Engineering Structures 23 (2001) 94–101www.elsevier.com/locate/engstruct

Experimental investigation into using lead to reduce vertical loadtransfer in infilled frames

M.K. Sahota, J.R. Riddington*

University of Sussex, School of Engineering and Information Technology, Falmer, Brighton BN1 9QT, UK

Received 7 February 1998; accepted 17 July 1999

Abstract

Reinforced concrete structural frames can be infilled with masonry to produce structures that will effectively resist in-planeracking loading. Creep and shrinkage of the columns can however result in vertical load transfer onto the infill that is difficult topredict and which varies with time. Results from an experimental investigation into the use of a copper–tellurium lead layer toreduce this load transfer are presented. These results indicate that a lead layer can cause a significant reduction in load transfer,whilst not causing any deterioration in the short term racking performance of the infilled frame, and that the load transfer onto theinfill can be predicted using finite element analyses. 2000 Elsevier Science Ltd and Civil-Comp Ltd. All rights reserved.

Keywords:Lead; Infilled frames; Creep; Racking; Masonry; Brickwork; Reinforced concrete

1. Introduction

An efficient and effective way of bracing a structureto resist in-plane horizontal shear loading is to constructit using infilled frames. In an infilled frame the wall andframe act together, with the masonry wall acting as adiagonal brace, to produce a structure that can be bothstiff and strong. However in order to act as an efficientbrace the infill has to be a tight fit in the frame. Unfortu-nately having a tightly fitting infill can be a problemwhen the frame is formed from reinforced concrete. Thecolumns of such a frame tend to shorten as a result ofcreep and shrinkage, whilst clay brick walls can expand.The column creep is caused by the long term compress-ive load that always acts on columns. This change inlength of the columns relative to the infill height canresult in very significant vertical load transfer from thecolumns onto the infill; the amount of which cannot becalculated with any degree of confidence since creeprates and the amount of expansion of the infill cannotbe accurately predicted. The amount of load transferredonto the columns will also change with time and willbe affected by the age of the frame when the infill is

* Corresponding author.E-mail address:j.r.riddington@sussex.ac.uk (J.R. Riddington).

0141-0296/01/$ - see front matter 2000 Elsevier Science Ltd and Civil-Comp Ltd. All rights reserved.PII: S0141-0296 (00)00025-0

constructed. Although the resulting compression of theinfill due to the load transfer may increase its strengthwith regard to its possible failure due to shear and diag-onal tension, it may also result in the crushing of theloaded corners of the infill at a reduced lateral load. Theload transfer also has an adverse affect on the beams ofthe frame, as a result of shear forces that are induced inthem at their connections with the columns. Since thepossible benefits of the load transfer cannot be takenadvantage of when designing infilled frames, due to thelevel of uncertainty associated with the load transfer,whilst allowance should be made for the possible nega-tive effects, the authors believe that it would be desirableto reduce if possible the amount of the load transfer.

Previous research [1] has indicated that a reduction inload transfer might be achieved by incorporating a layerof a visco-elastic material such as lead between the topof the wall and the underside of the top beam of theframe, as shown in Fig. 1. To be effective this layer hasto behave elastically when the structure is subjected tolateral loading, which is by its nature short term loading(wind and earthquake), whilst being able to creep suf-ficiently to accommodate the difference in movement ofthe columns and the infill, which occurs over a muchlonger period of time.

This paper presents the results from an experimentalinvestigation in which infilled frames were tested with

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Fig. 1. Corner section of an infilled frame containing a lead layer.

and without a lead layer, in order to assess the effective-ness of using a lead layer to reduce load transfer ontothe infill as a result of column shortening. Three half-scale infilled frames were tested, of which two containeda lead layer. The frames were tested in two stages. Inthe first the columns were progressively shortened inorder to investigate the effectiveness of the lead in pre-venting load transfer onto the infill. In the second theframes were subjected to short term racking loading soas to check whether a lead layer adversely affects aninfilled frame’s racking stiffness or strength.

The results from finite element analyses of the frameswhen subjected to column shortening are also presented.The objectives of this work were twofold, the first todetermine whether finite element analyses are capable ofpredicting the influence of a lead layer on the behaviourand the second to determine the influence of the infillproperties on the behaviour.

2. Infilled frames tested

Three half-scale square masonry infilled steel frameswith overall dimensions of 1890 mm×1890 mm weretested. The masonry infill was formed from three holeperforated clay engineering bricks manufactured byRedland Bricks, with nominal dimensions 215×102.5×65mm, together with mortar joints that were nominally10mm thick. A designation (ii) mortar [2] was usedwhich was formed from masonry cement and buildingsand in a weight ratio of 22:77. The frames were formedfrom Grade 43 steel [3] joist sections 152 mm×89 mm.The frames were formed of steel rather than reinforcedconcrete because the method used to simulate the short-ening of the columns as a result of shrinkage and creepput the columns into tension. Using a steel rather thana concrete frame resulted in different interface propertiesbetween the infill and frame in addition to differencesin their elastic properties and cross-sectional areas. Interms of what was being investigated with these tests,

the effects of these difference was not considered to besignificant. The steel sections were welded together toform the frames with the corners of each frame beingstiffened by welding plates 100 mm×100 mm×25 mmto each side of the frame.

It is known that the behaviour of lead varies greatlywith the type and quantity of the alloying metals. A sep-arate investigation by Riddington and Sahota [4] hasindicated that an alloy containing 0.06% copper and0.04% tellurium might be the most suitable alloy for thisparticular application. Consequently this alloy of leadwas used in this investigation, with it being extruded into5 mm thick layers for inclusion in the infilled frames.Longitudinal grooves were incorporated in the layers, asshown in Fig. 2, in order to reduce the effect of the lat-eral restraint provided by the beam above and the infillbelow it, on the creep rate of the lead. Prior to use, thelayers were preloaded to 20 N/mm2, since it had beenfound [4] that this work hardening improved the elasticproperties of the material. The lead layer was attachedto the underside of the top beam of the frame using acontact adhesive prior to the construction of the infill.

The columns of the frames were designed so that theycould simulate the shortening of concrete columns thatoccurs as a result of creep and shrinkage. This was achi-eved by splitting the columns of the frame in half andthen connecting the two halves with a threaded rod. Onone side of the gap the rod had a thread with a pitch of2.0 mm, whilst on the other side the rod had a threadwith a pitch of 2.25 mm. This small difference in pitchprovided a means for shortening the columns by smalland controlled amounts. One full revolution of the rodproduced a closure of 0.25 mm in the gap in the column.To enable the rod to be turned a 40 mm nut was weldedto the rod at the point where the pitch of the threadchanged. A calibrated dial was also attached to the rodto enable small amounts of rotation to be measured accu-rately against a pointer attached to the column.

3. Infill material properties

Average values of mortar properties found by testingmortar specimens at the time when the infilled frameswere tested in racking are given in Table 1. Compressiontests of mortar cylinders were undertaken in order toobtain the compressive strength, elastic modulus and

Fig. 2. Cross section of the lead layer.

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Table 1Average values of mortar properties

Compressive strength (N/mm2) 14.0Elastic modulus (kN/mm2) 9.21Poisson’s ratio 0.19Tensile strength (N/mm2) 2.0

Poisson’s ratio values for the mortar. The tensile strengthof the mortar was obtained from split cylinder testresults. The cylinder specimens were 40 mm in diameterand 80mm long and they were formed from mortar thathad been laid between two bricks for ten minutes. Testshave shown [5] that this method of specimen productioncan produce property values that are reasonably rep-resentative of the properties of the mortar actually inthe joints.

Tests were undertaken to obtain various properties ofthe bricks, the brickwork and the joint between the infillmortar and the steel frame. The results are shown inTable 2.

The elastic moduli of the bricks and the brickworkwere found from stress–strain results obtained fromcompressive tests on bricks and brickwork stacksrespectively. The joint bond tensile strength value wasobtained from the results given by direct tensile tests onbrick couplets, with the value being taken as 1.9 timesthe ultimate load divided by the cross-sectional area ofthe bed joint. This multiplication factor [5] accounts forthe uneven stress distribution along the joint when acouplet is subjected to the form of loading used. Thestress value obtained is the maximum stress acting acrossthe joint at failure if it is assumed that a brittle form offailure occurs. The value obtained (0.33 N/mm2) closelymatched the value (0.34 N/mm2) given by four pointflexural tests that were undertaken on nine brick highstack specimens. The flexural test value is the maximumtensile stress acting across the joint at failure if a stressdistribution given by simple bending theory is assumed.The bond shear strength and coefficient of internal fric-tion were found from results obtained from triplet testson specimens, using the Coulomb relationship:

t5t01msc

where: t is the shear strength,t0 is the bond shearstrength,m is the coefficient of internal friction of the

Table 2Average values of brick, brickwork and infill-frame joint properties

Brick elastic modulus (kN/mm2) 22.1Brickwork elastic modulus (kN/mm2) 15.4Brickwork joint bond tensile strength (N/mm2) 0.33Brickwork joint bond shear strengtht0 (N/mm2) 0.31Brickwork joint coefficient of internal frictionm 0.88Mortar to steel coefficient of friction 0.61

mortar joint andsc is the stress normal to the mortarjoint.

The coefficient of friction between the infill mortarand steel frame was obtained from the results of triplettests conducted on specimens formed from a smalllength of the steel frame section sandwiched betweentwo bricks using mortar joints. For these specimens bondshear strength was found to be negligible.

4. Long term creep tests

As stated earlier the infilled frames were tested in twostages. During the first stage the columns were progress-ively shortened over a period of approximately threemonths in order to investigate the effectiveness of a leadlayer in reducing load transfer from a frame to an infillwhen the columns of a frame shorten as a result of creepand shrinkage. The frames were then left for a furtherthree months in order to determine what dissipation ofload occurred when the length of the columns was notchanged. Two of the three infilled frames tested includeda lead layer. The rates of shortening applied to the col-umns of the three infilled frames were as follows:

O Frame 1 — without lead: shortened by 0.0125mm/day

O Frame 2 — with lead: shortened by 0.0125 mm/dayO Frame 3 — with lead: shortened by 0.0050 mm/day.

These shortening rates were determined after consider-ation of the extensive literature on reinforced concretecreep and shrinkage rates. The literature indicates thatthe 0.0125 mm/day shortening rate is close to the upperlimit of the straining rate that any reinforced concretecolumn is likely to experience in practice. The authorsare only aware of one example where a faster strainingrate has been measured in a full sized structure. Samra[6] reported a case where the measured straining rateover a 90 day period was slightly greater than that givenby the 0.0125 mm/day shortening rate. In this case theloading on the columns of a water tower was beingincreased at a particularly fast rate, due to the rapid rateof construction of the structure. Even the 0.005 mm/dayshortening rate used with Frame 3 produces a strainingrate which is relatively high for a concrete column. Theshortening rate was not increased to account for theextension of the columns that resulted from the tensileload generated in the columns. Calculations based on theloads that were generated in the test columns indicatethat this extension was less than 15% of the appliedshortening.

Electrical strain gauges were mounted along the centrelines of the columns of the steel frames and these wereused to monitor the build up of load in the columns andhence the load in the infills. The load data was recorded

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twice daily by means of a data logger. The results forthe loads developed in the columns during this first stageof testing are shown in Fig. 3.

Fig. 3 clearly shows that for the infilled frames testedwith the same rate of column shortening, the frame with-out the lead layer developed much higher axial forces inits columns than the infilled frame with the lead layer.This clearly shows that a lead layer can be used to reduceload transfer onto the infill of an infilled frame. The dif-ference in the loads transferred to the infills for thesetwo frames was not however as large as had beenexpected. This was because the load generated in theframe without the lead layer was lower, whilst the loadgenerated in the frame with lead was somewhat higher,than had been expected. Detailed finite element analysesof the frames, which will be described later, indicatedthat the lower than expected load in the frame withoutlead was a result of the flexibility of the mortar used toconstruct the infill. A structural infill would normally beconstructed using a high strength and stiffness mortar.Such a mortar was not used for the tested infills becauseit was thought that if it was used, the resulting infillswould have been too strong to be taken to failure whenthe frames were tested in racking, using the apparatusthat was available. The higher than expected load in theinfill with the lead layer was related to the alloy of leadused and the profile of the lead layer. The copper–tel-lurium alloy was chosen because of its good elasticproperties and resistance to recrystallisation after workhardening. Its creep rate when subjected to compressiveloading is however relatively low compared with otherlead alloys. If required, the amount of load transfer could

Fig. 3. Load developed in the columns as they were shortened.

be reduced by using a different load alloy or by usinga different profile or thickness of lead layer. It will beshown later that finite element analyses can be used topredict reasonably accurately the load transfer.

The results shown in Fig. 3 also indicate an increasein the rate of load transfer onto the infills for all threeframes during the last month when the columns werebeing shortened. This period corresponded with the onsetof winter and it is thought that the increase can be attri-buted to the expansion of the clay bricks during this per-iod, as result of the increased humidity levels.

Fig. 3 also shows the load in the columns of the threeframes during the three months after the shortening ofthe columns had stopped. Comparing the two infilledframes that had been shortened at the higher rate, it canbe seen that the drop off rate was much higher in theinfilled frame with lead.

5. Short term racking tests

The purpose of these second stage tests was to deter-mine whether the inclusion of a lead layer into an infilledframe adversely affects its stiffness and strength whenit is subjected to in-plane lateral loading. The infilledframes that had been used for the long-term creep testswere also used for these short-term racking tests. Theload that had been applied to the infilled frames by theshortening of the frame columns, which was different ineach frame, was taken off prior to the racking testing ofthe frames. No information on the influence of the leadlayer on racking strength would have been obtained from

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the tests if the frames had been subjected to these dif-fering levels of precompression, since it had alreadybeen established [1] that prestressing the columns of aninfilled frame can affect its racking strength.

The infilled frames were tested under cyclic lateralload with the load being applied by two hydraulic jacks,one at either end of the frame. The load was measuredby two 500 kN load cells that were connected to a datalogger. In addition to monitoring the applied load,measurements were also made of the forces generatedin the frame members using strain gauges and of thelateral deflection of the frame. The loading and supportarrangement is shown in Fig. 4. Pins passed througheach corner of the frame. The top pins were attached tothe hydraulic jacks by threaded rods in an arrangementthat did not permit compressive loading to be carried.To create the cyclic loading the two jacks were operatedin turn, so as to apply an alternating horizontal tensionload to the top corners of the frame. At the bottom cor-ners the pins were attached to vertical threaded rodswhich were fixed to the bottom beam of the test framein a manner that allowed both compression and tensionto be carried. The bottom pins were also fixed to hori-zontal threaded rods. These were attached to the verticalmembers of the test frame in a manner so that only ten-sion loading could be carried. As a result of this arrange-ment, when a horizontal tension load was being applied

Fig. 4. Racking tests loading arrangements.

by a jack at one top corner, a vertical compression reac-tion load was generated at the bottom corner below theloaded corner, and vertical and horizontal tension reac-tion loads were generated at the bottom corner diagon-ally opposite the loaded corner.

The load was first applied in small increments to oneside of the infilled frame. The load was then taken offin increments before being applied in the opposite direc-tion to the other side of the frame. Further cycles of loadwere then applied with the maximum load being appliedeach cycle being increased until complete failure of theinfill occurred. At every increment of load, the loadvalue, axial load in the frame members and framedeflection were recorded. During the loading any crack-ing that occurred was recorded.

Only the results from Frames 1 and 2 are presentedbecause Frame 3 failed prematurely as a result of a weld-ing failure. It was found that Frame 1 (without lead) andFrame 2 (with lead) failed at a very similar ultimate load,

Table 3First crack and ultimate load values

Infilled frame Initial cracking load Ultimate load

1. Without lead 80 kN 245 kN2. With lead 140 kN 250 kN

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as shown in Table 3, and in a very similar pattern, asshown in Fig. 5(a) and (b).

As shown in Table 3, the first tension crack appearedin the frame without lead at a much lower load than inthe frame with lead. As a result the infilled frame withthe lead layer was stiffer than the frame without lead at

Fig. 5. (a) Crack pattern at failure for the frame without lead. (b)Crack pattern at failure for the frame with lead.

higher load levels, as can be seen in the load–deflectiongraphs shown in Fig. 6(a) and (b).

Overall it can be concluded that the inclusion of thelead layer had no adverse effect on the racking perform-ance of the infilled frame tested.

6. Finite element analysis of the infilled frames

The ANSYS finite element package was used to ana-lyse the infilled frames with and without a lead layer,when they were subjected to long term column shorten-ing. The reason for undertaking these analyses was todetermine whether the behaviour of infilled frames con-taining a lead layer could be predicted with a reasonabledegree of accuracy; since if it could, it would permitfurther research work on the topic to be undertaken usingfinite element simulations.

The problem is complex because as the columns

Fig. 6. (a) Load–deflection graph for the frame without lead. (b)Load–deflection graph for the frame with lead.

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become shorter, the top and bottom beams of the frametend to bend away from the infill, remaining in contactwith the infill only close to the columns. The length ofcontact and stress distribution along this contact lengthdepend on many factors, which include the axial load inthe columns, the bending stiffness of the frame, the elas-tic properties of the lead and infill materials, and theamount of creep that has occurred in the lead.

The infill and the lead layer were represented by fournode plane stress rectangular elements, whilst two nodeoffset beam elements were used to represent the framemembers. Two node contact elements were introducedalong the infill–beam and infill–column interfaces inorder that the separation of the frame members from theinfill that occurs at certain points along their interfacecould be simulated. The top and bottom layers of mortarin the infill were also represented separately in the analy-ses using four node plane strain rectangular elements.This was because it was found that the stiffness of themortar in the corners of the infill significantly affectedthe analysis results when there was no lead layer. Planestrain elements were used because it was felt that theybetter represented the constraint conditions that affectthese mortar layers. The mortar is restrained fromspreading out of the plane by the bricks and the steelbeam above and below it.

The elements used to represent the lead layer werecapable of simulating creep. The creep characteristics

Fig. 7. Comparison between measures and computed axial forces inthe columns for Frame 3.

Fig. 8. Finite element prediction of load generated in the columns if there were to be a stiffer infill.

that were used in the infilled frame analyses wereobtained from separate finite element creep analyses ofthe profiled layer, using creep data obtained from creeptests on lead cylinders [7]. The shortening of the col-umns was simulated in the analyses by applying pro-gressively thermal strain to the columns.

It was found that the build up of load in the columnswas predicted reasonably accurately for all three infilledframes. The results for the frame with lead, shortenedat the slowest rate, is shown in Fig. 7. As previouslymentioned, the increase in the load transfer rate recordedduring the last month whilst the columns were beingshortened, was attributed to an expansion of the infillmaterial, caused by moisture take-up at the onset of win-ter. No attempt was made to model this expansion inthe analyses.

As mentioned previously, the tested infills were con-structed using a mortar with a lower stiffness andstrength than might be expected to be used in practice.To investigate the effect of having a higher stiffness andstrength mortar, analyses were undertaken on infilledframes where the mortar and infill elastic moduli wereincreased to the relatively high values of 19.4 and 24.6kN/mm2, respectively. These were values that had beenobtained in another test programme [8], when a desig-nation (i) mortar [2] (1 part cement to 1/4 part lime to3 parts sand by volume) had been used with a solid clayengineering brick. The specimen forming and testingmethods described in Section 3 had been used to obtainthese values. The results from these analyses for the loadthat would have been generated in the columns, if thismortar type had been used, and the columns had beenshortened at a rate of 0.0125 mm/day, are shown in Fig.8. Comparing Fig. 8 with Fig. 3, it can be seen that thedifference in the load generated between the frame with-out, and the frame with a lead layer, is much greaterwhen a higher strength mortar is used. Increased mortarand infill stiffness results in reduced contact lengthsbetween the infill and the lead layer and as a conse-quence higher stresses and creep rates in the lead.

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7. Conclusions

It is concluded that:

1. The inclusion of a lead layer in an infilled frame struc-ture will reduce significantly the compressive loadthat is transferred onto the infill as a result of col-umn shortening.

2. In the tests undertaken, the inclusion of the copper–tellurium lead layer did not have any adverse effecton the racking performance of the infilled frame.

3. Finite element analyses were capable of predictingwith a good degree of accuracy the vertical load trans-fer that occurred in the infilled frames tested, whenthey were subjected to column shortening.

Acknowledgements

The authors wish to acknowledge the support for thisresearch given by the International Lead Zinc Research

Organisation, the Lead Development Association, Bri-tish Lead Mills, Redland Bricks and the Engineering andPhysical Sciences Research Council.

References

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[2] British Standards Institution, BS5628:Part 1:1992, Code of prac-tice for structural use of masonry: Part 1. Unreinforced masonry.London: BSI, 1992.

[3] British Standards Institution, BSEN10025:1990, Specification forhot rolled products of non-alloy structural steels and their technicaldelivery conditions. London: BSI, 1990.

[4] Riddington JR, Sahota MK. Stability of lead alloy compressivework hardening. Materials and Design 1999;20(1):13–7.

[5] Riddington JR, Jukes P. Determination of material properties foruse in masonry FE analyses. Proc British Masonry Society1995;7(2):314–9.

[6] Samra RP. New analysis for creep behaviour in concrete columns.Journal of Structural Engineering 1995;121(3):399–407.

[7] Sahota MK, Riddington JR. Compressive creep properties of leadalloys. Materials and Design 2000;21(3):159–67.

[8] Jukes P. An investigation into the shear strength of masonry joints.DPhil. thesis, University of Sussex, 1997.