pci journal - 1983 - sep-oct

President's Message

PCI Committee WorkStrength and Commitment

This year I have been fortunate to attend Committee Days in addition to beingpresent at numerous other PCI functions. This involvement has given me newinsight into the wide scope of activities performed by PC] members throughout thecountry.

One group, our Professional members, has continued to perform diligently andexpertly on behalf of our industry with their activities coordinated by ourHeadquarters staff.

Producer and Associate members have also continued their dedicated efforts onPCI committees which have made significant progress in helping our industry togrow and prosper.

PCI's Plant Certification Committee is taking giant strides in improving qualitycontrol among our Producer members. The Marketing Committee, working with thelargest budget ever, continues to provide excellent promotional leadership for PCI.All of our committees--Architectural Precast Concrete Committees, BoardCommittees, Program Planning Committee, Producer Activities Committees,Structural Prestressed Concrete Committees and Technical Activities Committee,along with their subcommitteeshave continued to expand the body of knowledgeand service which make them so important to our organization and our industry.

We cannot, however, rest on our laurels. There is still much work left to be done. Ifyou or one of your colleagues has some special expertise that would be of value toour committee work, by all means please contact PCI Headquarters.

Committee work is rewarding in itself and awakens all participants to new ideasand the professionalism of our national headquarters staff. PCI is indeed fortunate tohave individuals of such high caliber working daily for our organization and ourindustry.

Hard-working, dedicated PCI members on all our various committees are thestrong "right hand" of any PCI president. To all of you, "Thanks!"

PCI JOURNALL'September-October 195317

T welve outstanding projects have beennamed winners in the 1983 PCIAwards Program, and another was recog-nized with a special jury award.

Five prominent architects and en-gineers selected the winners on the basisof exceptional achievement in aestheticexpression, function, and economy, aswell as ingenuity in the use of materials,methods, and equipment.

The PCI Awards Program has beenheld each year since 1963 to recognizeexcellence in design using precast pre-stressed concrete andlor architecturalprecast concrete. Because the designproblems faced by engineers and arch-itects are so diverse, no single first placeaward is given. Each winning project re-ceives equal recognition for excellence;the special jury award is given to projectswhich display an '`honorable mention"level of achievement.

Jury chairman this year was RobertBroshar, FAIA, president of the AmericanInstitute of Architects and principal, Thor-son-Brom-Broshar-Snyder, Architects.

Other members of the jury were:Thomas H. Beeby, AIA, partner, Ham-mond, Beeby & Bapka, Inc.; Walter Po-dolny, Jr., Bridge Division, U.S. Depart-ment of Transportation, Federal HighwayAdministration; Macy DuBois, president,Royal Architectural Institute of Canadaand principal, DuBois Plumb & As-sociates; and John H. Wiedeman, presi-dent, American Society of Civil Engineersand principal, Wiedeman and Singleton,Inc.

.-' , _--tea Counterclockwise from top left: JohnWiedeman and Walter Podolny, Jr., MacyDuBois, Robert Broshar (jury chairman);and Thomas H. Beeby.

r'Cl Awards Program Winr.

Dauphin Island Bridge

Mobile County, Alabama

Engineer: Figg and Muller Engineers, Inc., Tallahassee, Florida.General Contractor. Brown and Root, Inc., Mandeville, Louisiana.Owner: State of Alabama Highway Department, Montgomery, Alabama.

This 17,814 ft (5430 m) long prestressed concrete bridge replaced the original bridgedestroyed by a hurricane in September, 1979. The bridge combines 13,924 ft (4240m) of precast prestressed concrete full-deck trestle spans with high-level precastsegmental concrete bridge spans. The segmental portion is composed of twenty-six118 ft (36 m) approach spans (using 160 uniform depth box girder segments) and athree-span 822 ft (250 m) unit (employing 92 variable depth segments). Construction ofthe approaches was by the span-by-span method on temporary trusses and of the mainspan and two side spans by the balanced cantilever method.

The following major features were incorporated in the structure: The 400 ft (120 m) main navigational span is of variable depth and one of the

longest precast segmental bridge spans in the world. The 822 ft (250 m) main span unit is epoxy jointed. Traffic rides on the as-cast

surfaces in both main and approach spans. External post-tensioning tendons inside the box girder segments were used for

the high level approaches without employing epoxy in the match-cast joints. In the approach spans, up to 90 ft (27 m) high precast hollow box piers were

match-cast and post-tensioned vertically.

Jury comment: "We were impressed with the execution of the long precast segmentsand the innovative use of twin piers to account for thermal movements in thestructure."

PCI JOURNAL/September-October 1983 19

Mu omah County Maintenanceand Operations FacilityPortland, OregonArchitect: Zimmer Gunsul Frasca Partnership, Portland, Oregon.Structural Engineer. kpff consulting engineers, Portland, Oregon.General Contractor: Donald M. Drake Co., Portland, Oregon.Owner. Multnomah County, Portland, Oregon.Precast prestressed concrete: Morse Bros. Prestress, Inc., Clackamas, Oregon.

T he inherent durability and strength of prestressed concrete allowed the county tobury a 186,000 sq ft (17280 m2) vehicle maintenance and operations facility 30 11 (9m) in the ground and begin the massive reclamation project. This solution alsominimized the visual impact in a neighborhood of existing and proposed residential andopen space uses.

Unique double tee wall panels, weighing up to 65,000 lb (29 t), provided the stabilityneeded in this substantially below-grade facility. The 44 ft (13 m) high panels weredesigned for 42 lb per cu ft (680 kg/m3) equivalent fluid pressure, as required by thesoils consultants specifications.

Double tees used in the second floor accommodate a specified live load of HS20highway loading or 250 psf (12 kPa). Hollow-core slabs were used in the roof system.Solar panels on the south side of the roof provide almost all the facilitys air conditioningneeds and about half its heating and hot water requirements. Further details of thisproject appear in the September-October 1981 PCI JOURNAL, pp. 66-81.

Jury comment: "An interesting and convincing solution to a difficult challenge.Integration with the site is exceptional. The structural system is well-executed andproves the value of total prestressed concrete structures. "

20

food Canal Floating BridgeJefferson and Kitsap Counties, WashingtonArchitectlEngineer: Parsons Brickerhoff Quade & Douglas/Raymond Technical Facilities, San

Francisco, California.General Contractor. J. A. Jones Construction Co., Charlotte, North Carolina.Owner: State of Washington.Prestressed concrete: Associated Sand and Gravel Co., Inc. (A subsidiary of Hydro Conduit

Corp.), Everett, Washington.

M assive precast prestressed concrete floating pontoons support a precastprestressed I-girder superstructure on the only floating bridge in the world to spana tidal waterway. After the western half of the original structure was destroyed in anextremely severe storm in 1979, the bridge design was redeveloped for greaterstrength and durability under extraordinary environmental conditions.

Twelve pontoons, each 360 ft (110 m) long, 60 ft (18 m) wide and 18 ft (5.5 m) deep,were fabricated in a graving dock and towed 55 miles (88 km) to the site. The pontoonsare tied to 24 precast concrete bucket-shaped anchors, weighing 1500 tons (1361 t)each, with 40 tons (36 t) of 3-in. (76 mm) cable.

The new portion of the bridge spans 3775 ft (1151 m), and is 10 ft (3 m) wider, 4 ft(1.2 m) deeper and 50 percent stronger than its predecessor.

Jury comment: "A novel application of precast concrete as a structural solution indeveloping the floating pontoons. The design is useful in transporting the elements aswell as in function."

Eric Harvie Bridge

Calgary, Alberta

Engineer: Simpson Lester Goodrich Engineering Partnership, Calgary, Alberta.Architect. Graham McCourt Architects, Calgary, Alberta.General Contractor. Genstar Structures Limited, Calgary, Alberta.Owner: City of Calgary, Alberta.Precast prestressed concrete: Genstar Structures Limited, Calgary, Alberta.

A combination of precast pretensioned components and cable-stayed techniquesmade possible this very elegant pedestrian bridge. The structure is located inCar-bum Park, an environmentally sensitive area in the City of Calgary, Alberta,Canada.

The bridge has a central span of 262 ft 6 in. (80 m) and two side spans of 65 ft 7 in.(20 m). The deck is composed of five precast pretensioned T-shaped box girdershaving a total deck width of 10 ft 4 in. (3.2 m) and a depth of 3 ft 7 in. (1.1 m).

The superstructure is supported by two towers resting on cast-in-place reinforcedconcrete piers. The upper portion of the tower is made up of four 55 ft (17 m) hightapering precast reinforced square columns.

Precast concrete provides clean, sharp lines, accentuating the basic structural form.Excellent camber control creates a smooth bridge profile; the grouping of four columnsto form the towers provides a strong sense of support; and the precast concretefurnishes a low-maintenance structure which was desirable to the client. Further detailsof this project appear in the July-August 1983 PCI JOURNAL, pp. 154-159.

Jury comment: "The cable-stayed, precast prestressed design presents a cheerfuland spirited solution for a crossing in a romantic river park setting.

22

Number 5 Newsprint Machine BuildingBritish Columbia, Canada

Designer/Structural Engineer: Swan Wooster Engineering Co.. Ltd., Vancouver, British Columbia.General Contractor. Commonwealth Construction Co., Ltd., Burnaby, British Columbia.Owner: Crown Zellerbach Canada Ltd., Vancouver, British Columbia.Precast prestressed concrete: Genstar Structures Ltd., Richmond. British Columbia.

E ngineering, production, and erection of this 85,700 sq ft (7960 m2) newsprint papermill took just 31 weeks. Prestressed and precast concrete components wereselected to achieve the tight construction schedule while producing a finished structurethat would withstand high temperature and humidity resulting from operations within thebuilding and providing tough fire-resistance ratings of 4 hours in critical areas.

Transverse stability was provided by combining precast units with post-tensionedmoment connections. In addition to gravity and lateral loads, the precast structure wasalso designed to support one 54-ton (49 t) reel crane and two 82-ton (74 t) overheadcranes.

Openings in the root structure I-beams spanning 92 ft (28 m) contributed to theaesthetic quality of the roof, reduced beam weights, and provided a convenience forrouting the ceiling air circulation ducts.

Jury comment: "Simplicity gives it monumentality. Design and construction are wellexecuted. "


Vail Lionshead Parking CenterVail, Colorado

Architect: Robbins & Ream, Inc., San Francisco, California.Structural Engineer: KKBNA, Wheat Ridge, Colorado.General Contractor: Hyder Construction Co., Denver, Colorado.Owner: Town of Vail, Vail, Colorado.Precast prestressed concrete: Stanley Structures, Denver, Colorado.

-s

Constructing a parking facility for 1135 cars while retaining the village-like characterof this winter sports community was the challenge faced by the architects. Apartially underground, totally precast prestressed concrete structure combined withlandscaped berms and plaza areas proved to be the ideal solution.

Plant manufacture provided strict quality control of alignment, color and texture of theexposed aggregate, architectural precast concrete panels. Variations and irregularitiesin concrete quality often found in concrete cast on-site in cold weather are virtuallyeliminated.

Circular cut-outs in the central bearing walls allow views and cross-ventilation, whileaiding security and providing a feeling of openness inside the structure.

Jury comment: "The architect should be commended for successfully blending thisfacility into the landscape. The circular cut-outs are delightful. The entire structuremaintains a sense of lightness and playfulness. "

24

Blue Cross and Blue Shield of TexasHeadquarters BuildingRichardson, Texas

Architect: Omniplan Architects, Dallas, Texas.Engineer: Datum Structures Engineering, Inc., Dallas, Texas.General Contractor: J. W. Bateson Co., Inc., Dallas, Texas.Owner: Blue Cross and Blue Shield of Texas, Richardson, Texas.Precast prestressed concrete: TXI Structural Products, Inc., Dallas, Texas.

M assive, 60-ton (54 t) precast prestressed concrete bents were used on the BlueCross and Blue Shield of Texas Regional Headquarters Building to resist bothlateral and gravity loads as welt as to create the architecural expression of the building.

The inverted U-shaped bents reduced the number of elements needed to constructthe building, eliminated joint lines, and helped solve the normal problems involved increating a moment-resisting frame.

On-site erection costs as well as interim financing were reduced through themonolithic casting of the beam and column units.

According to the architect, the plant-manufactured components produced high qualityfinishes and detailing which could not be matched by site-cast concrete, even understrict controls.

A column-free interior was provided using long span double tees in the floor and roofsystems.

Jury comment: "The structure is the building_ We were impressed with the use of thestructural bents which became the buildings architecture. A well-done, honestexpression. "

* t I If iiit

PCI JOURNAUSeptember-October 1983

25

1-205 Columbia River Bridge (NorthChannel) Between Portland, Oregon andVancouver, WashingtonEngineer: Sverdrup & Parcel and Associates, Inc., Bellevue, Washington.Value-Engineering Redesign: T. Y. Lin International, San Francisco, California.General Contractor: S. J. Groves & Sons Co., Minneapolis, Minnesota, and Guy F. Atkinson

Construction Co., South San Francisco, California.Owner: Oregon and Washington Departments of Transportation.

in cantilever prestressed segmental box girder spans, built with a combination ofprecast and cast-in-place construction, make up the superstructure of the 1-205

Columbia River Bridge over the North Channel.This majestic structure, actually two parallel bridges side-by-side, carries interstate

highway traffic over a navigable river between two states, thereby providing a by-passof congested areas for regional traffic and improving access to a nearby airport.

Each structure of the precast portion of the North Channel section required 296precast two-cell box girder segments which varied in size. The segments werematch-cast using the short-line method of production and barged to the job site.Post-tensioning was required transversely (in both the deck and cross-girders) andlongitudina^ly.

Segment geometry had to provide for curvature of both horizontal and verticalalignments. In addition, segment depths tapered from 17 to 12 ft (5.2 to 3.7 m) as spanlengths shortened to complement the descending roadway. Further details of thisproject appear in the March-April 1982 PCI JOURNAL, pp. 56-77.

Jury comment: "A graceful solution to a monumental crossing incorporating bothprecast and cast-in-place segmental construction.

Lake Washington'ocational Technical Institute

Kirkland, Washington

Architect: Cummings Schlatter Associates, Kirkland, Washington.Structural Engineer: Andersen Bjornstad Kane Jacobs, Inc., Seattle, Washington.Owner: Lake Washington School District 414, Kirkland, Washington.Precast concrete: Concrete Technology Corp., Tacoma, Washington.

^^R 11^^ ^Ilil^l 1Ni[^IA ui^. ^'

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Repetitive use of standard precast prestressed concrete structural components inthis vocational technical institute resulted in savings in construction time and bothinitial and operational costs.

The prestressed concrete framing also allowed longer spans with greater rigidity andreduced overall structural depth. Open spaces, thus provided, are completely flexible tomeet changing classroom and shop requirements.

Use of prestressed concrete reduces the building's life cycle costs in a number ofways. Fire resistant precast concrete components contribute to lower insurancepremiums and require virtually no maintenance. Mass of the prestressed concretebeams and hollow-core slabs cause them to act as passive solar collectors to helpreduce heating costs.

Jury comment: "The use of precast concrete as the weathering surface, structuralsystem and interior skin works extremely well. The issue of systemization is dealt within a very positive way. "

PCI JOURNAL,September-October 1983 27

The Westin HotelBoston,

Massachusetts

Architect: The Architects Collaborative, Inc., Cambridge, Massachusetts.Engineer: Lev Zetlin Associates, Inc.. New York, New York.General Contractor: Turner Construction Co., Boston, Massachusetts.Owner: Westin Hotels, Seattle, Washington, and Urban Investment and Development Co., Boston,

Massachusetts.Architectural precast concrete: Lone Star/San-Vel Concrete Corp., Littleton, Massachusetts.

T he Westin Hotel in Boston had to meet the owner's requirements of economy andrapid construction, while remaining compatible with the historic masonry buildingssurrounding it.

Cladding the 36-story luxury hotel with 1374 architectural precast concrete panelswas the solution to both challenges. The architects used rustication panels to break upthe mass of the tower and express the basic 14 ft 6 in. (4.4 m) module. The scale of thehotel bays harmonizes with the bow fronts and bays of the surrounding 19th centurybrick residential architecture.

Jury comment: "The use of texture and the scale of the precast concrete wall panelswas done exceedingly well. The refinement of detailing and blending the hotel with itshistoric neighbors were impressive."

28

Linn Cove Viaduct

Linville, North Carolina

Engineer: Figg and Muller Engineers, Inc., Tallahassee, Florida.Foundation Design/Construction Administration: Eastern Direct Federal Division of the Federal

Highway Administration.General Contractor: Jasper Construction, Inc., Plymouth, Minnesota.Owner: National Park Service, Denver, Colorado.

T his 1234 11 (379 m) long, S-shaped viaduct is a post-tensioned precast segmentalconcrete bridge erected by progressive placement in one-directional cantilever. It islocated at Grandfather Mountain, Blue Ridge Parkway, an environmentally sensitivearea featuring rough, wooded terrain and extensive rock outcroppings.

Environmental restrictions dictated that the structure be built from the top withminimum disruption to the area. (It is believed that this is the first major bridge in theworld to be completely erected in such a manner.)

The bridge has eight spans with four at a maximum of about 180 ft (55 m). Some 153match-cast segments, no two of which are exctly alike, and 37 box pier segments wereused.

Horizontal alignment includes spiral curves going into circular curves with radii assmall as 250 11 (76 m). Superelevations on each curve are a full 10 percent.

Special winter erection techniques developed for this project will be important forother precast segmental bridges in North America. Further details of this project appearin the September-October 1982 PCI JOURNAL, pp. 120-130.

Jury comment: "A bold and imaginative engineering solution in response topreserving a beautiful environment."

St. Bernard CondominiumsTaos, New MexicoArchitect: Antoine Predock, FAIA, Albuquerque, New Mexico.Structural Engineer: Randy Holt & Associates, Inc., Albuquerque, New Mexico.General Contractor: Solar Homes, Inc., Albuquerque, New Mexico.Owner: Solar Ski Taos Limited, Albuquerque, New Mexico.Precast concrete: Stanley Structures, Inc., Albuquerque, New Mexico.

Speed of construction and thermal storage capacity made precast concrete thelogical building material for this condominium project located at the base of the skislopes of Taos Ski Valley.

Plant-manufactured precast prestressed concrete units were erected without delayduring the winter, and produced a building that is in harmony with surrounding cliffformations.

Extensive use of prestressed concrete helped to meet local fire code requirementsand provided the owners with lower insurance rates based on the inherent fireresistance of the structure.

Jury comment: "The massing retains the natural character of the mountain site. The

blending of detailing and continuity of the design are appealing.

30

Special Jury Award

Woodrow Wilson Memorial Bridge

On Interstate 95, south of Washington, D.C.

Engineer: Greiner Engineering Sciences, Inc., Baltimore, Maryland.General Contractor: Cianbro Corporation, Pittsfield, Maine.Owner: Federal Highway Administration (Maintained/operated by State Highway Administration of

Maryland, Virginia Department of Highways and Transportation, and the District of Columbia.)Precast concrete: Shockey Bros, Inc., Winchester, Virginia.

At least two lanes of traffic have been kept open at all times during rehabilitation ofthe decking on this bridge. Precast panels of lightweight concrete have theirdesign strength prior to placement and rapid setting polymer concrete is used for theclosure pours, allowing up to six lanes to be kept open during peak traffic hours.

Typical panels are 46 ft 7 1/2 in. (14.2 m) wide, 10 to 12 ft (30 to 3.7 m) long, and Bin.(203 mm) thick, with a 5 in. (127 mm) haunch at the exterior girder. The widened bridgedeck provides two roadways, each 44 ft (13,4 m) wide.

Transverse post-tensioning, performed at the precasting plant, provides addedstrength and allows the new deck to cantilever laterally from the existing steel stringersuperstructure, thus widening the bridge without supplementing the main structuralgirders. After placement, the panels are post-tensioned longitudinally to providelong-term durability and continuity under live loading. Polymer concrete bearing padson sliding steel plates prevent excessive strains on the supporting structure.

Jury comment: "A premier example of a new use for precast prestressed concreteand an innovative solution to the problem of maintaining traffic during rehabilitation."

Knowledge Gained FromInstrumentation of the

Kishwaukee River Bridge

K. Nam ShiuStructural EngineerStructural Experimental SectionConstruction Technology LaboratoriesPortland Cement AssociationSkokie, Illinois

Henry G. RussellDirector

Structural Development DepartmentConstruction Technology Laboratories

Portland Cement AssociationSkokie, Illinois

T he Kishwaukee River Bridge, shownin Fig. 1, consists of two long spanpost-tensioned concrete segmental boxgirder bridges. The bridges, made ofprecast segments, were erected by thebalanced cantilever method of con-struction using a launching truss sys-tern. This application of the launchingtruss for bridge erection was the first inthe United States.2

In the balanced cantilever method ofconstruction, bridge segments undergodifferent Stress conditions during andafter construction. Therefore, thebridge design has to include every con-struction event that results in signifi-cant stress changes. In addition, thetime-dependent effects of creep,shrinkage, and temperature must betaken into account.

Verification of design assumptionsusing measured data is needed and isessential to the successful prediction of

long-term bridge deformations. There-fore, an investigation to obtain fielddata on an existing bridge was made toverify analytical and design proceduresfor predicting time-dependent behaviorof post-tensioned box girder bridges.

The investigation comprised threeparts:

1. Field measurements on Kishwau-kee River Bridge.

2. Laboratory measurements of con-crete used in the bridge.

3. Comparisons between field mea-surements and calculated defor-mations.

Field measurements included lon-gitudinal concrete strains, vertical de-flections, and temperatures of threebridge segments. Initial readings weretaken as soon as possible after place-ment of concrete. Readings presented adetailed record of bridge behavior overa period of5 years.

32

0 1

Fig. 1. Kishwaukee River Bridge.

Synopsis

The objectives of the instrurnen-tation program were to measureand evaluate the behavior of a longspan box girder bridge. Threebridge segments were instru-mented. Field measurements weretaken from the beginning of bridgeconstruction for a total period of 5years. Properties of concrete usedin the instrumented segments weredetermined in the laboratory.

Using the measured concreteproperties and detailed constructionrecords, time-dependent deforma-

tions of the bridge segments werecalculated by a step-by-step numer-ical procedure. Calculated deforma-tions were compared with measureddata to verify the numerical proce-dure. Subsequently, the numericalprocedure was used to determineinformation that could not be ob-tained by field measurements.Findings with regard to axial defor-mations, prestress losses, momentredistribution, concrete stresses,and temperature effects are pre-sented.


Laboratory tests were made to deter-mine the properties of concrete used inthe bridge. Measured concrete proper-ties included variation of compressivestrength, modulus of elasticity, Pois-sons ratio, and coefficient of thermalexpansion with time. Measured time-dependent properties were creep andshrinkage of concrete. Information ob-tained from laboratory measurementswas used in the analytical studies.

A computer program using a step-by-step numerical procedure to determinetime-dependent behavior of box girderswas used. The computer program wasdeveloped at the University of Illinoisat Urbana by Marshall and Camble.3The actual construction schedule wasused in the analyses. Comparisons be-tween field measurements and calcu-lated results verified the numericalprocedure. The numerical procedurewas then used to determine informationthat could not be obtained from fieldmeasurements.

FINDINGS

The following findings were obtainedfrom the evaluation of time-dependentdeformations on the Kishwaukee RiverBridge:

1. Longitudinal deformations calcu-lated by a step-by-step numerical pro-cedure were in good agreement withmeasured values.

2. Based on temperature mea-surements taken on four randomly se-lected days, the maximum measuredvariation of air temperature betweenseasons was 60 F (33 C). Within a 24-hour period, a maximum temperaturedifference of 25 F (14 C) was measured.

3. Measured temperature distribu-tions across sections of the bridge werenonlinear. Temperature distributionprofiles varied with fluctuations of airtemperatures inside and outside thebox girder.

4. Maximum measured temperature

differentials between the top slab andbottom slab were 15 F (8 C) and 20 F(-11 Q.

5. Calculated prestress losses causedby concrete creep and shrinkage duringthe first 1850 days after erection wereless than values determined byAASHTO Specifications.;

6. Calculated prestress losses causedby relaxation of post-tensioning tendonswere greater than values recommendedby AASHTO Specifications.d

7. Five years after bridge comple-tion, redistributed moments from con-crete creep were calculated to be 14percent of support moments and 56percent of midspan moments. However,redistribution of moments is unlikely toresult in flexural tensile stresses in thesuperstructure.

S. Compressive concrete stresses atthe top and bottom of each instru-mented segment tended to equalize asa result of time-dependent deforma-tiOns.

RECOMMENDATIONS

1. Time-dependent behavior analysisof post-tensioned segmental concretebridges should be based on actual con-crete material properties. Best predic-tion of time-dependent behavior is ob-tained when data for concrete storedoutdoors are used. However, goodagreement is obtained when data forconcrete cured indoors are used.

2. Based on measured temperaturedifferentials between the top slab andbottom slab of the bridge, a designtemperature differential of 9 F (-5 C)may not be sufficient for midwesternstates. A maximum temperature differ-ential of 20 F (-11 C) was recordedduring four randomly selected days ofmeasurements.

3. Calculated prestress losses due toconcrete creep and shrinkage were lessthan those calculated using AASHTOSpecifications4 which do not take into

34

170 250` 250 250'

KISHWAUKEERIVER .

ELEVATION

INSTRUMENTEDSPAN

u y/// SO^JTH BOUND I LANES _ iiSOUTH NORTHABUTMENT ABUTMENT

IIINORTH IBOUND ILANES

PLAN

Fig. 2. Elevation and plan of Kishwaukee River Bridge (Note: 1 ft = 0.305 m).

account concrete age at erection andmember thickness. Further investiga-tion of creep and shrinkage effects onprestress losses is recommended.

4. Estimation of prestress losses forthe relaxation of post-tensioning ten-dons according to AASHTO Specifica-tions should be further investigated.Current specifications seem to under-estimate prestress losses due to relaxa-tion in post-tensioning tendons.

5. Moment redistribution due tctime-dependent effects should he ac-counted for in design of long span con-tinuous bridges. An increase of positivemoments by as much as 56 percent atmidspan was predicted in the analysisof the Kishwaukee River Bridge.

FIELD MEASUREMENTS

The Kishwaukee River Bridges con-sists of two identical continuous singlecell box girders made with precast con-crete segments. Each girder has threemain spans of 250 ft (76.2 m) and twoapproach spans of 170 ft (51.8 m) for atotal length of 1090 ft (332 m). Eleva-

tion and plan views of the bridge areshown in Fig. 2.

Each span was constructed by canti-levering segments out from both sidesof the main pier. A 150-ton (136 t)launching truss was used to facilitatepositioning and erection of each seg-ment Fig. 3 shows the positioning of abridge segment before temporarypost-tensioning. At the completion ofconstruction, spans were made con-tinuous for live loads.

Each main span consisted of 34 pre-cast segments with one cast-in-placeclosure segment at midspan. Longitu-dinal post-tensioning was provided inthe top and bottom slab of the box sec-tion. The top slab was also prestressedtransversely. Straight threaded 1y-in.(32 mm) diameter bars were used aslongitudinal tendons. Other details ofconstruction have been reported previ-ously.

Three segments in one 250-ft (76.2 m)span of the south bound lane bridgewere selected for instrumentation asshown in Fig. 2. Locations of the se-lected segments are identified in Fig. 4


as SB1-N1, SB1-N9, and SB1-N16.These segments were located next tothe pier support, at quarter span, andnear midspan. A detailed description ofthe field instrumentation is given else-where.5

Field measurements included read-ings of longitudinal concrete strains,vertical deflections, air temperatures,and concrete surface temperatures.

Fig. 3. Launching truss facilitates erectionand positioning of bridge segment prior totemporary post-tensioning.

Longitudinal StrainsLongitudinal strains were measured

in three instrumented segments. Loca-tions of strain measurements are shownin Fig. 5. Measurements were madewith a Whittemore mechanical straingage and 24 Carlson strain meters. TheWhittemore strain gage measured lon-gitudinal surface deformations betweenfixed reference points. Reference pointswere glued to the inside concrete sur-face of the box segment soon aftersteam curing.

The Carlson strain meters measuredinternal concrete strains and tempera-.tures. Meters were installed by tying tothe reinforcement cage, shown in Fig.6, before casting. All Carlson meterswere embedded in concrete, To have adetailed account of the bridge behavior,longitudinal strain readings were takenbefore and after every known significantevent that changed the strain historiesof bridge segments.

The strain history of Segment SB1-Nlis shown in Fig. 7. It is noted that thelarge increase in compressive strain atabout 100 days corresponds to erectionof the bridge segment. A similar longi-tudinal strain history pattern was ob-served in the other two instrumentedsegments.

It is also noted that seasonal strainvariations of about 50 millionths can beobserved in Fig. 7 even though all re-corded strains had been corrected fortemperature to 73 F (23 C). The correc-

SBI-NI S8I-N9 S8I-NF6

p ier Closure

125

Fig. 4. Locations of instrumented segments (Note: 1 ft = 0.305 m).

36

20 8t"

X = WHITTEMORE STRAIN GAGEMEASUREMENT

= CARLSON STRAIN METER

A= SURFACE TEMP. MEASUREMENT

Fig. 5. Locations of longitudinal strain measurements in bridge segment.Measurements were made with a Whittemore mechanical strain gage and 24 Carlsonstrain meters (Note: 1 ft = 0.305 m).

StrainMeter

W s,

f: 2

Fig. 6. Installation of Carlson strain meter by tying to reinforcement cage. Meters weresubsequently embedded in concrete.

PCI JOURNAL./September-October 1983 37

w E4 5 6

3 7

2 1 8

1000

500

00

10000

E

500mU)

ma^

E.0

00 I^

78 1979 1980 1 1981 J 9 82 1983

5

3

2

4

1

00 500 1000

Time , days

Fig. 7. Carlson strain data for Segment SB1-N1.

15002000

38

Air

Temperature , 60of

40

20

0400 pm12:00 pm 8!00 am4:00pm

Time

Fig. 8. Air temperature variation over 24 hours for different seasons (Note: 1F = 0.56C).

tion was based on the assumption thatconcrete and strain gages expand andcontract freely according to the linearcoefficient of thermal expansion. Theseasonal strain fluctuation was espe-cially obvious for longitudinal strainsmeasured in the top slab of the boxgirder. This phenomenon was attrib-uted to effects of varying outdoor tem-peratures and relative humidities at thebridge site.

Both Carlson strain meter readingsand the Whittemore gage readingsyielded consistent and similar results.In this paper, only data from the Carl-son strain meters are discussed.

Temperature

Temperature measurements includedair temperatures, concrete surface tem-peratures, and internal concrete tem-peratures. Air temperatures were mea-sured by a thermometer. Readings in-cluded air temperature inside and out-side the box section. Surface tempera-tures were measured by a portable sur-face thermocouple. Internal concrete

temperatures were calculated fromCarlson strain readings. Locations oftemperature measurements are given inFig. 5. From temperature mea-surements, temperature gradientsthrough the top slab of the box sectionwere obtained. Relative humidities in-side and outside the box girder werealso recorded.

In addition to regular field mea-surements, temperature readings weretaken in four 24-hour periods. Each24-hour period represented one season.Variations of measured air temperaturesat the bridge over the four 24-hour pe-riods are plotted in Fig. 8. From thecollected data, the maximum seasonaltemperature variation of 60 F (33 C)was observed. Within a 24-hour period,the maximum recorded temperaturedifference was 25 F (14 C).

Measured temperature distributionsacross bridge sections were nonlinear.Temperature profiles of the bridge insummer and winter of 1979 are shownin Fig. 9. Maximum measured temper-ature differentials between the top slaband bottom slab were 15 F (8 C) and

PCI JOURNAUSeptember-October 1983 39

-86F

g^-92F 75F

80F 73 F

Air Cell

77F 73FI8 - 77F - 76F

f Ti--76F 63Fz

4=f5 pm 5:00 am

(a) Summer of 1979

I 3 23F 18F

9 1 > :: 20F --21F

Ig F 20F

Air Cell

+ --- 18F 20F18 19F 20F

T g 25F 21 F

6 10 pm 6: 00 am

(b) Winter of 1979

Fig. 9. Temperature distribution across Segment SRI-N1(Note: i ft = 0.305 m; 1F = 0.56C).

II'-4"

II-4

40

Indoors--' Outdoors

1.00

0.75

0.50

0.25

0

Specific 075

Creep ,millionths /psi 0.50

0.25

0

0.75

0.50

0.25

0

Segment SBI NI ace of Lood,ng days

28rr^^^^r180

360

Segment SBI N9

28

28f 1 90

180

I

Segment SBI N 16

28-^'^^ 90

IBfl

0 500 1000 1500 2000

Time , days

Fig. 10, Specific creep versus time (Note: 1 millionth/psi = 145 millionths/Nlmm2).

20F (-11 C). The design differ-entials were 18 F (10 C) and 9 F(-5 C). Thus, the negative measuredtemperature differential was larger thanthe design value.

LABORATORYMEASUREMENTS

Properties of concrete used in the in-strumented segments were determined.Tests were performed on concrete cyl-inders made at the precasting plantwhere bridge segments were cast. The

tests determined variations of concretecompressive strength, modulus of elas-ticity, Poissons ratio, and coefficient ofthermal expansion of concrete withtime.

Creep tests for concrete cylindersunder constant temperature of 73 F(23 C) and 50 percent relative humiditywere initiated at three different ages.Tests were conducted conforming toASTM Designation: C512-82.8 All creepcylinders were subjected to a constantstress of 2000 psi (13.8 MPa). Whenevercreep measurements were made, com-

PCI JOURNAUSeptember-Oclober 1983 41

Indoors-- -- Outdoors

78 1979 1980 1981 1982 1983

Segment SBI NI

Segment SBI N9

Segment SBIN116

('IITTI

750

500

250

0

Shrinkage 500Strain ,millionths 250

0

500

250

00 500 10001500

2000

Time , days

Fig. 11. Shrinkage strain of concrete versus time.

panion shrinkage measurements werealso obtained.

Similar creep tests for cylinderscured under an outdoor environmentwere started at concrete age of 28 days.In addition to regular creep readings,measurements on outdoor creep cylin-ders were made once every season.Measurements were needed to evaluateeffects of seasons on time-dependentconcrete properties.

Variation of concrete specific creepwith time for each instrumented seg-ment is shown in Fig. 10. Specificcreep is defined as the amount of creepstrain under unit stress in millionths

per psi (millionths/NImm2). Concreteshrinkage was excluded from specificcreep readings. Solid curves in Fig. 10represent specific creep of concreteloaded at different concrete ages. Thedashed lines represent relationships ofspecific creep versus time for concretecured outdoors. All readings were ad-justed for temperature effects to 73 F(23 C) for comparison purposes.

As shown in Fig. 10, relationships ofspecific creep versus time for concretesloaded at the same age were similar. Inaddition, specific creep was lower forconcrete cured outdoors than for con-crete cured indoors.

42

Shrinkage measurements of concretecylinders began 7 days after casting. Acomparison of shrinkage data forspecimens cured indoors and thosecured outdoors is shown in Fig. 11. Thesolid line in Fig. 11 represents shrink-age of concrete cured under laboratoryconditions. The dashed line in the fig-ure indicates shrinkage of concretecured outdoors. Fig. 11 shows thatshrinkage of concrete cured outdoorswas substantially less than concretecured indoors. In addition, there was adistinct seasonal fluctuation of concreteshrinkage under outdoor conditions.

COMPUTER ANALYSIS

Time-dependent deformations werecalculated by a step-by-step numericalprocedure. Total shortening was con-sidered to include instantaneous de-formation, shrinkage deformation, andcreep deformation. Analyses accountedfor time-dependent effects of concreteproperties, relaxation of prestressingsteel, member thicknesses, elastic re-covery, age of loading, shrinkage, andcreep of concrete under a variablestress history. The actual casting anderection schedule of bridge segmentswas used. Further details of the numer-ical procedure are given elsewhere.3

Analyses were performed for threedifferent sets of material properties. InAnalysis No. 1, experimentally deter-mined properties of concrete specimensstored outdoors were used. In AnalysisNo. 2, concrete properties determinedfrom specimens cured indoors wereused. For Analysis No. 3, materialproperties recommended by the Euro-pean Concrete Committee7 were used.

COMPARISON OFMEASURED AND

CALCULATED STRAINS

Measured and calculated results were

compared with regard to longitudinalconcrete strains. Comparisons verifiedthe numerical procedure.

Comparisons between measured andcalculated strains from the three analy-ses of Segment SB1-N1 are shown inFig. 12. Detailed comparisons are pre-sented elsewhere.s- s The best agree-ment was obtained with Analysis No. 1which used concrete properties of cyl-inders stored outdoors. Good compari-son was obtained with Analysis No. 2using properties of laboratory curedconcrete cylinders. In Analysis No. 3,calculated strains were consistentlysmaller than measured values.

Comparisons of measured and calcu-lated strains using Analysis No. 1 forSegments SB1-N9 and SBI-N16 areshown in Fig. 13. Good agreementbetween measured and calculatedstrains was obtained. Comparisonsbetween measured and calculatedstrains for Segments SB1-N9 and SB1-N16 using Analysis No. 2 also showedgood agreement. These comparisonsshow that the numerical procedure wasable to predict deformations reasonablyclose to measured deformations. Con-sequently, the numerical procedurewas used to determine information thatcould not be obtained directly by fieldmeasurements.

SIGNIFICANCE OF RESULTS

Discussions of results with regard toaxial deformations, prestress losses,moment redistribution, concretestresses, and temperature effects arepresented.

Axial Deformations

Comparisons of measured and calcu-lated axial strains for the three instru-mented segments of the bridge areshown in Figs. 12 and 13. Calculateddeformations for Segment SB1-N1 at2000 days since erection were 380 mil-


Measured Analysis

Analysis 2

!T t Analysis 3

Top Slab

750

250C0

0

N 750

C500

y 250NN

0 aE0U

750

Web

500

250Bottom Slab

010500100015002000

Time , days

Fig. 12. Comparison of measured and calculated strains for Segment SB1-N1.

44

Measured--- Analysis

s,Segment SBI N9

750Compressive

Strains ,

millionths 500

250

Bottom Slob

i'Web

"^^ Top Slab

0 C0

500 1000

1500 2000

Time , days

s+Segment SBI N 16

750Compressive

Strains,Bottom Slab

millionths 500Web

250 Top Slab

0 uir i

0 500 1000 1500 2000

Time , days

Fig. 13. Comparison of measured and calculated strains for Segments SBI-N9 andS61-N16.

PCI JOURNALlSeptember-October 1983 45

Tension

Stress

ksi

150

e

50

q 0 500100015002000

Time , days

Fig. 14. Calculated average tendon stresses (Note: 1 ksi = 6.89 MPa).

lionths in the top slab and 780 mil-lionths in the bottom slab. An axial de-formation of 780 millionths represents ashortening of 2.3 in. (59 mm) in a 250-ft(76.2 m) span. In Segment SB1-N16,axial strains of 197 and 486 millionthswere calculated for the top and bottomof the box section, respectively.

However, the majority of the axialdeformation occurred during construc-tion of the cantilever spans. Calculatedaxial deformations for Segment SBI-N1after completion of construction rangedfrom 143 to 233 millionths at 2000 dayssince erection. This accounts for 30 to40 percent of the total measured short-ening. Consequently, most of the de-formation was accommodated when theclosure segment was cast at midspan.Therefore, only shortening that oc-curred after completion of the bridge

need be considered in determiningoverall longitudinal movements.

Prestress LossesThe numerical procedure was used to

determine variation of prestressingforce with time. Calculated changes ofprestressing force with time for thethree instrumented segments are shownin Fig. 14. Calculated values are basedon Analysis No. 1 which used materialproperties of concrete cured outdoors.Tendon stresses are the average valuesfor tendons in each instrumented seg-ment. It is noted that total prestresslosses are relatively small and are ap-proximately equal for each segment.

Individual prestress losses caused bytime-dependent effects as predicted byAnalysis No. 1 are summarized in Table

46

Table 1. Summary of prestress losses for Segments SB1-N1, SB1-N9, and SB1-N16.

Prestress losses (psi)

Segment Shrinkage Creep Relaxation

Analysis AASHTO Analysis AASf-TO Analysis AASHTO

SB1-NISB1-N9SB1-N16

879*880*585*

484048404840

546054306820

636066609340

362040203750

300030003000

Metric Equivalent: 1 psi = 6.89 Pa.Values represent maximum prestress losses from concrete shrinkage calculated for the second yearafter segment erection.

1. For comparison, prestress losses cal-culated according to AASHTO Specifi-cations4 are also listed. It is noted thatanalytical values shown in Table 1 rep-resent prestress losses at approximately1850 days after the segments wereerected.

Table I shows that calculated lossesfrom creep and shrinkage are less thanthose calculated according to AASHTOSpecification 5. 4 Differences betweencalculated and AASHTO specified val-ues are especially large for prestresslosses due to concrete shrinkage. Thereason for the small calculated prestresslosses was that concrete under outdoorconditions experienced less creep andshrinkage as shown in Figs. 10 and 11.

In addition, concrete cured outdoorsexhibited seasonal shrinkage variationswhich were also reflected in the calcu-lated prestress losses. Therefore,AASHTO Specifications overestimatedprestress losses due to concrete creepand shrinkage. However, calculatedrelaxation losses were slightly largerthan the 3000 psi (21 MPa) given inAASHTO Specifications.

It should be also noted that the in-strumented segments were erected atapproximately 100 days after casting.However, AASHTO Specifications donot consider ages of concrete segmentsat time of erection. Furthermore, theSpecifications do not take into account

member thickness. Therefore, differ-ences between prestress losses ac-cording to AASHTO Specifications andthe calculated values are expected.

Moment RedistributionAs a result of time-dependent defor-

mations, redistribution of bending mo-ments takes place in a continuousstructure. Using Analysis No. 1, time-dependent redistribution of moments inthe Kishwaukee River Bridge was cal-culated. Distribution of bending mo-ment at the time of bridge completionand at 1850 days after completion areshown in Fig. 15.

The general redistribution trend wasto shift moment from negative momentregions to positive moment regions.The magnitudes of positive bendingmoments increased with time whilemagnitudes of negative bending mo-ments decreased with time, The posi-tive increase of bending momentthroughout each span was found to besimilar. However, the percentage ofmoment change at the pier and at mid-span varied significantly. Due to therelatively high initial negative mo-ments, a small percentage decrease innegative moment corresponded to alarge percentage increase in positivemoment. Calculated results indicatedan average 14 percent decrease in mo-ment at the pier and an average 56 per-

PCI JOURNALJSeptember-October 1983 47

IUco

- rOU,^k^c

- 80,00(

- 60,00(Bending

Moment ,

k --ft

'0.00(

+ 20,00(

+ 40,00(.

Fig. 15. Redistribution of bending moments (Note: 1 k-ft = 1.35 kN-m).

2.0 -

1.5

1.0

05

0

Compressive 1.5

Concrete1.0

Stress,.

ksi 05

0

1.5

1.0

0.5

0 I t i +0500100015002000

Time , days

Fig. 16. Calculated concrete stresses from Analysis No. 1 (Note: I ksi = 6.89 MPa).

cent increase of midspan moment be-tween piers.

The large increase in positive mo-ments is especially significant at thequarter span where the direction ofbending moment may change as a re-sult of redistribution. Therefore, de-signs must account for time-dependentincrease in positive moments.

Concrete Stresses

As indicated from Analysis No. 1,there was a large increase of positivemoment at midspan of the bridge.However, the calculated concretestresses from Analysis No. 1 indicatedno danger of cracking as a result ofmoment redistribution. Calculated


100

75

50

Air

Temperature , 25of

0

75

50

25

0

Summer Fall

in

out

Out

Winter Spring

outout

in

Noon Mid Noon Noon Mid Noonnight night

Fig. 17. Daily variations of air temperatures inside and outside the box girder in fourseasons (Note: 1F 0.56C).

stresses by Analysis No. 1 for each ofthe instrumented segments are shownin Fig, 16.

In general, compressive stresses inthe top slabs increased with time whilecompressive stresses in the bottomslabs decreased with time. These dataare consistent with the redistribution ofmoment occurring throughout thebridge. In Fig. 16, there is a substantialdecrease in compressive stress at thebottom slab of Segment SB1-N16.However, the stress reduction was notsufficient to cause concern aboutcracking at midspan. The calculatedcompressive stress at the bottom ofSegment SB1-N16 at 1850 days aftererection was approximately 620 psi(4272 Pa).

It should be noted that the stressesare calculated based on actual deadloads of the bridge rather than designservice loads. Consequently, stressesresulting from additional dead load or

live load will be additive to valuesshown in Fig. 16.

Temperature Effects

Variations of air temperatures insideand outside the box girder over 24hours in four randomly selected daysare shown in Fig. 17. Air temperaturesinside the box girder were more stableand exhibited less variation than theoutside air temperatures. In addition, atime lag was observed in the responseof the inside air temperature to the out-side environment changes. Conse-quently, big temperature differentialsacross the top and bottom slabs andthrough the webs can exist when thereis a sudden change of outside air tem-perature.

Thermal response of structures totemperature can either he in the forn ofinduced stresses or induced deforma-tions. Therefore, longitudinal strains of

50

w E4 5 b

3 7

2 1 g

550

500

Strain, 450

millionths400

500

450

Summer

Fall i

55

Winter Spring

zZZ55

Mid Noon NoonMidNoonnight night

400Noon

Fig. 18. Daily variations of longitudinal concrete strains of Segment SB1-N1 in fourseasons.

the instrumented segments were re-corded together with temperature mea-surements. Figs. 18 through 20 showthe variation of concrete strains withina day for the four seasons for SegmentsSB1-N1, SB1-N9, and SB1-N16, re-spectively. The strain data have beencorrected for temperature of concreteon the basis that the bridge respondsfreely to temperature change.

Data in Figs. 18 to 20 show compara-tively large changes in strain for Seg-ment SB1-N1 due to temperature ascompared with responses recorded inSegments SB1-N9 and SB1-N16. Largestrain changes due to temperature indi-cated that the bridge segment re-

sponded in the form of thermal move-ments. Comparison between measureddeformations of the three segments in-dicated that both thermal stresses anddeformations were induced in thebridge. Such thermal stresses should beincluded in design.

ACKNOWLEDGMENTS

This investigation was sponsored bythe State of Illinois, Department, ofTransportation in cooperation with theFederal Highway Administration,under Project IHR-307. The work wasperformed in the Structural Develop-ment Department of the Construction


W E

4 5 6

3 7

2 I 8

Summer Fa I I

650

550

450Strain,

millionths 350

250

550

450

350

250

5 5

I

Winter

5

Spring

5

Noon Mid- Noon Noon Mid- Noonnight night


Technology Laboratories, James I.Daniel, structural engineer, andThomas L. Weimnann, project assistantof the Structural Experimental Section,helped in all tests. Their efforts are sin-cerely appreciated and acknowledged.

Dr. W. L. Gamble and Dr. V. L. Mar-shall were responsible for the analyticalstudies. The analytical work was per-formed at the University of Illinoisunder an agreement with the Construc-tion Technology Laboratories.

Richard Taylor of the State of IllinoisDepartment of Transportation coordi-nated the project. Members of the Ad-visory Committee who reviewed prog-

ress of the project were Robert Ap-plernan, Gayle Lane, Frank Grahski,Donald R. Schwartz, Colin Strang,Richard Taylor, and John Ross. Theircontributions to the project are ap-preciated.

REFERENCES

1. Nair, R. S., and Iverson, J. K., "Designand Construction of the KisbwaukeeRiver Bridges," PCI JOURNAL, V. 27,No. 6, November-December 1982, pp.22-47.

2. "Truss Launches Quickly in U.S. BoxGirder Debut," Engineering News Rec-ord, October 19, 1978, pp. 45-58.

52

W E

456

2I8

650

550

450

Strain

millionths 350

250

550

450

350

250

Summer Fall

5 5

Winter Spring

I I

5

NoonMid-NoonNoonMid-Noonnight night


3. Marshall, V., and Gamble, W. L.,"Time-Dependent Deformations in Seg-mental Prestressed Concrete Bridges,"Structural Research Series No. 495, CivilEngineering Studies, University of I1-linois, Urbana, October 1981, 242 pp.

4. Standard Specifications for HighwayBridges, Twelf h Edition, American As-sociation of State Highway and Trans-portation Officials, 1977.

5. Shin, K. N., Daniel, J. I., and Russell,H. G., "Time-Dependent Behavior ofSegmental Cantilever Concrete Bridges,"Final Report to the State of Illinois, De-partment of Transportation, by Construe-

tion Technology Laboratories, a Divisionof the Portland Cement Association,Skokie, Illinois, March 1983; to be pub-lished through National Technical Infor-mation Services, Springfield, Virginia.

6. "Standard Method of Test for Creep ofConcrete in Compression," C512-82,American Society for Testing and Materi-als, Philadelphia, Pennsylvania

7. CEB (European Concrete Committee),International Recommendations for theDesign and Construction of ConcreteStructures, Principles and Recommen-dations, Comite Europeen du Beton,Paris, France, June 1970.


Summary Paper

Feasibility Study ofStandard Sections for

Segmental PrestressedConcrete Box Girder Bridges

Felix KulkaConsulting Engineer(At time study was conducted,Mr. Kulka was President ofT. Y. Lin International,San Francisco, California)

S. J. ThomanStructural Engineer

T. Y Lin InternationalSan Francisco, California

Segmental prestressed concrete boxgirder bridges were introduced inNorth America in the late sixties andearly seventies, following their suc-cessful entry into the European marketduring the post World War II recon-struction period. Several bridges of this

NOTE: This Summary Paper is a condensation of theresults of an investigation commissioned by the Fed-eral Highway Administration on the feasibility of usingstandard sections for segmental prestressed concretebox girder bridges. The study was initiated in 1980and completed in July of 1982. The full length report,entitled -Feasibility of Standard Sections for Seg-mental Prestressed Concrete Box Girder Bridges"{FHWA RO-82;024) by F. Kulka, S. J. Thoman, andT. Y. Lin is available from the National Technical In-formation Service, Springfield, Virginia 22161.

type, both precast and cast in place,were built successfully in the UnitedStates and Canada during this time, andthe approximately 70 projects whichhave been designed to date indicatethat the segmental prestressed concretebox girder bridge is a very viable alter-native for medium to long span bridgestrictures in North America.

At the same time, it is recognized thatthe design and construction of seg-mental bridges still largely follow prac-tices in Europe and that a closer iden-tification with American constructionpractice is in order. Standardization ofcertain aspects of segmental box girderbridges appears to he one way to ex-

54

Synopsis

Presents the highlights of a study which investigatedthe feasibility of developing standard sections forsegmental prestressed concrete box girder bridges.The report is based on an extensive survey ofsegmental box girder bridges in the United States andCanada. Recommendations are given for specific itemsthat could be standardized, while also discussing areaswhich might not be appropriate to standardization.

pand their economical use by instillingconfidence among bridge engineersand by producing a cost effectivenessthrough uniformity in design, thuspermitting precasters and contractors toinvest in forms and equipment on abroader basis than is done today.

This report deals with the feasibilityof standardizing segmental prestressedconcrete box girder bridges in theUnited States. The study relied heavilyon a survey of bridge engineers in theUnited States and Canada, which pro-duced valuable information on allbridges of this type. Statistical studieswere conducted to determine correla-tions and uniformity of significant pa-rameters, particularly with respect togeometry.

Analytical design studies, mainly todetermine the economical use of mate-rials, were made to augment the statis-tical analyses. The results wereevaluated both qualitatively and quan-titatively, and an advisory technical re-view committee was formed to reviewthe content of the study and its recom-mendations.

The report takes the position thatstandardization of segmental pre-stressed concrete box girder bridges ispossible and should be initiated. The

specific areas which should be standar-dized are listed and discussed in thereport, as are those which are not cur-rently subject to standardization andthose which are questionable.

Scope of Study

Standardization of highway construc-tion elements is a long-standing prac-tice in the American highway industry.Development of the AASHTO-PCI I-girders is one example; precast con-crete culverts, traffic barriers, and pilesare other examples. It is fairly wellagreed that standardization has meritsin cost savings, reduction of construc-tion time, and improved product qual-ity.

It was felt that for standardization ofbox girder sections to succeed, a uni-form approach should be used in orderto permit bridge engineers to designsuch sections with a sufficient degree ofuniformity and to allow precasters andcontractors to bid and build them asthey would any other advanced type ofstructure.

The object of this study, then, was toconsider all the advantages and disad-vantages of standardization and makeappropriate recommendations for future


development. In doing so, care wastaken not to let standardization limitcompetition, rather, standardizationwas approached with a view towardsexploiting all the alternatives, therebyimproving design and increasing com-petition. The scope of the study in-cluded:

1. An assessment of the state of theart of segmental bridge construc-tion.

2. Development of design constraintsas affected by construction limita-tion s.

3. An analysis of costs and benefits ofstandard sections.

4. Development of specific recom-mendations concerning the feasi-bility of standard sections for seg-mental prestressed concrete boxgirder bridges.

Study ApproachIt was felt essential that the recom-

mendations concerning possible stan-dardization be based on experienceswith existing practices rather than onarbitrary judgments.

Accordingly, a questionnaire con-cerning prestressed concrete segmentalbox girder bridges was sent to bridgeengineers in all states and territoriesplus the provinces of Canada. The sur-vey included bridge site, state of com-pletion, cross section, design and de-tails, construction, costs and other per-tinent information. The response wasexcellent, and the information collectedprovided a good sampling for furtherin-depth studies.

The data obtained were categorizedand statistical studies were made toevaluate significant parameters, leadingto a rational assessment of the state ofthe art of segmental bridge design andconstruction. Analytical studies wereperformed in cases where data were notavailable, permitting the establishmentof qualitative and quantitative relation-ships.

State of the Art ofBox Girder Bridges

Cast-in-place, conventionally formedbox girder bridges had been used inNorth America for many years when, inthe late sixties, segmental box girderconstruction was introduced to thecontinent. This type of structure was aEuropean development of the post-World War II era, when the reconstruc-tion of war-torn European countriesdemanded methods of constructionwhich would overcome the scarcity oflabor and which would produce manystructures in the shortest possible time.The development of cast-in-place seg-mental construction is generally attri-buted to Germany, while precast seg-mental construction is primarily aFrench innovation.

Since the volume of construction waslarge and there was sufficient invest-ment available, the box girder becamepopular even though it is not necessar-ily the most economical section for allconditions. The box girder can, how-ever, safely accommodate spans up to800 ft (244 in) and resist a wide range ofstresses. Furthermore, its resistance totorsion made the box girder particularlysuitable for cantilever construction,which proved to be a good method forrapid construction and for achievinglong spans without the use of falseworkor shoring.

The Lievre River Bridge in Quebec(completed in 1967) was the first pre-cast prestressed segmental bridge builtin North America. This was followedshortly by the Bear River Bridge nearDigby, Nova Scotia. The first majorsegmental box girder bridge in theUnited States was the JFK MemorialCauseway in Corpus Christi, Texas(completed in 1973).

As a result of a fairly active programof promotion, more than 50 segmentalbridges have been constructed in NorthAmerica since that time. Their recordwith respect to economy and successful

56

i d

0 ^ C/

q/

Hawaiian < a_Islands.'----- -

'I

rte] - _.

f I ^

Koror Babelthaup

Puerto Rico

Fig. 1. Distribution of segmental prestressed concrete box girder bridges designed orconstructed in the United States or Canada. (Note that bridges in the United Statesinclude also those in the Commonwealth of Puerto Rico and the Trust Territories in theSouth Pacific.)

construction was not uniform, owingmostly to a wide variety of site condi-tions, design practices, specificationsand bidding requirements. The 1980dollar cost per square foot of bridgedeck of some 37 segmentally con-structed box girder bridges appears tovary widely from $30 to $150 (8323 to$1615/m 2 ). Nevertheless, sufficientcases of successful and economical con-struction exist to make the segmentalbox girder a very viable choice in theconcrete bridge market.

The conditions surrounding the pres-ent state of segmental box girder con-struction raise the obvious question ofstandardizing at least some aspects ofits design and detailing. Ideally, stan-dardization could bring about cost ben-efits by permitting contractors to investin forms, installations and equipmentwhich could be reused more often, thusreducing the cost of mobilization. De-tails and joinery could he simplified inthe process of standardization, andoverall safety and integrity could he

PCI JOUR NAL'September- October 1983 57

ME

15

O V] G Lo 0 0a' (0 P- ti CO CO

r r r r r

Bid Year

Fig. 2. Number of bridges bid in successive years.

20

15cn

Lm

0 10

L0E7z

added to the structure by making avail-able past experience and knowledge tothose new in the industry.

The map in Fig. 1 shows the dis-tribution of existing segmental bridgesin North America (which also includesthe Commonwealth of Puerto Rico andthe Trust Territories in the SouthPacific) in 1981. It can be seen that thevast majority of these bridges lie in theeastern part of the United States, whichis consistent with the distribution ofother bridges as well. Bridges shown onthis map are located in Canada and 18U.S. states and territories.

The histograph in Fig. 2 shows thebridges as they were hid. It can be seenthat there is a steady increase in thenumber of bridges from 1970 to theeighties. The peaks and valleys are nottoo important, since the time at whichthe bridge incidence was plotted canvary with respect to the completion of

design or start of construction. Thethree bridges before 1970 were built inCanada, which preceded the UnitedStates in segmental bridge construction.Nevertheless, Fig. 2 shows a steady in-crease in the use of segmental bridgesand dramatically so in the late seven-ties. Indeed, it may be concluded thatthis method of bridge construction ishere to stay.

The five major types of constrictionwhich have been employed are the bal-anced cantilever type (precast and castin place), span-by-span construction,progressive placing, and incrementallaunching.

In the balanced cantilever construc-tion method the segments are cantile-vered out from each side of their sup-port, so as to balance the momentwhich is induced in the pier. The seg-ments can either be precast or cast inplace.

58

in progress or completed

35 1- has not been bid or was not selected

30 _QQ. 3 V

2 5 o s mU 7C 61 :s Cl

20 a c C Gw a a roU l a U V

>15 c m I -n tom m m W

E U Uz 10 o

C

Construction MethodFig. 3. Frequency of various construction methods.

In precast construction the segmentsare manufactured at a factory or at theproject site. The segments are thentransported to the bridge superstructureand lifted into their final positionwhere they are post-tensioned againstthe previously erected segments.

In cast-in-place construction a formtraveler is employed to carry the formsinto which the segments are cast intheir final position. After the concretehas reached sufficient strength, thesegment is post-tensioned against thealready completed superstructure.

The span-by-span method features asuperstructure constructed in one di-rection, one span at a time, incorporat-ing either precast or cast-in-place seg-ments.

The progressive cantilever method issimilar to the balanced cantilevercast-in-place construction method, ex-cept that the segments cantilever out-ward from only one side of the pier,while the sidespan is cast on falsework.

In the incremental launching methodthe segments are cast near the bridge

abutment. Once a segment has reachedsufficient strength, it is post-tensioned,then vertical and horizontal hydraulicjacks are engaged to lift the segmentsand push them out longitudinally fromthe abutments.

Fig. 3 shows the number of segmen-tal bridges classified according to con-struction method. Balanced cantilever,both cast in place and precast, com-prises by far the largest percentage ofbridges. It is interesting to note that inreviewing bidding history, more con-tractors favored cast in place rather thanprecast segments when they had achoice in the method of construction.One reason for this is that contractorsare in general more experienced withcast-in-place construction methods.

Of the 33 bridges designed in precastsegments, 15 were constructed; theothers were not built or changed to adifferent type. Of the 27 balanced can-tilever bridges 24 were constructed asdesigned; the others were either notbuilt or changed to a different type. In-cremental launching and progressive

PCI JOURNALlSeptember-October 1983 59

;^ in progress or completed

4 yw-

has not been bid or was not selected

e 0.09m2 =1.0sf0

d 3- 0.0)a)

m r c m I

Oc2 a

020) }

ro as

Q

ao

UfbO

_c mdE 0

N U U

IIIIICt0

G^C

ay 1 va

a m MN . p

JC.0)

Construction Method

Fig. 4. Total square footage of bridge deck for various construction methods.

placing represent only one bridge each,hence, cannot necessarily be consid-ered representative.

The bridge deck area for the variousmethods of construction is shown inFig, 4. It may be seen that span-by-spanconstruction encompasses about 30percent of the total bridge deck area,but as shown in Fig. 3, represents onlyabout 10 percent of the total number ofbridges. That indicates that this methodobviously was applied to very largeprojects. In balanced cantilever con-struction the total bridge deck area isabout equally divided between cast-in-place and precast structures as de-signed, but the proportion of bridgesactually constructed to the total designedis higher for the cast-in-place structure.

The project size greatly influencesthe method of construction to he used.Table 1 shows the average length ofsegmental bridges surveyed. Here thetotal deck area of bridges .for each typeof construction was normalized to an

equivalent 40-ft (12 m) width, and theresulting total bridge length was thendivided by the number of particularbridges, thus obtaining an averagelength of bridge for each constructiontype. The numbers show that the aver-age length for span-by-span construc-tion is about 40 percent larger than forbalanced cantilever. In other words, it

Table 1. Average bridge lengths forvarious construction methods.

Constructionmethod

Average length for a40-ft (12 m) roadway

Incrementallaunching 1087 ft (331 m)Progressiveplacing 1165 if (355 in)Span-by-span 5347 ft (1630 in)Balancedcantilever

(precast) 3133 ft (955 m)(cast in place) 2818 ft (850 rn)

60

in emental launching

progressive placing

v0

A

4A 0.30m=1.OftAAA

span-byspan0 o M 0

. i C 0"

Ubalanced cantilever, PC

. ME -4^ balanced cantilever, cip !^

100 200 300 400 500 600 700 800

(ft)

Span Length

Fig. 5. Span ranges of box girder bridges for various construction methods.

200

Co a

150 a

C UI

0 100 U r

o oa e^Q ^ ,,

tO

50 m c 0.09m2-1.Osf

ar '' A m 0.30m=1.OftEa - A

0 a CV

Construction Method

Fig. 6. Cost of segmental prestressed concrete box girder bridges for differentconstruction methods.


U,a

0 20

aE

Z 10

Cross-Sectional Configuration

Fig. 7. Frequency of various cross-sectional configurations of box girder bridges.

2.50.30m3/m2=1.Ocy/sf0.30m =1 .Oft

wing section2.0

1.5mnC0U ..

wm 7 1.0E Vv7OJ

rder

0.5

50

100 150

Span Length (ft)

Fig. 8. Weight of mild steel reinforcement for various girder types.

62

2101-cell2-cells ---

1900.30m2/m =1.0sf/ft0.30m= 1.0ft

160 /d

o //0- 130 (a / A

100 /

_` W= 70Ar U

7 0 W - 50

iitEEEE 1iic

W - 30^40

10 20 30 40(ft)

Girder Depth

Fig. 9. Internal surface forming area for various deck widths and girder depths.

takes a larger project with many shortspans, as for example a causeway, forthis method to be economical as com-pared to other structural sections.

The distribution of constructionmethods for segmental bridges with re-spect to span length is shown in Fig. 5.It can be noted that the balanced can-tilever method was used primarily forspans greater than 200 ft (61 m). Thespan-by-span method was used forspans between 80 and 180 ft (24 and 55m) as were the incremental launchingand progressive placing methods.For spans longer than 450 ft (137 m),only cast-in-place segments wereemployed, most likely because of theincreased weight of precast segmentsneeded for long spans.

Fig. 6 shows the costs of thesebridges. It may be seen that there is noobvious uniformity to be discernedfrom these cost figures, Partially, thereason for this is the fact that accuratecosts are very difficult to establish. Firstof all, cost figures are not readily avail-able; secondly, when they are availableit is not totally clear what the costscover. However, costs do vary widelyprincipally as the result of lack in uni-formity of design and constructionpractices. In any event, the figuresmight demonstrate qualitatively the factthat costs of the bridges varied consid-erably within the construction methoditself, in addition to differences be-tween the various constructionmethods.

PCI JOURNALISeptember-October 1983 63

1 1 -Cell

2 -cells -- -

W=70'10m /

U0

/

// W 5O8

E5 ^y W=30'

o>

m= 1.Oft0.300.09m3/m = 1.Ocy /ft

10 20 30 40(t t)

Girder Depth

Fig. 10. Volume of concrete for various deck widths and girder depths.

1001 -cell2-cells -----

$0

4.45N =1.01b0.30m -1.0ft

c /m 60of

/

a "40 J(

o /

20

30 40 (ft) 50 60 70

Width of Deck

Fig. 11. Weight of transverse prestressing steel for various deck widths.

64

W

T WT 1Dr-

L..A

AA

1.0

2.0

A

A

0.003L+0.026 (A)

I-

3:I-m..r ^'m vaLaQ

correlation = 0.85A cast-in-place = 0.87 precast = 0.70

0.30m = i.Oft3.0

3: ... 0.002L + 0.024 ()

.-:;-:r_A

200 300 400 500 600 700 800(ft)

Span Length

Fig. 12. Variation of web parameter with span length.

Bridge Cross Sections

The cross sections of bridges used inthe United States and Canada con-tained single cells, double cells, triplecells, and twin single cells. The histo-gram in Fig. 7 shows the number andshape of cells incorporated into thecross section of the box girders used todate. It is evident that the single cell ora combination of single cells is the mostwidely used section, representing about90 percent of all bridges surveyed.

In order to establish the cost effec-tiveness of the single cell and doublecell cross-sectional configurations, pre-liminary designs were made to studyrequired material quantities. Theweight of prestressing steel, weight of

mild steel, the volume of concrete, andthe area of internal forming for variousgirder depths and roadway widths werecompared. Figs. 8 through 11 show re-lationships between material quantitiesand girder depths for the various topflange widths of the box sections.

In Fig. 10 it may be seen that the vol-ume of concrete in a single cell sectionis less than that of a double box for a30-ft (9 m) width, but as the width in-creases the difference diminishes. In a70-ft (21 m) width the volume of con-crete is greater for a single cell than fora double cell. This conclusion may alsobe reached by realizing that a longerspan requires the top flange width to beincreased in thickness in order to carrythe heavier traffic loading.


correlation = 0.83W cast-in-place - 0.87

precast=0.520.30m=1.Oft

....:r ST

l^ vSW /

(A) 0.005L-0.654AL

2.0(DEI-

&

0

0 1.0

t

A'

AA

0.003L+0.234 ()

f

A

N

200 300 400 500 600 700

Span Length (ft)

Fig. 13. Variation of soffit parameter with span length.

4.0 i

3.0

The weight of mild reinforcing steelis less for the single cell section thanfor the double cell section for all sec-tion widths. This, again, is reasonable,since much of the mild steel is nominalreinforcing and the loads are carriedlargely by the post-tensioning tendons.The internal surface forming area isconsiderably less in the single cell sec-tion, which translates into great econ-omy for formwork. The elimination ofinterior webs also produces a more con-structable section. The requiredamount of transverse post-tensioning is,of course, higher for the single-cellsection than for the double-cell section.

It can therefore be deduced that thesingle cell section is more economicalthan the multiple section in all aspects,except for the transverse prestressingsteel. This is true up to a width of ap-proximately 70 ft (21 m), at which pointtwin single cells should be considered.

Statistical Studies ofDimension Parameters

In order to determine the degree ofuniformity in dimensions, parametersin the transverse and longitudinal di-rection were studied statistically for the

66

31- CLcorrelation = 0.600.30m =1 .Oft

mm 7> 2.0

0.064CL + 0.81

1.0

4

0UO

5 10 15 20(ft)

Length of Cantilever

Fig. 14. Variation of cantilever deck thickness with cantilever length.

bridges surveyed. Linear regressioncurves were fitted through the datapoints using a least square criterion.

Correlation coefficients were calcu-lated to determine the uniformity be-tween the parameters. The parameterswith correlation coefficients greaterthan approximately 0.80 were consid-ered to be related, indicating uni-formity. Such uniformity would suggestthat the parameters lend themselves tostandardization. Note that precast andcast-in-place bridges were consideredtogether and also independently.

To study the web dimensions for aparticular span length, the web area forthose bridges surveyed was normalizedby the bridge width. This accounted forthe varied number of traffic lanes andloading conditions. The web parameterwas defined as the total area of the webdivided by the bridge width. The re-lationship between web parameter and

span length is shown in Fig. 12. Thecorrelation coefficient was 0.85 whencombined and 0.87 and 0.70 whenstudied independently for cast-in-placeand precast bridges, respectively.These values indicate uniformity,which suggests the feasibility of stan-dardization.

It is interesting to note that the func-tion for these precast bridges wasbelow and somewhat parallel to cast-in-place bridges. This indicates that forthe same span length the precast seg-ments incorporate thinner webs thantheir cast-in-place equivalents, whichmay be related to weight reductionstrived for in plant production.

The study of the soffit parameter,shown in Fig. 13, was defined by di-viding the soffit cross-sectional area (lo-cated near the pier) by the bridgewidth, which normalized the differentbridges surveyed. Quantitatively, when

PCI JOURNAUSeptemher-October 1983 67

20

7 Dcorrelation = 0.95

A cast-in-place precast

0.30m=1.Oft

11

15

m0a Z.m0i0 10

0.042L+0.45

slope 1:22 to 1:23 ^

200 250 300 350

400 450(ft)

Span Length

Fig. 15. Variation of girder depth with span length for balanced cantilever construction.

the structural system is continuous overa support, the bottom soffit near thesupport must develop a compressiveforce to resist the induced moment.Since this induced moment is related tothe span length, the bottom soffit areamust also increase with increasing spanlength. The correlation coefficient con-sidering both precast and cast-in-placesegments was 0.83, indicating good cor-relation.

In Fig. 14, the deck thickness at thecantilever base is plotted against thelength of cantilever. The figure repre-sents the results of a study of the deckthickness at the transverse cantileversupport as a function of the cantileverlength. AIthough a low correlationcoefficient of 0.60 was calculated, thedeck thickness could intuitively bestandardized For a particular bridgewidth.

The low correlation may be attri-buted to the varying amount of trans-verse prestressing in the deck, whichwas not included in the study. Also, thedeck thickness of the cantilever at itssupport may he controlled by dimen-sioning requirements to accommodatethe longitudinal tendon anchorages, in-stead of providing the amount of resis-tance to induced forces.

A high correlation was found be-tween span length and girder depth forbalanced cantilevers with constantdepth sections, as shown in Fig. 15. Acorrelation coefficient of 0.95 was cal-culated for the bridges considered. Re-sults show that the average span-to-depth ratio was between 22 and 23 forspan ranges between 130 and 450 ft (40and 137 m). Also, the majority of con-stant depth structures are precast as op-posed to cast in plaee.

correlation50 r A cast-in-place =0.84

precast =0.600.30m =1.Oft

MIOP L0

0 4.0s

aW

AL

A

L

^ Qa 3.0

c ..a

20

2.00a-

-0.004L+O.9 (A)

if ^

A 0.003E+1.13 ()

300 400 500 600 700 800(ft)

Span Length

Fig. 16. Pier to midspan girder depth ratio for various span lengths for balancedcantilever construction.

The longitudinal haunch ratios, de-fined as the pier-to-midspan-depthratios, were studied for those bridgesemploying balanced cantilever con-struction. The results are shown in Fig.16. The cast-in-place haunch ratiosvaried from 1.7 for the shorter spans to4.3 for the longer spans. A correlationcoefficient of 0.84 was calculated forcast-in-place bridges, indicating highuniformity. The low correlation coeffi-cient for precast construction maysuggest difficulties or reluctance as-sociated with using precast haunched

segments. Also, the infrequent use ofhaunched precast concrete segmentsresulted in insufficient data for statisti-caI analysis.

Preliminary Designs forVarious ConstructionMethods

Preliminary designs were made todetermine the cost effectiveness of thevarious construction methods. Quan-tities of materials rather than cost fig-


5.0

0.30m31m2 =1.Ocf/sf0.30m = 1.0ft

4.0

U

OU

3.0

E

0

incremental rlaunching

balanced cantileverprismatic, precast/& cast-in-place Cast-in-place

v/c-precast

balanced & progressivecantilever non-prismatic

ressive placing

span-by-span

100 200 300 400 500 600 700 800(ft)

Span Length

Fig. 17. Volume of concrete for various construction methods.

ures were used, since the latter are toovariable.

Fig. 17 shows the volume of concreteplotted against span lengths for thevarious construction methods. Span-by-span construction is more efficientin the lower span ranges, with balancedcantilever being more efficient in thehigher ranges. Incremental launching iscost-effective up to about 200-ft (61 m)spans, but becomes inefficient beyondthat point, apparently because of theneed to employ concentric prestressing.Progressive placing shows economy ofconcrete volume up to about 200 ft (61m).

The weight of prestressing steel ver-sus span lengths is shown in Fig. 18.The relationship is similar to that ofvolume of concrete for the variousmethods ofconstnrction.

From these curves and from otherdata presented it may he concluded thatbalanced cantilever is the most preva-lent method of construction for spansover 150 It (46 m). Up to 300 ft (91 m),precast construction is advantageousbecause such spans permit a constantdepth of section. Once a parabolichaunch is necessary to accommodatethe span, the cast-in-place section be-comes more appropriate. It has been

70

20incremental launching

0.30m =1 Oft47.9NJm2=1.Olb/sf

15/

/N ^^ /5,

nonprismatic1fl progressive placing-''

^! prismatico

L 5 ^'

balanced & progressivea,

cantileverti- span-by-span

100 200 300 400 500 600 700 800

Span Length (ft)

Fig. 18. Weight of prestressing steel for various construction methods.

Table 2. Parameters feasible for standardization.

Parameter Yes No Maybe Parameter Yes No Maybe

Cross sectiondimensions

Span depth ratios

Shape of box Haunch ratios

Number of cells Radius ofcurvature Segment length Construction method Joint details Span limitations Post-tensioning details Roadway details Design guides Reinforcement Design criteria Specifications

used for spans up to about 800 ft (244m). Span-by-span construction is 're-stricted to the shorter spans, perhaps upto 150 ft (46 in).

Items suitable for standardization aresummarized in Table 2.

Fig. 19 shows an interesting projec-

tion of use, which was based on an ad-ditional questionnaire sent to bridgeengineers in the United States andCanada, It shows that the projected use,in their opinion, will feature to a greatextent spans between 80 and 120 ft (24and 37 m). This indicates that segmen-


95

80

60m r,m ^^ c

0=E40o

20

o 0 0 00 o o MD o

0 Ui 4 W u]N

o o 0 0

0 0 0 0 0 0'CD 0 IL) o MD aa m r c*) V W m

Span Ranges (ft)

Fig. 19. Comparison of projected total square footage of bridge deck within spanranges for steel, reinforced and prestressed concrete structures.

Fig. 20. Configurations of various bridge sections.

72

1247.9Nlm2=1.Olb/sf0.30m =1.Oft

10

wing section

81-girder

C

^ a 6o ^. box girder

zas

4 -T-girder

2

50

100 150(ft)

Span Length

Fig. 21, Volume of concrete for various girder types.

tal box girder construction will have tocompete with other types of construc-tion which have shown economies inthese particular span ranges. For thatpurpose a comparative study was madebetween the box section and other sec-tional forms.

Comparative Studies ofBridge Sections

An analytical study produced thecomparison of quantities for the boxsection, I-girder, T-section, and wingsection, as shown in Fig. 20.

PCI JOUR NAL/September-October 1983

Fig. 21 plots the volume of concreteversus span length, showing that thevolume is lowest for the T-section,There is a cross-over point at about 90 ft(27 m) between the box section and theI-section. Comparing the weight of lon-gitudinal post-tensioning steel, asshown in Fig. 22, the I-section is low-est, but there is a cross-over point atabout 110 ft (33 m) with the box section.Comparing the mild steel required, theT-section is again lowest (see Fig. 23).The I-section and the box section havea cross-over point at about 85 ft (26 m).Assessing the cost, as shown in Fig, 24,

73

47.9N/m2= 1.0lbisf0.30m=1.Oft

6

5

4 w

N N- m 3N ^Q)

5-

0.c 2a)

rder

50 100 150(ft)

Span Length

Fig. 22. Weight of longitudinal prestressing steel for various girder types.

the T-girder appears to be the mosteconomical in the lower spans. The boxsection becomes most economicalabove

pci journal - 1983 - sep-oct

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