analysis of Results Figure 16 plots the load-deflectionrelationship of the truss specimen This plot indicates thatthe specimen had a linear elastic behavior up to the crackingload which is determined to be 1580 kN (355 kip) using thetangents method This load is 7 higher than the 1468 kN(330 kip) predicted cracking loadThemeasured deflection atthe cracking load was found to be 34mm (133 in) which is33 higher than the 25mm (10 in) predicted deflectionThisis primarily due to the premature cracking that was observedat almost all acute-angled corners whichmight have resultedin stiffness reduction Also the deviations between specified

10 Journal of Structures

85 in

75 in

85

in

85 in

9 in 9 in 9 in 9 in

65

in11 in

North South

North 1 South 1North 2 South 2

Figure 10 Actual truss member concrete dimensions after casting (1 in = 254mm)

Figure 11 Truss specimen before testing

Figure 12 First cracks observed at 445 kN (100 kip)

Figure 13 Excessive cracking at the bottom flange before failure

Figure 14 Failure of the truss specimen

Figure 15 Shearing of anchor bars at the failure location

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 100 200 300 400 500 600

Load

(kN

)

Midspan deflection (mm)

Figure 16 Load versus deflection curve (black lines indicate end ofelastic behavior)

and actual dimensions could have influenced the behavior ofthe specimen Figure 16 also indicates that the ultimate loadwas 1713 kN (385 kip) which is 38 lower than the predictedcapacity of 1779 kN (400 kip) due to the premature pullout of

Journal of Structures 11

the vertical threaded rod as a result of inadequate anchoragein the bottom flange

Figure 17 shows themeasured slippage of 178mm (07 in)diameter prestressing strands during loading This plot indi-cates that all the recorded readings are significantly less than025mm (001 in) which is the limit for initial slippageThe highest recorded value was even less than 0025mm(0001 in) which is the measurement accuracy of the usedLVDTs indicating no slippage up to failure load This meansthat 178mm (07 in) diameter prestressing strands were fullydeveloped within the 45m (15 ft) distance (ie half of thespecimen length) which is the predicted development lengthusing ACI 318-11 It should be noted that the high valuerecorded by the south LVDT at the failure load is incorrectdue to the sudden movement of the specimen at the momentof failure

Figure 18 plots the measured strain in four vertical mem-bers of the truss specimen versus the applied load Theseverticals are 38mm (15 in) diameter threaded rods that havea yield strength of 724MPa (105 ksi) and ultimate strengthof 862MPa (125 ksi)Themaximummeasured strain reached26 (at the south rod 1 where the failure occurred) Also allmeasured strains in the four threaded rods had significantlyexceeded the yield strain of 036 Figure 19 plots the forcesin the four threaded rods versus the applied load This plotindicates that the forces in all the four threaded rods reachedthe yield force of 689 kN (155 kip) which is 14 more thanthe predicted design force of 605 kN (136 kip)

Figure 20 plots the measured strains in the four rein-forced concrete diagonal members of the truss specimenversus the applied loadThis plot indicates that the measuredstrains varied significantly among the four diagonalmembersdue to the variation in their angles and concrete dimensions(eg the width of the south diagonal 1 was 165mm (65 in)while the width of the north diagonal 2 was 280mm(11 in)) However they were all much lower than the ultimatedesign strain of concrete (03) The maximum ultimatestrain reached 01 at the south diagonal 1 where failurehappened and the minimum ultimate strain reached 0045at the north diagonal 2 Also the straight line relationshipsin all diagonal members indicate their linear elastic behaviorup to failure loadTherefore the truss failure due to crushingof the diagonals is a low probability mode of failure Figure 21plots the forces in all four diagonal members versus theapplied loadThis plot indicates that the ultimate compressiveforce varied from 1179 kN (265 kip) at the north diagonal2 to 1446 kN (325 kip) at the south diagonal 1 wherefailure occurred The forces in all diagonal members exceptthe north diagonal 2 exceeded the design force of 1192 kN(268 kip) which indicates that the design of the diagonals wasadequate

4 Conclusions and Recommendations

This research aimed to develop a precast concrete truss sys-tem for roof applications that is lightweight and aestheticallypleasing can span up to 48m (160 ft) and can be fabricated

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025

Load

(kN

)

Slippage (mm)

North strand slippageSouth strand slippage

Figure 17 Strand slippage measurements (1 kip = 445 kN 1 in =254mm)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 05 10 15 20 25 30

Load

(kN

)

Strain in TR ()

N rod 1

N rod 2

S rod 1

S rod 2

Figure 18 Tensile strains in the four threaded rods (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

mdash 100 200 300 400 500 600 700 800

Load

(kN

)

Force in TR (kN)

N rod 1

N rod 2

S rod 1

S rod 2

Figure 19 Tension forces in the four threaded rods (black lineindicates yield force 1 kip = 445 kN)

12 Journal of Structures

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025 030

Load

(kN

)

Strain in diagonals ()

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 20 Compressive strains in the diagonal members (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 200 400 600 800 1000 1200 1400 1600

Load

(kN

)

Force in diagonals (kN)

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 21 Compressive forces in the diagonal members (black lineindicates yield force 1 kip = 445 kN)

using standard precast practices in the USA Based on thepresented work three main conclusions can be made

(1) Fabrication of the proposed truss system is practicaland efficient The proposed method of fabricationwas evaluated experimentally by producing a 9m(30 ft) long truss specimen using economical andcommercially available components standard bridgegirder form foam block-outs vertical threaded rodsand conventional reinforcement The success of theproposed method is also attributed to the use of highstrength self-consolidating concrete (SCC) that fillsthe complex form encapsulates the reinforcementand provides a smooth finished surface without anymechanical consolidation

(2) Structural testing of the truss specimen that wasdesigned and detailed to resist the forces generatedin an example building indicated the adequacy ofthe proposed design method and connection details

Few recommendations were made as shown belowto further improve the performance of the proposedsystem

(3) The 2D frame models and 3D finite element modelscan be used to accurately predict the behavior of theproposed system Forces obtained from these modelscan be used to design truss members using the strut-and-tie method and in accordance to the existingbuilding code FEA can be used to accurately predictstress concentration at truss member connections

Based on the analytical and experimental investigationsseveral recommendations could be made to improve theproposed truss system

(1) Use chamfered edges and curved corners instead ofthe sharp ones to avoid stress concentrations and addnominal reinforcement to control cracking at theselocations

(2) Use adequate top flange and bottom flange confer-ment reinforcement to help anchoring diagonal andvertical reinforcement whichmust be fully developedto prevent pullout

(3) Avoid using foam block-outs due to the difficultyof gluing them to the form and stripping themUsing light gage steel pans or fiber glass panes ishighly recommended for efficient and economicalfabrication as they can have multiple reuses

(4) Diagonal bars should be tied together after beingplaced in the forms to accommodate the tolerancesespecially when bar lengths and bent diameters arenot exact This practice will allow the diagonalsto slide against each other Another suggestion forfabrication is to assemble all the reinforcements in astiff manner outside the form to precisely match theform dimensionsThen the assembly is tied togetherlifted by a crane and placed in the form in one step

(5) Self-consolidating concrete with high flowability(average spread of 800mm (28 in) plusmn 50mm (2 in))passing ability (nominal maximum size aggregate is10mm (38 in)) resistance to segregation (VSI notmore than 10) and low viscosity (119879

50lt 2 sec) is

required to simplify production and ensure properfilling of the complex truss form without mechanicalconsolidation

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The technical support of the PrecastPrestressed ConcreteInstitute (PCI) advisory committee members is acknowl-edged and the financial support of the PCI Daniel PJenny Fellowship is appreciated Assistance was provided by

Journal of Structures 13

Musa Alawneh as a scholarship partner Support was alsogiven by University of Nebraska-Lincoln (UNL) graduatesstudents and staff Kelvin Lein AfshinHatami Shaddi AssadMohamed El-Kady and Micheal Asaad

References

[1] PCI PCI Design Hnadbook PrecastPrestressed Concrete Insti-tute (PCI) Chicago Ill USA 2010

[2] PCI ldquoThe 50 most significant precast concrete projectsrdquoASCENT Magazine 2004

[3] P H Shah and H R May ldquoPrecast vierendeel trusses pro-vide unique structural facade for parking structurerdquo Pre-castPrestressed Concrete Institute Journal vol 22 no 4 pp 24ndash39 1977

[4] W T Carroll F W Beaufait and R H Bryan ldquoPrestressedconcrete trussesrdquo ACI Journal vol 75 no 8 pp 367ndash373 1978

[5] MDeSutter United States of America Patent 7275348 B2 2007[6] A Trygestad andM DeSutter ldquoBookmen stacks cobalt condos

use ER-POST for column-free spacerdquo PCI Journal vol 52 no3 pp 58ndash71 2007

[7] ASCESEI 7 Minimum Design Loads for Buildings and OtherStructures American Society of Civil Engineers (ASCE)Reston Va USA 2010

[8] ACI 318 Building Code Requirments for Structural Concrete(ACI 318-11) and Commentary American Concrete Institute(ACI) Farmington Hills Mich USA 2011

[9] ASTMA193 ldquoStandard specification for alloy-steel and stainlesssteel bolting for high temperature or high pressure service andother special purpose applicationsrdquo ASTM Standard A193M-14ASTM International West Conshohocken Pa USA 2014

[10] AISC Steel Construction Manual American Institute of SteelConstruction (AISC) Chicago Ill USA 14th edition 2012

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RotatingMachinery

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Civil EngineeringAdvances in

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Electrical and Computer Engineering

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Advances inOptoElectronics

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Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

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Chemical EngineeringInternational Journal of Antennas and

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International Journal of

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Navigation and Observation

International Journal of

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DistributedSensor Networks

International Journal of

8 Journal of Structures

A B

DD

C

Detail A

3998400-3 5

8

998400998400

1998400-0998400998400

30998400-0998400998400

Detail A

3 ties at8998400998400 spacings

46

SectionSection Section Section

DA B C

5998400 998400

11 2

99840099840011 2

998400998400

1998400-9998400998400

1998400-9998400998400

1998400-11998400998400

D116998400998400

36998400998400 spacing

21

2

99840099840021

2

998400998400

21

2

998400998400

61

2

998400998400

71

2

998400998400

71 2

998400998400

21

2

998400998400

1998400-11998400998400

4998400998400

6998400998400

6998400998400

8998400998400

4998400998400

4998400998400

2998400998400

11998400998400

2998400-51 2

998400998400111 2

998400998400

PL 23998400998400times6

998400998400times1

2

998400998400

w3 1

2

998400998400 studs

7998400998400 spacings

41

2

99840099840041

2

9984009984007998400998400

7998400998400

3998400998400

5998400998400

6998400998400

1998400-11998400998400

Detail A

Topreinforcements

reinforcements

36998400998400 spacings

Bottom

W2

Diagonals rebarrefer to the details below

11

2

998400998400emptythreaded rod

and a structural nut

8998400998400

8998400998400

PL 8998400998400times8

998400998400times1

2

998400998400

opening locatedat center for TRdia of openingcorresponds to

the TR dia

Plate welded to thediagonal rebar

6998400998400

1 2

998400998400

4998400-2

998400998400

11

2

998400998400emptythreaded rods w6998400998400

threads at the endsGR 120 B7 weathered

steel

Threaded rod

A-A

3998400-101 2

998400998400

3998400-2998400998400

4998400 -85

8

998400998400

2998400-6998400998400

2998400-6998400998400

8998400998400

8998400998400

R4 1

2 998400998400

59984009984008998400998400

11 2

99840099840011 2

998400998400

Diagonal steel reinforcement details

4998400-4

998400998400

Figure 8 Elevation and cross-sections of the specimen (1ft = 0302m 1 in = 254mm)

The top flange had 2 number 25 (8) bars tied using number10 (3) stirrups at 150mm (6 in) spacing After the form wasclosed the top flange reinforcement assembly was placed andlumber yokes were used to tie the forms from the top

The specimen was cast on March 11 2013 using specified55MPa (8000 psi) self-consolidating concrete The mixturewas designed using type III Portland cement with 30replacement of class C fly ash and a mixture of 10mm(38 in) crushed limestone and natural sand and gravel

The SCC had an average spread of 800mm (28 in) 11987950

less than 2 sec J-ring spread reduction less than 50mm(2 in) and visual stability index (VSI) of 10 Nine 100mm times200mm (4 in times 8 in) cylinders were taken to evaluate thecompressive strength at release testing and 28 days CastingSCC started at the middle of the specimen Two pipe cameraswere attached at the bottom of the truss one at each endto record the concrete flow around the reinforcement andstrands Another camera was recording the casting process

Journal of Structures 9

Figure 9 All reinforcements after being installed in the form

from the top The high flowability and passing ability of SCCmade it easy to fill the bottom flange with no consolidationproblems However as soon as SCC started to fill trussdiagonals and verticals two foam panels started to float asthe buoyant forces broke the bond between the steel formsand foam block-outs Several actions were taken to keep thefoam block-outs in place against the uplifting force Lumberpieces were used as spacers between the foam and top flangereinforcement and between the top reinforcement and thesteel form to prevent further movements of the block-outsThis experiment indicated that foam panels were not a goodchoice for forming

After curing the specimen with wet burlap for 3 days theconcrete strength reached 524MPa (7600 psi) forms werestripped and strands were released on March 14 2013 Steelforms were easy to strip however foam block-outs wereembedded in the concrete and did not come out easily Thepressure of the concrete on the foam made it difficult topull out of the concrete in addition to having a thin layerof concrete between the foam and the steel form in somelocations which had to be chipped out Foam block-outs hadto be cut into small pieces using an electric saw Removing theblock-outs at the corners was evenmore challenging A smallchipping hammerwas used to carefully remove the remainingfoam without damaging the concrete The movement of thefoam block-outs while casting resulted in deviations in thedimensions angles and locations of two diagonal membersand two vertical members as shown in Figure 10

Prestressing strands were released using gradual deten-sioning and specimen ends were inspected for cracking Fewcracks appeared at south and north ends that were mostlyhorizontal and extended for the full thickness of the web andfew inches in the longitudinal directionThese cracks are dueto the bursting force of prestressing and were not controlledproperly because the end zone reinforcements were notplaced as close as they should be to the bulkheads Fewshrinkage cracks occurred in the top flange at the locationswere the lumber pieces were placed to prevent the floatingof the foam These cracks were not critical for the proposedtesting as they occurred only in the top flange which is acompression member

34 Specimen Testing Two roller supports were placed onconcrete blocks and spaced 89m (295 ft) apart on center tosupport the 9m (30 ft) long truss specimen The rollers werecentered on the 150mm (6 in) wide bearing plates embeddedin the truss at both ends A steel frame with a 1780 kN(400 kip) loading jack was placed as shown in Figure 11 toload the specimen at midspan section

To clearly see and track crack propagation of trusselements during loading one side of the specimen waspainted in white while strain gages were attached to the otherside Linear variable differential transforms (LVDTs) wereused to monitor the slippage of strands during testing Adeflection gage was installed to measure midspan deflectionsduring loading The specimen was tested on March 29 2013Concrete cylinders were tested and the compressive strengthwas found to be 724MPa (10500 psi) at that time Duringloading the specimen was visually inspected at 2225 kN(50 kip) load increments and cracking was being marked upAt 2225 kN (50 kip) the deflection reached 5mm (019 in)with no visible cracking The loading continued to 445 kN(100 kip) and deflection reached 10mm (039 in) Minorhorizontal cracks were observed at the corners betweendiagonals and topbottom flanges as shown in Figure 12

At 667 kN (150 kip) the deflection reached 15mm(057 in) and cracking continued at acute-angled cornersbetween the solid web and bottom flangeThemiddle verticalstarted to crack as well at the top and bottom At 890 kN(200 kip) deflection reached 20mm (075 in) and crackingoccurred at all the acute-angled corners At 1112 kN (250 kip)deflection reached 24mm (093 in) and cracking severityhad not significantly increased except at the webbottomflange corner After 1112 kN (250 kip) the load increasedcontinuously without interruptions up to failure at 1712 kN(385 kip) Excessive cracking was observed at the bottomflange around its connection to vertical rods and diagonalsas shown in Figure 13 The failure was dramatic as the one-threaded rod was pulled out of the bottom flange causing theadjacent diagonal to snap as shown in Figure 14 This failureoccurred as one of the 6 anchor bars welded to the washerplate was completely sheared as shown in Figure 15 Despitethe high load carrying capacity that the specimen achieved itis believed that having longer anchor bars and hat bars forbottom flange confinement around the anchor bars wouldhave postponed or even eliminated this mode of failure Alsochamfering sharp edges and using curved corners wouldhave reduced stress concentrations andminimized prematurecracking at these locations

35 Analysis of Results Figure 16 plots the load-deflectionrelationship of the truss specimen This plot indicates thatthe specimen had a linear elastic behavior up to the crackingload which is determined to be 1580 kN (355 kip) using thetangents method This load is 7 higher than the 1468 kN(330 kip) predicted cracking loadThemeasured deflection atthe cracking load was found to be 34mm (133 in) which is33 higher than the 25mm (10 in) predicted deflectionThisis primarily due to the premature cracking that was observedat almost all acute-angled corners whichmight have resultedin stiffness reduction Also the deviations between specified

10 Journal of Structures

85 in

75 in

85

in

85 in

9 in 9 in 9 in 9 in

65

in11 in

North South

North 1 South 1North 2 South 2

Figure 10 Actual truss member concrete dimensions after casting (1 in = 254mm)

Figure 11 Truss specimen before testing

Figure 12 First cracks observed at 445 kN (100 kip)

Figure 13 Excessive cracking at the bottom flange before failure

Figure 14 Failure of the truss specimen

Figure 15 Shearing of anchor bars at the failure location

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 100 200 300 400 500 600

Load

(kN

)

Midspan deflection (mm)

Figure 16 Load versus deflection curve (black lines indicate end ofelastic behavior)

and actual dimensions could have influenced the behavior ofthe specimen Figure 16 also indicates that the ultimate loadwas 1713 kN (385 kip) which is 38 lower than the predictedcapacity of 1779 kN (400 kip) due to the premature pullout of

Journal of Structures 11

the vertical threaded rod as a result of inadequate anchoragein the bottom flange

Figure 17 shows themeasured slippage of 178mm (07 in)diameter prestressing strands during loading This plot indi-cates that all the recorded readings are significantly less than025mm (001 in) which is the limit for initial slippageThe highest recorded value was even less than 0025mm(0001 in) which is the measurement accuracy of the usedLVDTs indicating no slippage up to failure load This meansthat 178mm (07 in) diameter prestressing strands were fullydeveloped within the 45m (15 ft) distance (ie half of thespecimen length) which is the predicted development lengthusing ACI 318-11 It should be noted that the high valuerecorded by the south LVDT at the failure load is incorrectdue to the sudden movement of the specimen at the momentof failure

Figure 18 plots the measured strain in four vertical mem-bers of the truss specimen versus the applied load Theseverticals are 38mm (15 in) diameter threaded rods that havea yield strength of 724MPa (105 ksi) and ultimate strengthof 862MPa (125 ksi)Themaximummeasured strain reached26 (at the south rod 1 where the failure occurred) Also allmeasured strains in the four threaded rods had significantlyexceeded the yield strain of 036 Figure 19 plots the forcesin the four threaded rods versus the applied load This plotindicates that the forces in all the four threaded rods reachedthe yield force of 689 kN (155 kip) which is 14 more thanthe predicted design force of 605 kN (136 kip)

Figure 20 plots the measured strains in the four rein-forced concrete diagonal members of the truss specimenversus the applied loadThis plot indicates that the measuredstrains varied significantly among the four diagonalmembersdue to the variation in their angles and concrete dimensions(eg the width of the south diagonal 1 was 165mm (65 in)while the width of the north diagonal 2 was 280mm(11 in)) However they were all much lower than the ultimatedesign strain of concrete (03) The maximum ultimatestrain reached 01 at the south diagonal 1 where failurehappened and the minimum ultimate strain reached 0045at the north diagonal 2 Also the straight line relationshipsin all diagonal members indicate their linear elastic behaviorup to failure loadTherefore the truss failure due to crushingof the diagonals is a low probability mode of failure Figure 21plots the forces in all four diagonal members versus theapplied loadThis plot indicates that the ultimate compressiveforce varied from 1179 kN (265 kip) at the north diagonal2 to 1446 kN (325 kip) at the south diagonal 1 wherefailure occurred The forces in all diagonal members exceptthe north diagonal 2 exceeded the design force of 1192 kN(268 kip) which indicates that the design of the diagonals wasadequate

4 Conclusions and Recommendations

This research aimed to develop a precast concrete truss sys-tem for roof applications that is lightweight and aestheticallypleasing can span up to 48m (160 ft) and can be fabricated

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025

Load

(kN

)

Slippage (mm)

North strand slippageSouth strand slippage

Figure 17 Strand slippage measurements (1 kip = 445 kN 1 in =254mm)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 05 10 15 20 25 30

Load

(kN

)

Strain in TR ()

N rod 1

N rod 2

S rod 1

S rod 2

Figure 18 Tensile strains in the four threaded rods (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

mdash 100 200 300 400 500 600 700 800

Load

(kN

)

Force in TR (kN)

N rod 1

N rod 2

S rod 1

S rod 2

Figure 19 Tension forces in the four threaded rods (black lineindicates yield force 1 kip = 445 kN)

12 Journal of Structures

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025 030

Load

(kN

)

Strain in diagonals ()

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 20 Compressive strains in the diagonal members (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 200 400 600 800 1000 1200 1400 1600

Load

(kN

)

Force in diagonals (kN)

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 21 Compressive forces in the diagonal members (black lineindicates yield force 1 kip = 445 kN)

using standard precast practices in the USA Based on thepresented work three main conclusions can be made

(1) Fabrication of the proposed truss system is practicaland efficient The proposed method of fabricationwas evaluated experimentally by producing a 9m(30 ft) long truss specimen using economical andcommercially available components standard bridgegirder form foam block-outs vertical threaded rodsand conventional reinforcement The success of theproposed method is also attributed to the use of highstrength self-consolidating concrete (SCC) that fillsthe complex form encapsulates the reinforcementand provides a smooth finished surface without anymechanical consolidation

(2) Structural testing of the truss specimen that wasdesigned and detailed to resist the forces generatedin an example building indicated the adequacy ofthe proposed design method and connection details

Few recommendations were made as shown belowto further improve the performance of the proposedsystem

(3) The 2D frame models and 3D finite element modelscan be used to accurately predict the behavior of theproposed system Forces obtained from these modelscan be used to design truss members using the strut-and-tie method and in accordance to the existingbuilding code FEA can be used to accurately predictstress concentration at truss member connections

Based on the analytical and experimental investigationsseveral recommendations could be made to improve theproposed truss system

(1) Use chamfered edges and curved corners instead ofthe sharp ones to avoid stress concentrations and addnominal reinforcement to control cracking at theselocations

(2) Use adequate top flange and bottom flange confer-ment reinforcement to help anchoring diagonal andvertical reinforcement whichmust be fully developedto prevent pullout

(3) Avoid using foam block-outs due to the difficultyof gluing them to the form and stripping themUsing light gage steel pans or fiber glass panes ishighly recommended for efficient and economicalfabrication as they can have multiple reuses

(4) Diagonal bars should be tied together after beingplaced in the forms to accommodate the tolerancesespecially when bar lengths and bent diameters arenot exact This practice will allow the diagonalsto slide against each other Another suggestion forfabrication is to assemble all the reinforcements in astiff manner outside the form to precisely match theform dimensionsThen the assembly is tied togetherlifted by a crane and placed in the form in one step

(5) Self-consolidating concrete with high flowability(average spread of 800mm (28 in) plusmn 50mm (2 in))passing ability (nominal maximum size aggregate is10mm (38 in)) resistance to segregation (VSI notmore than 10) and low viscosity (119879

50lt 2 sec) is

required to simplify production and ensure properfilling of the complex truss form without mechanicalconsolidation

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The technical support of the PrecastPrestressed ConcreteInstitute (PCI) advisory committee members is acknowl-edged and the financial support of the PCI Daniel PJenny Fellowship is appreciated Assistance was provided by

Journal of Structures 13

Musa Alawneh as a scholarship partner Support was alsogiven by University of Nebraska-Lincoln (UNL) graduatesstudents and staff Kelvin Lein AfshinHatami Shaddi AssadMohamed El-Kady and Micheal Asaad

References

[1] PCI PCI Design Hnadbook PrecastPrestressed Concrete Insti-tute (PCI) Chicago Ill USA 2010

[2] PCI ldquoThe 50 most significant precast concrete projectsrdquoASCENT Magazine 2004

[3] P H Shah and H R May ldquoPrecast vierendeel trusses pro-vide unique structural facade for parking structurerdquo Pre-castPrestressed Concrete Institute Journal vol 22 no 4 pp 24ndash39 1977

[4] W T Carroll F W Beaufait and R H Bryan ldquoPrestressedconcrete trussesrdquo ACI Journal vol 75 no 8 pp 367ndash373 1978

[5] MDeSutter United States of America Patent 7275348 B2 2007[6] A Trygestad andM DeSutter ldquoBookmen stacks cobalt condos

use ER-POST for column-free spacerdquo PCI Journal vol 52 no3 pp 58ndash71 2007

[7] ASCESEI 7 Minimum Design Loads for Buildings and OtherStructures American Society of Civil Engineers (ASCE)Reston Va USA 2010

[8] ACI 318 Building Code Requirments for Structural Concrete(ACI 318-11) and Commentary American Concrete Institute(ACI) Farmington Hills Mich USA 2011

[9] ASTMA193 ldquoStandard specification for alloy-steel and stainlesssteel bolting for high temperature or high pressure service andother special purpose applicationsrdquo ASTM Standard A193M-14ASTM International West Conshohocken Pa USA 2014

[10] AISC Steel Construction Manual American Institute of SteelConstruction (AISC) Chicago Ill USA 14th edition 2012

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

Journal of Structures 9

Figure 9 All reinforcements after being installed in the form

from the top The high flowability and passing ability of SCCmade it easy to fill the bottom flange with no consolidationproblems However as soon as SCC started to fill trussdiagonals and verticals two foam panels started to float asthe buoyant forces broke the bond between the steel formsand foam block-outs Several actions were taken to keep thefoam block-outs in place against the uplifting force Lumberpieces were used as spacers between the foam and top flangereinforcement and between the top reinforcement and thesteel form to prevent further movements of the block-outsThis experiment indicated that foam panels were not a goodchoice for forming

After curing the specimen with wet burlap for 3 days theconcrete strength reached 524MPa (7600 psi) forms werestripped and strands were released on March 14 2013 Steelforms were easy to strip however foam block-outs wereembedded in the concrete and did not come out easily Thepressure of the concrete on the foam made it difficult topull out of the concrete in addition to having a thin layerof concrete between the foam and the steel form in somelocations which had to be chipped out Foam block-outs hadto be cut into small pieces using an electric saw Removing theblock-outs at the corners was evenmore challenging A smallchipping hammerwas used to carefully remove the remainingfoam without damaging the concrete The movement of thefoam block-outs while casting resulted in deviations in thedimensions angles and locations of two diagonal membersand two vertical members as shown in Figure 10

Prestressing strands were released using gradual deten-sioning and specimen ends were inspected for cracking Fewcracks appeared at south and north ends that were mostlyhorizontal and extended for the full thickness of the web andfew inches in the longitudinal directionThese cracks are dueto the bursting force of prestressing and were not controlledproperly because the end zone reinforcements were notplaced as close as they should be to the bulkheads Fewshrinkage cracks occurred in the top flange at the locationswere the lumber pieces were placed to prevent the floatingof the foam These cracks were not critical for the proposedtesting as they occurred only in the top flange which is acompression member

34 Specimen Testing Two roller supports were placed onconcrete blocks and spaced 89m (295 ft) apart on center tosupport the 9m (30 ft) long truss specimen The rollers werecentered on the 150mm (6 in) wide bearing plates embeddedin the truss at both ends A steel frame with a 1780 kN(400 kip) loading jack was placed as shown in Figure 11 toload the specimen at midspan section

To clearly see and track crack propagation of trusselements during loading one side of the specimen waspainted in white while strain gages were attached to the otherside Linear variable differential transforms (LVDTs) wereused to monitor the slippage of strands during testing Adeflection gage was installed to measure midspan deflectionsduring loading The specimen was tested on March 29 2013Concrete cylinders were tested and the compressive strengthwas found to be 724MPa (10500 psi) at that time Duringloading the specimen was visually inspected at 2225 kN(50 kip) load increments and cracking was being marked upAt 2225 kN (50 kip) the deflection reached 5mm (019 in)with no visible cracking The loading continued to 445 kN(100 kip) and deflection reached 10mm (039 in) Minorhorizontal cracks were observed at the corners betweendiagonals and topbottom flanges as shown in Figure 12

At 667 kN (150 kip) the deflection reached 15mm(057 in) and cracking continued at acute-angled cornersbetween the solid web and bottom flangeThemiddle verticalstarted to crack as well at the top and bottom At 890 kN(200 kip) deflection reached 20mm (075 in) and crackingoccurred at all the acute-angled corners At 1112 kN (250 kip)deflection reached 24mm (093 in) and cracking severityhad not significantly increased except at the webbottomflange corner After 1112 kN (250 kip) the load increasedcontinuously without interruptions up to failure at 1712 kN(385 kip) Excessive cracking was observed at the bottomflange around its connection to vertical rods and diagonalsas shown in Figure 13 The failure was dramatic as the one-threaded rod was pulled out of the bottom flange causing theadjacent diagonal to snap as shown in Figure 14 This failureoccurred as one of the 6 anchor bars welded to the washerplate was completely sheared as shown in Figure 15 Despitethe high load carrying capacity that the specimen achieved itis believed that having longer anchor bars and hat bars forbottom flange confinement around the anchor bars wouldhave postponed or even eliminated this mode of failure Alsochamfering sharp edges and using curved corners wouldhave reduced stress concentrations andminimized prematurecracking at these locations

35 Analysis of Results Figure 16 plots the load-deflectionrelationship of the truss specimen This plot indicates thatthe specimen had a linear elastic behavior up to the crackingload which is determined to be 1580 kN (355 kip) using thetangents method This load is 7 higher than the 1468 kN(330 kip) predicted cracking loadThemeasured deflection atthe cracking load was found to be 34mm (133 in) which is33 higher than the 25mm (10 in) predicted deflectionThisis primarily due to the premature cracking that was observedat almost all acute-angled corners whichmight have resultedin stiffness reduction Also the deviations between specified

10 Journal of Structures

85 in

75 in

85

in

85 in

9 in 9 in 9 in 9 in

65

in11 in

North South

North 1 South 1North 2 South 2

Figure 10 Actual truss member concrete dimensions after casting (1 in = 254mm)

Figure 11 Truss specimen before testing

Figure 12 First cracks observed at 445 kN (100 kip)

Figure 13 Excessive cracking at the bottom flange before failure

Figure 14 Failure of the truss specimen

Figure 15 Shearing of anchor bars at the failure location

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 100 200 300 400 500 600

Load

(kN

)

Midspan deflection (mm)

Figure 16 Load versus deflection curve (black lines indicate end ofelastic behavior)

and actual dimensions could have influenced the behavior ofthe specimen Figure 16 also indicates that the ultimate loadwas 1713 kN (385 kip) which is 38 lower than the predictedcapacity of 1779 kN (400 kip) due to the premature pullout of

Journal of Structures 11

the vertical threaded rod as a result of inadequate anchoragein the bottom flange

Figure 17 shows themeasured slippage of 178mm (07 in)diameter prestressing strands during loading This plot indi-cates that all the recorded readings are significantly less than025mm (001 in) which is the limit for initial slippageThe highest recorded value was even less than 0025mm(0001 in) which is the measurement accuracy of the usedLVDTs indicating no slippage up to failure load This meansthat 178mm (07 in) diameter prestressing strands were fullydeveloped within the 45m (15 ft) distance (ie half of thespecimen length) which is the predicted development lengthusing ACI 318-11 It should be noted that the high valuerecorded by the south LVDT at the failure load is incorrectdue to the sudden movement of the specimen at the momentof failure

Figure 18 plots the measured strain in four vertical mem-bers of the truss specimen versus the applied load Theseverticals are 38mm (15 in) diameter threaded rods that havea yield strength of 724MPa (105 ksi) and ultimate strengthof 862MPa (125 ksi)Themaximummeasured strain reached26 (at the south rod 1 where the failure occurred) Also allmeasured strains in the four threaded rods had significantlyexceeded the yield strain of 036 Figure 19 plots the forcesin the four threaded rods versus the applied load This plotindicates that the forces in all the four threaded rods reachedthe yield force of 689 kN (155 kip) which is 14 more thanthe predicted design force of 605 kN (136 kip)

Figure 20 plots the measured strains in the four rein-forced concrete diagonal members of the truss specimenversus the applied loadThis plot indicates that the measuredstrains varied significantly among the four diagonalmembersdue to the variation in their angles and concrete dimensions(eg the width of the south diagonal 1 was 165mm (65 in)while the width of the north diagonal 2 was 280mm(11 in)) However they were all much lower than the ultimatedesign strain of concrete (03) The maximum ultimatestrain reached 01 at the south diagonal 1 where failurehappened and the minimum ultimate strain reached 0045at the north diagonal 2 Also the straight line relationshipsin all diagonal members indicate their linear elastic behaviorup to failure loadTherefore the truss failure due to crushingof the diagonals is a low probability mode of failure Figure 21plots the forces in all four diagonal members versus theapplied loadThis plot indicates that the ultimate compressiveforce varied from 1179 kN (265 kip) at the north diagonal2 to 1446 kN (325 kip) at the south diagonal 1 wherefailure occurred The forces in all diagonal members exceptthe north diagonal 2 exceeded the design force of 1192 kN(268 kip) which indicates that the design of the diagonals wasadequate

4 Conclusions and Recommendations

This research aimed to develop a precast concrete truss sys-tem for roof applications that is lightweight and aestheticallypleasing can span up to 48m (160 ft) and can be fabricated

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025

Load

(kN

)

Slippage (mm)

North strand slippageSouth strand slippage

Figure 17 Strand slippage measurements (1 kip = 445 kN 1 in =254mm)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 05 10 15 20 25 30

Load

(kN

)

Strain in TR ()

N rod 1

N rod 2

S rod 1

S rod 2

Figure 18 Tensile strains in the four threaded rods (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

mdash 100 200 300 400 500 600 700 800

Load

(kN

)

Force in TR (kN)

N rod 1

N rod 2

S rod 1

S rod 2

Figure 19 Tension forces in the four threaded rods (black lineindicates yield force 1 kip = 445 kN)

12 Journal of Structures

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025 030

Load

(kN

)

Strain in diagonals ()

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 20 Compressive strains in the diagonal members (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 200 400 600 800 1000 1200 1400 1600

Load

(kN

)

Force in diagonals (kN)

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 21 Compressive forces in the diagonal members (black lineindicates yield force 1 kip = 445 kN)

using standard precast practices in the USA Based on thepresented work three main conclusions can be made

(1) Fabrication of the proposed truss system is practicaland efficient The proposed method of fabricationwas evaluated experimentally by producing a 9m(30 ft) long truss specimen using economical andcommercially available components standard bridgegirder form foam block-outs vertical threaded rodsand conventional reinforcement The success of theproposed method is also attributed to the use of highstrength self-consolidating concrete (SCC) that fillsthe complex form encapsulates the reinforcementand provides a smooth finished surface without anymechanical consolidation

(2) Structural testing of the truss specimen that wasdesigned and detailed to resist the forces generatedin an example building indicated the adequacy ofthe proposed design method and connection details

Few recommendations were made as shown belowto further improve the performance of the proposedsystem

(3) The 2D frame models and 3D finite element modelscan be used to accurately predict the behavior of theproposed system Forces obtained from these modelscan be used to design truss members using the strut-and-tie method and in accordance to the existingbuilding code FEA can be used to accurately predictstress concentration at truss member connections

Based on the analytical and experimental investigationsseveral recommendations could be made to improve theproposed truss system

(1) Use chamfered edges and curved corners instead ofthe sharp ones to avoid stress concentrations and addnominal reinforcement to control cracking at theselocations

(2) Use adequate top flange and bottom flange confer-ment reinforcement to help anchoring diagonal andvertical reinforcement whichmust be fully developedto prevent pullout

(3) Avoid using foam block-outs due to the difficultyof gluing them to the form and stripping themUsing light gage steel pans or fiber glass panes ishighly recommended for efficient and economicalfabrication as they can have multiple reuses

(4) Diagonal bars should be tied together after beingplaced in the forms to accommodate the tolerancesespecially when bar lengths and bent diameters arenot exact This practice will allow the diagonalsto slide against each other Another suggestion forfabrication is to assemble all the reinforcements in astiff manner outside the form to precisely match theform dimensionsThen the assembly is tied togetherlifted by a crane and placed in the form in one step

(5) Self-consolidating concrete with high flowability(average spread of 800mm (28 in) plusmn 50mm (2 in))passing ability (nominal maximum size aggregate is10mm (38 in)) resistance to segregation (VSI notmore than 10) and low viscosity (119879

50lt 2 sec) is

required to simplify production and ensure properfilling of the complex truss form without mechanicalconsolidation

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The technical support of the PrecastPrestressed ConcreteInstitute (PCI) advisory committee members is acknowl-edged and the financial support of the PCI Daniel PJenny Fellowship is appreciated Assistance was provided by

Journal of Structures 13

Musa Alawneh as a scholarship partner Support was alsogiven by University of Nebraska-Lincoln (UNL) graduatesstudents and staff Kelvin Lein AfshinHatami Shaddi AssadMohamed El-Kady and Micheal Asaad

References

[1] PCI PCI Design Hnadbook PrecastPrestressed Concrete Insti-tute (PCI) Chicago Ill USA 2010

[2] PCI ldquoThe 50 most significant precast concrete projectsrdquoASCENT Magazine 2004

[3] P H Shah and H R May ldquoPrecast vierendeel trusses pro-vide unique structural facade for parking structurerdquo Pre-castPrestressed Concrete Institute Journal vol 22 no 4 pp 24ndash39 1977

[4] W T Carroll F W Beaufait and R H Bryan ldquoPrestressedconcrete trussesrdquo ACI Journal vol 75 no 8 pp 367ndash373 1978

[5] MDeSutter United States of America Patent 7275348 B2 2007[6] A Trygestad andM DeSutter ldquoBookmen stacks cobalt condos

use ER-POST for column-free spacerdquo PCI Journal vol 52 no3 pp 58ndash71 2007

[7] ASCESEI 7 Minimum Design Loads for Buildings and OtherStructures American Society of Civil Engineers (ASCE)Reston Va USA 2010

[8] ACI 318 Building Code Requirments for Structural Concrete(ACI 318-11) and Commentary American Concrete Institute(ACI) Farmington Hills Mich USA 2011

[9] ASTMA193 ldquoStandard specification for alloy-steel and stainlesssteel bolting for high temperature or high pressure service andother special purpose applicationsrdquo ASTM Standard A193M-14ASTM International West Conshohocken Pa USA 2014

[10] AISC Steel Construction Manual American Institute of SteelConstruction (AISC) Chicago Ill USA 14th edition 2012

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

10 Journal of Structures

85 in

75 in

85

in

85 in

9 in 9 in 9 in 9 in

65

in11 in

North South

North 1 South 1North 2 South 2

Figure 10 Actual truss member concrete dimensions after casting (1 in = 254mm)

Figure 11 Truss specimen before testing

Figure 12 First cracks observed at 445 kN (100 kip)

Figure 13 Excessive cracking at the bottom flange before failure

Figure 14 Failure of the truss specimen

Figure 15 Shearing of anchor bars at the failure location

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 100 200 300 400 500 600

Load

(kN

)

Midspan deflection (mm)

Figure 16 Load versus deflection curve (black lines indicate end ofelastic behavior)

and actual dimensions could have influenced the behavior ofthe specimen Figure 16 also indicates that the ultimate loadwas 1713 kN (385 kip) which is 38 lower than the predictedcapacity of 1779 kN (400 kip) due to the premature pullout of

Journal of Structures 11

the vertical threaded rod as a result of inadequate anchoragein the bottom flange

Figure 17 shows themeasured slippage of 178mm (07 in)diameter prestressing strands during loading This plot indi-cates that all the recorded readings are significantly less than025mm (001 in) which is the limit for initial slippageThe highest recorded value was even less than 0025mm(0001 in) which is the measurement accuracy of the usedLVDTs indicating no slippage up to failure load This meansthat 178mm (07 in) diameter prestressing strands were fullydeveloped within the 45m (15 ft) distance (ie half of thespecimen length) which is the predicted development lengthusing ACI 318-11 It should be noted that the high valuerecorded by the south LVDT at the failure load is incorrectdue to the sudden movement of the specimen at the momentof failure

Figure 18 plots the measured strain in four vertical mem-bers of the truss specimen versus the applied load Theseverticals are 38mm (15 in) diameter threaded rods that havea yield strength of 724MPa (105 ksi) and ultimate strengthof 862MPa (125 ksi)Themaximummeasured strain reached26 (at the south rod 1 where the failure occurred) Also allmeasured strains in the four threaded rods had significantlyexceeded the yield strain of 036 Figure 19 plots the forcesin the four threaded rods versus the applied load This plotindicates that the forces in all the four threaded rods reachedthe yield force of 689 kN (155 kip) which is 14 more thanthe predicted design force of 605 kN (136 kip)

Figure 20 plots the measured strains in the four rein-forced concrete diagonal members of the truss specimenversus the applied loadThis plot indicates that the measuredstrains varied significantly among the four diagonalmembersdue to the variation in their angles and concrete dimensions(eg the width of the south diagonal 1 was 165mm (65 in)while the width of the north diagonal 2 was 280mm(11 in)) However they were all much lower than the ultimatedesign strain of concrete (03) The maximum ultimatestrain reached 01 at the south diagonal 1 where failurehappened and the minimum ultimate strain reached 0045at the north diagonal 2 Also the straight line relationshipsin all diagonal members indicate their linear elastic behaviorup to failure loadTherefore the truss failure due to crushingof the diagonals is a low probability mode of failure Figure 21plots the forces in all four diagonal members versus theapplied loadThis plot indicates that the ultimate compressiveforce varied from 1179 kN (265 kip) at the north diagonal2 to 1446 kN (325 kip) at the south diagonal 1 wherefailure occurred The forces in all diagonal members exceptthe north diagonal 2 exceeded the design force of 1192 kN(268 kip) which indicates that the design of the diagonals wasadequate

4 Conclusions and Recommendations

This research aimed to develop a precast concrete truss sys-tem for roof applications that is lightweight and aestheticallypleasing can span up to 48m (160 ft) and can be fabricated

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025

Load

(kN

)

Slippage (mm)

North strand slippageSouth strand slippage

Figure 17 Strand slippage measurements (1 kip = 445 kN 1 in =254mm)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 05 10 15 20 25 30

Load

(kN

)

Strain in TR ()

N rod 1

N rod 2

S rod 1

S rod 2

Figure 18 Tensile strains in the four threaded rods (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

mdash 100 200 300 400 500 600 700 800

Load

(kN

)

Force in TR (kN)

N rod 1

N rod 2

S rod 1

S rod 2

Figure 19 Tension forces in the four threaded rods (black lineindicates yield force 1 kip = 445 kN)

12 Journal of Structures

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025 030

Load

(kN

)

Strain in diagonals ()

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 20 Compressive strains in the diagonal members (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 200 400 600 800 1000 1200 1400 1600

Load

(kN

)

Force in diagonals (kN)

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 21 Compressive forces in the diagonal members (black lineindicates yield force 1 kip = 445 kN)

using standard precast practices in the USA Based on thepresented work three main conclusions can be made

(1) Fabrication of the proposed truss system is practicaland efficient The proposed method of fabricationwas evaluated experimentally by producing a 9m(30 ft) long truss specimen using economical andcommercially available components standard bridgegirder form foam block-outs vertical threaded rodsand conventional reinforcement The success of theproposed method is also attributed to the use of highstrength self-consolidating concrete (SCC) that fillsthe complex form encapsulates the reinforcementand provides a smooth finished surface without anymechanical consolidation

(2) Structural testing of the truss specimen that wasdesigned and detailed to resist the forces generatedin an example building indicated the adequacy ofthe proposed design method and connection details

Few recommendations were made as shown belowto further improve the performance of the proposedsystem

(3) The 2D frame models and 3D finite element modelscan be used to accurately predict the behavior of theproposed system Forces obtained from these modelscan be used to design truss members using the strut-and-tie method and in accordance to the existingbuilding code FEA can be used to accurately predictstress concentration at truss member connections

Based on the analytical and experimental investigationsseveral recommendations could be made to improve theproposed truss system

(1) Use chamfered edges and curved corners instead ofthe sharp ones to avoid stress concentrations and addnominal reinforcement to control cracking at theselocations

(2) Use adequate top flange and bottom flange confer-ment reinforcement to help anchoring diagonal andvertical reinforcement whichmust be fully developedto prevent pullout

(3) Avoid using foam block-outs due to the difficultyof gluing them to the form and stripping themUsing light gage steel pans or fiber glass panes ishighly recommended for efficient and economicalfabrication as they can have multiple reuses

(4) Diagonal bars should be tied together after beingplaced in the forms to accommodate the tolerancesespecially when bar lengths and bent diameters arenot exact This practice will allow the diagonalsto slide against each other Another suggestion forfabrication is to assemble all the reinforcements in astiff manner outside the form to precisely match theform dimensionsThen the assembly is tied togetherlifted by a crane and placed in the form in one step

(5) Self-consolidating concrete with high flowability(average spread of 800mm (28 in) plusmn 50mm (2 in))passing ability (nominal maximum size aggregate is10mm (38 in)) resistance to segregation (VSI notmore than 10) and low viscosity (119879

50lt 2 sec) is

required to simplify production and ensure properfilling of the complex truss form without mechanicalconsolidation

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The technical support of the PrecastPrestressed ConcreteInstitute (PCI) advisory committee members is acknowl-edged and the financial support of the PCI Daniel PJenny Fellowship is appreciated Assistance was provided by

Journal of Structures 13

Musa Alawneh as a scholarship partner Support was alsogiven by University of Nebraska-Lincoln (UNL) graduatesstudents and staff Kelvin Lein AfshinHatami Shaddi AssadMohamed El-Kady and Micheal Asaad

References

[1] PCI PCI Design Hnadbook PrecastPrestressed Concrete Insti-tute (PCI) Chicago Ill USA 2010

[2] PCI ldquoThe 50 most significant precast concrete projectsrdquoASCENT Magazine 2004

[3] P H Shah and H R May ldquoPrecast vierendeel trusses pro-vide unique structural facade for parking structurerdquo Pre-castPrestressed Concrete Institute Journal vol 22 no 4 pp 24ndash39 1977

[4] W T Carroll F W Beaufait and R H Bryan ldquoPrestressedconcrete trussesrdquo ACI Journal vol 75 no 8 pp 367ndash373 1978

[5] MDeSutter United States of America Patent 7275348 B2 2007[6] A Trygestad andM DeSutter ldquoBookmen stacks cobalt condos

use ER-POST for column-free spacerdquo PCI Journal vol 52 no3 pp 58ndash71 2007

[7] ASCESEI 7 Minimum Design Loads for Buildings and OtherStructures American Society of Civil Engineers (ASCE)Reston Va USA 2010

[8] ACI 318 Building Code Requirments for Structural Concrete(ACI 318-11) and Commentary American Concrete Institute(ACI) Farmington Hills Mich USA 2011

[9] ASTMA193 ldquoStandard specification for alloy-steel and stainlesssteel bolting for high temperature or high pressure service andother special purpose applicationsrdquo ASTM Standard A193M-14ASTM International West Conshohocken Pa USA 2014

[10] AISC Steel Construction Manual American Institute of SteelConstruction (AISC) Chicago Ill USA 14th edition 2012

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

Journal of Structures 11

the vertical threaded rod as a result of inadequate anchoragein the bottom flange

Figure 17 shows themeasured slippage of 178mm (07 in)diameter prestressing strands during loading This plot indi-cates that all the recorded readings are significantly less than025mm (001 in) which is the limit for initial slippageThe highest recorded value was even less than 0025mm(0001 in) which is the measurement accuracy of the usedLVDTs indicating no slippage up to failure load This meansthat 178mm (07 in) diameter prestressing strands were fullydeveloped within the 45m (15 ft) distance (ie half of thespecimen length) which is the predicted development lengthusing ACI 318-11 It should be noted that the high valuerecorded by the south LVDT at the failure load is incorrectdue to the sudden movement of the specimen at the momentof failure

Figure 18 plots the measured strain in four vertical mem-bers of the truss specimen versus the applied load Theseverticals are 38mm (15 in) diameter threaded rods that havea yield strength of 724MPa (105 ksi) and ultimate strengthof 862MPa (125 ksi)Themaximummeasured strain reached26 (at the south rod 1 where the failure occurred) Also allmeasured strains in the four threaded rods had significantlyexceeded the yield strain of 036 Figure 19 plots the forcesin the four threaded rods versus the applied load This plotindicates that the forces in all the four threaded rods reachedthe yield force of 689 kN (155 kip) which is 14 more thanthe predicted design force of 605 kN (136 kip)

Figure 20 plots the measured strains in the four rein-forced concrete diagonal members of the truss specimenversus the applied loadThis plot indicates that the measuredstrains varied significantly among the four diagonalmembersdue to the variation in their angles and concrete dimensions(eg the width of the south diagonal 1 was 165mm (65 in)while the width of the north diagonal 2 was 280mm(11 in)) However they were all much lower than the ultimatedesign strain of concrete (03) The maximum ultimatestrain reached 01 at the south diagonal 1 where failurehappened and the minimum ultimate strain reached 0045at the north diagonal 2 Also the straight line relationshipsin all diagonal members indicate their linear elastic behaviorup to failure loadTherefore the truss failure due to crushingof the diagonals is a low probability mode of failure Figure 21plots the forces in all four diagonal members versus theapplied loadThis plot indicates that the ultimate compressiveforce varied from 1179 kN (265 kip) at the north diagonal2 to 1446 kN (325 kip) at the south diagonal 1 wherefailure occurred The forces in all diagonal members exceptthe north diagonal 2 exceeded the design force of 1192 kN(268 kip) which indicates that the design of the diagonals wasadequate

4 Conclusions and Recommendations

This research aimed to develop a precast concrete truss sys-tem for roof applications that is lightweight and aestheticallypleasing can span up to 48m (160 ft) and can be fabricated

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025

Load

(kN

)

Slippage (mm)

North strand slippageSouth strand slippage

Figure 17 Strand slippage measurements (1 kip = 445 kN 1 in =254mm)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

00 05 10 15 20 25 30

Load

(kN

)

Strain in TR ()

N rod 1

N rod 2

S rod 1

S rod 2

Figure 18 Tensile strains in the four threaded rods (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

mdash 100 200 300 400 500 600 700 800

Load

(kN

)

Force in TR (kN)

N rod 1

N rod 2

S rod 1

S rod 2

Figure 19 Tension forces in the four threaded rods (black lineindicates yield force 1 kip = 445 kN)

12 Journal of Structures

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025 030

Load

(kN

)

Strain in diagonals ()

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 20 Compressive strains in the diagonal members (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 200 400 600 800 1000 1200 1400 1600

Load

(kN

)

Force in diagonals (kN)

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 21 Compressive forces in the diagonal members (black lineindicates yield force 1 kip = 445 kN)

using standard precast practices in the USA Based on thepresented work three main conclusions can be made

(1) Fabrication of the proposed truss system is practicaland efficient The proposed method of fabricationwas evaluated experimentally by producing a 9m(30 ft) long truss specimen using economical andcommercially available components standard bridgegirder form foam block-outs vertical threaded rodsand conventional reinforcement The success of theproposed method is also attributed to the use of highstrength self-consolidating concrete (SCC) that fillsthe complex form encapsulates the reinforcementand provides a smooth finished surface without anymechanical consolidation

(2) Structural testing of the truss specimen that wasdesigned and detailed to resist the forces generatedin an example building indicated the adequacy ofthe proposed design method and connection details

Few recommendations were made as shown belowto further improve the performance of the proposedsystem

(3) The 2D frame models and 3D finite element modelscan be used to accurately predict the behavior of theproposed system Forces obtained from these modelscan be used to design truss members using the strut-and-tie method and in accordance to the existingbuilding code FEA can be used to accurately predictstress concentration at truss member connections

Based on the analytical and experimental investigationsseveral recommendations could be made to improve theproposed truss system

(1) Use chamfered edges and curved corners instead ofthe sharp ones to avoid stress concentrations and addnominal reinforcement to control cracking at theselocations

(2) Use adequate top flange and bottom flange confer-ment reinforcement to help anchoring diagonal andvertical reinforcement whichmust be fully developedto prevent pullout

(3) Avoid using foam block-outs due to the difficultyof gluing them to the form and stripping themUsing light gage steel pans or fiber glass panes ishighly recommended for efficient and economicalfabrication as they can have multiple reuses

(4) Diagonal bars should be tied together after beingplaced in the forms to accommodate the tolerancesespecially when bar lengths and bent diameters arenot exact This practice will allow the diagonalsto slide against each other Another suggestion forfabrication is to assemble all the reinforcements in astiff manner outside the form to precisely match theform dimensionsThen the assembly is tied togetherlifted by a crane and placed in the form in one step

(5) Self-consolidating concrete with high flowability(average spread of 800mm (28 in) plusmn 50mm (2 in))passing ability (nominal maximum size aggregate is10mm (38 in)) resistance to segregation (VSI notmore than 10) and low viscosity (119879

50lt 2 sec) is

required to simplify production and ensure properfilling of the complex truss form without mechanicalconsolidation

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The technical support of the PrecastPrestressed ConcreteInstitute (PCI) advisory committee members is acknowl-edged and the financial support of the PCI Daniel PJenny Fellowship is appreciated Assistance was provided by

Journal of Structures 13

Musa Alawneh as a scholarship partner Support was alsogiven by University of Nebraska-Lincoln (UNL) graduatesstudents and staff Kelvin Lein AfshinHatami Shaddi AssadMohamed El-Kady and Micheal Asaad

References

[1] PCI PCI Design Hnadbook PrecastPrestressed Concrete Insti-tute (PCI) Chicago Ill USA 2010

[2] PCI ldquoThe 50 most significant precast concrete projectsrdquoASCENT Magazine 2004

[3] P H Shah and H R May ldquoPrecast vierendeel trusses pro-vide unique structural facade for parking structurerdquo Pre-castPrestressed Concrete Institute Journal vol 22 no 4 pp 24ndash39 1977

[4] W T Carroll F W Beaufait and R H Bryan ldquoPrestressedconcrete trussesrdquo ACI Journal vol 75 no 8 pp 367ndash373 1978

[5] MDeSutter United States of America Patent 7275348 B2 2007[6] A Trygestad andM DeSutter ldquoBookmen stacks cobalt condos

use ER-POST for column-free spacerdquo PCI Journal vol 52 no3 pp 58ndash71 2007

[7] ASCESEI 7 Minimum Design Loads for Buildings and OtherStructures American Society of Civil Engineers (ASCE)Reston Va USA 2010

[8] ACI 318 Building Code Requirments for Structural Concrete(ACI 318-11) and Commentary American Concrete Institute(ACI) Farmington Hills Mich USA 2011

[9] ASTMA193 ldquoStandard specification for alloy-steel and stainlesssteel bolting for high temperature or high pressure service andother special purpose applicationsrdquo ASTM Standard A193M-14ASTM International West Conshohocken Pa USA 2014

[10] AISC Steel Construction Manual American Institute of SteelConstruction (AISC) Chicago Ill USA 14th edition 2012

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

12 Journal of Structures

0

200

400

600

800

1000

1200

1400

1600

1800

2000

000 005 010 015 020 025 030

Load

(kN

)

Strain in diagonals ()

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 20 Compressive strains in the diagonal members (1 kip =445 kN)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 200 400 600 800 1000 1200 1400 1600

Load

(kN

)

Force in diagonals (kN)

N diagonal 1N diagonal 2

S diagonal 1S diagonal 2

Figure 21 Compressive forces in the diagonal members (black lineindicates yield force 1 kip = 445 kN)

using standard precast practices in the USA Based on thepresented work three main conclusions can be made

(1) Fabrication of the proposed truss system is practicaland efficient The proposed method of fabricationwas evaluated experimentally by producing a 9m(30 ft) long truss specimen using economical andcommercially available components standard bridgegirder form foam block-outs vertical threaded rodsand conventional reinforcement The success of theproposed method is also attributed to the use of highstrength self-consolidating concrete (SCC) that fillsthe complex form encapsulates the reinforcementand provides a smooth finished surface without anymechanical consolidation

(2) Structural testing of the truss specimen that wasdesigned and detailed to resist the forces generatedin an example building indicated the adequacy ofthe proposed design method and connection details

Few recommendations were made as shown belowto further improve the performance of the proposedsystem

(3) The 2D frame models and 3D finite element modelscan be used to accurately predict the behavior of theproposed system Forces obtained from these modelscan be used to design truss members using the strut-and-tie method and in accordance to the existingbuilding code FEA can be used to accurately predictstress concentration at truss member connections

Based on the analytical and experimental investigationsseveral recommendations could be made to improve theproposed truss system

(1) Use chamfered edges and curved corners instead ofthe sharp ones to avoid stress concentrations and addnominal reinforcement to control cracking at theselocations

(2) Use adequate top flange and bottom flange confer-ment reinforcement to help anchoring diagonal andvertical reinforcement whichmust be fully developedto prevent pullout

(3) Avoid using foam block-outs due to the difficultyof gluing them to the form and stripping themUsing light gage steel pans or fiber glass panes ishighly recommended for efficient and economicalfabrication as they can have multiple reuses

(4) Diagonal bars should be tied together after beingplaced in the forms to accommodate the tolerancesespecially when bar lengths and bent diameters arenot exact This practice will allow the diagonalsto slide against each other Another suggestion forfabrication is to assemble all the reinforcements in astiff manner outside the form to precisely match theform dimensionsThen the assembly is tied togetherlifted by a crane and placed in the form in one step

(5) Self-consolidating concrete with high flowability(average spread of 800mm (28 in) plusmn 50mm (2 in))passing ability (nominal maximum size aggregate is10mm (38 in)) resistance to segregation (VSI notmore than 10) and low viscosity (119879

50lt 2 sec) is

required to simplify production and ensure properfilling of the complex truss form without mechanicalconsolidation

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The technical support of the PrecastPrestressed ConcreteInstitute (PCI) advisory committee members is acknowl-edged and the financial support of the PCI Daniel PJenny Fellowship is appreciated Assistance was provided by

Journal of Structures 13

Musa Alawneh as a scholarship partner Support was alsogiven by University of Nebraska-Lincoln (UNL) graduatesstudents and staff Kelvin Lein AfshinHatami Shaddi AssadMohamed El-Kady and Micheal Asaad

References

[1] PCI PCI Design Hnadbook PrecastPrestressed Concrete Insti-tute (PCI) Chicago Ill USA 2010

[2] PCI ldquoThe 50 most significant precast concrete projectsrdquoASCENT Magazine 2004

[3] P H Shah and H R May ldquoPrecast vierendeel trusses pro-vide unique structural facade for parking structurerdquo Pre-castPrestressed Concrete Institute Journal vol 22 no 4 pp 24ndash39 1977

[4] W T Carroll F W Beaufait and R H Bryan ldquoPrestressedconcrete trussesrdquo ACI Journal vol 75 no 8 pp 367ndash373 1978

[5] MDeSutter United States of America Patent 7275348 B2 2007[6] A Trygestad andM DeSutter ldquoBookmen stacks cobalt condos

use ER-POST for column-free spacerdquo PCI Journal vol 52 no3 pp 58ndash71 2007

[7] ASCESEI 7 Minimum Design Loads for Buildings and OtherStructures American Society of Civil Engineers (ASCE)Reston Va USA 2010

[8] ACI 318 Building Code Requirments for Structural Concrete(ACI 318-11) and Commentary American Concrete Institute(ACI) Farmington Hills Mich USA 2011

[9] ASTMA193 ldquoStandard specification for alloy-steel and stainlesssteel bolting for high temperature or high pressure service andother special purpose applicationsrdquo ASTM Standard A193M-14ASTM International West Conshohocken Pa USA 2014

[10] AISC Steel Construction Manual American Institute of SteelConstruction (AISC) Chicago Ill USA 14th edition 2012

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of

Journal of Structures 13

Musa Alawneh as a scholarship partner Support was alsogiven by University of Nebraska-Lincoln (UNL) graduatesstudents and staff Kelvin Lein AfshinHatami Shaddi AssadMohamed El-Kady and Micheal Asaad

References

[1] PCI PCI Design Hnadbook PrecastPrestressed Concrete Insti-tute (PCI) Chicago Ill USA 2010

[2] PCI ldquoThe 50 most significant precast concrete projectsrdquoASCENT Magazine 2004

[3] P H Shah and H R May ldquoPrecast vierendeel trusses pro-vide unique structural facade for parking structurerdquo Pre-castPrestressed Concrete Institute Journal vol 22 no 4 pp 24ndash39 1977

[4] W T Carroll F W Beaufait and R H Bryan ldquoPrestressedconcrete trussesrdquo ACI Journal vol 75 no 8 pp 367ndash373 1978

[5] MDeSutter United States of America Patent 7275348 B2 2007[6] A Trygestad andM DeSutter ldquoBookmen stacks cobalt condos

use ER-POST for column-free spacerdquo PCI Journal vol 52 no3 pp 58ndash71 2007

[7] ASCESEI 7 Minimum Design Loads for Buildings and OtherStructures American Society of Civil Engineers (ASCE)Reston Va USA 2010

[8] ACI 318 Building Code Requirments for Structural Concrete(ACI 318-11) and Commentary American Concrete Institute(ACI) Farmington Hills Mich USA 2011

[9] ASTMA193 ldquoStandard specification for alloy-steel and stainlesssteel bolting for high temperature or high pressure service andother special purpose applicationsrdquo ASTM Standard A193M-14ASTM International West Conshohocken Pa USA 2014

[10] AISC Steel Construction Manual American Institute of SteelConstruction (AISC) Chicago Ill USA 14th edition 2012

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International Journal of

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Active and Passive Electronic Components

Control Scienceand Engineering

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

RotatingMachinery

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Shock and Vibration

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Civil EngineeringAdvances in

Acoustics and VibrationAdvances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation httpwwwhindawicom

Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Chemical EngineeringInternational Journal of Antennas and

Propagation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Navigation and Observation

International Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

DistributedSensor Networks

International Journal of