Behavior and strength of steel

Journal of Constructional Steel Research65(2009)1569–1581Contents lists available at ScienceDirect Journal of Constructional Steel Research journal homepage:/locate/jcsrBehavior and strength of steel reinforced concrete beam–column joints with single-side force inputsCheng-Cheng Chen a,Budi Suswanto a,b,∗,Yu-Jen Lin ca Department of Construction Engineering,National Taiwan University of Science and Technology,No.43,Sec.4,Keelung Road,106,Taipei,Taiwan,ROCb Department of Civil Engineering,Sepuluh Nopember Institute of Technology,Surabaya,Indonesiac Taiwan Building Technology Center,National Taiwan University of Science and Technology,Taipei,Taiwan,ROCa r t i c l e i n f o Article history:Received22February2009 Accepted13April2009 Keywords:Steel reinforced concrete(SRC) Beam–column jointsShear strengthSuperposition method a b s t r a c tFive large-scale beam–column subassemblies were fabricated and tested under cyclic loading to investigate the behavior of SRC Type I exterior and Type II corner beam–column joints.In addition,the applicability of strength superposition method on joint shear strength was assessed.It was found that: (1)the strength superposition method was able to estimate the SRC beam–column joint shear strength with reasonable accuracy;(2)the anchorage position of beam longitudinal bars has an obvious influence on the joint shear strength and crack pattern;(3)increased depth of cross-sectional steel leads to a higher shear strength for the beam–column joint;and(4)a combination of corner stirrups and shaped steel cross-sections was able to provide sufficient lateral support to longitudinal steel bars and adequate confinement to the concrete in the joint to replace the need for closed hoops.©2009Published by Elsevier Ltd1.IntroductionThe framework of steel reinforced concrete(SRC)is typically comprised of steel structural members characterized by a cross-sectional shape and accompanied by longitudinal and transverse steel bars for further reinforcement.The beam–column joints of SRC moment resisting frames(MRFs)bear significant shearing forces when subjected to earthquake type loading.Thus,the shear design of beam–column joints is an important aspect in the seismic design of SRC MRFs.Reinforced concrete(RC)beam–column joints are classified by ACI-ASCE352[1]as interior,exterior,and corner joints(where the latter pair each consists of two subtypes)as shown in Fig.1. The results of Chen and Lin[2]indicate many similarities in the appearance and mechanical behavior of SRC and RC beam–column joints;hence,the existing RC beam column-joint classification method is adopted here for SRC structures.Research into SRC beam–column joints was initiated by Wakabayashi[3,4]in Japan during the1970s.The structural members used for the columns mainly had wide flanged steel cross-sectional shapes.Teraoka et al.[5]examined seven SRC beam–column subassemblies with cross-H steel columns.Their research was mainly geared towards examining the effects ∗Corresponding author at:National Taiwan University of Science and Technol-ogy,Construction Engineering No.43,Sec.4,Keelung Road,106Taipei,Taiwan,ROC. Tel.:+886227376589;fax:+886227376606.E-mail address:budisw2000@(B.Suswanto).of their proposed diaphragm scheme.Chen and Lin[2]also carried out cyclic loading tests on five large-scale cruciform type beam–column subassemblies with two-side force inputs,as shown in Fig.2(a).Based on the test results and utilizing the concept of strength superposition,a joint shear strength evaluation method was proposed and shown to estimate the interior and Type II exterior joint shear strength with reasonable accuracy.Shear-oriented research into other types of joints is still pending.In this study,five large-scale beam–column subassemblies containing either SRC Type I exterior or Type II corner joints were fabricated and tested under cyclic loading conditions to investigate joint behavior.The applicability of the joint shear strength evaluation method proposed by Chen and Lin[2]to these joints was also examined.Since,shear force is transferred to the joint from only one side of the column,as illustrated in Fig.2(b), these cantilevered systems are said to experience‘‘single-side force inputs’’.2.Experimental program2.1.Test specimensThe systems detailed in Table1consisted of five beam–column subassemblies which included four of SRC composition and one of pure steel.The geometry and dimensions of the test specimens are shown in Fig.3;it is assumed that the zero-moment inflection points were located at the mid-span of beams and the mid-height of columns.The subassemblies,along with boundary and loading conditions,are designed to simulate part of a frame subjected to0143-974X/$–see front matter©2009Published by Elsevier Ltd doi:10.1016/j.jcsr.2009.04.0031570 C.-C.Chen et al./Journal of Constructional Steel Research65(2009)1569–1581(a)Interior joint.(b)Exterior joint I.(c)Exterior joint II.(d)Corner jointI.(e)Corner joint II.Fig.1.Types of beam–columnjoints.(a)Two-side force inputs.(b)Single-side force inputs.Fig.2.Beam–column joints with two-side and single-side force inputs.Table1an earthquake-induced moment.To facilitate the analysis of thebehavior and shear strength of the beam–column joints,thesesystems were designed in such a way that the joints are likely tofail first.A combination of thick beam flanges(32mm)and thincolumn flanges(9mm and11mm)was chosen to achieve this.The dimensions of the SRC column sections were500mm×500mm,as depicted in Fig.4(a)and Fig.4(b).H-shaped steelcruciform cross-sections,designated XH1(or XH300×150×6.5×9)and XH2(or XH396×199×7×11),comprised each column.The XH1units consisted of two wide flanged steel cross-sections,individually designated H1(or H300×150×6.5×9),intersectingperpendicularly at the mid-height of the web;XH2was similarlycomposed,with two mutually bisected H2(or H396×199×7×11)cross-sections.Both H1and H2were ASTM A36hot rolled shapes.Four#10longitudinal steel bars and#4transverse hoops with aspacing of120mm were used for all SRC columns.The steel barsused were rated ASTM A706Grade50,with a nominal yield stressof345MPa.The joint area specimen details are indicated in Fig.5.Headedlongitudinal steel bars were used in the beam;the anchorageposition A1for SRC-XH1,SRC-XH1-TB,and SRC-XH2was fixedjust behind the column’s transverse web,giving a developmentlength of280mm or8.8d b.The anchorage position A2for SRC-XH2-A2was located at the column’s transverse flange,giving adevelopment length of450mm or14d b.For the SRC specimens,corner stirrups were used in the jointrather than the closed hoops to provide both lateral supportto the longitudinal steel bars,and confinement of the concrete.Four layers of#4corner stirrups with standard180◦hooks wereincorporated into the joint area,as illustrated in Fig.6.Thecorner stirrups were specially designed for the joints to minimizepotential difficulties associated with placing closed hoops.The SRC beam sections measuring400mm×520mm shown inFig.7(a)and Fig.7(b)were used as longitudinal beams for SRC-XH1,SRC-XH1-TB,SRC-XH2,and SRC-XH2-A2.For the transverse beamsSRC-XH1-TB,400mm×580mm SRC beam sections were used asdetailed in Fig.8.Wide flanged steel shapes H3(or H300×120×19×32)and H4(or H300×160×19×32)were built with ASTMA572Grade50steel plates.Eight#10longitudinal steel bars andC.-C.Chen et al./Journal of Constructional Steel Research 65(2009)1569–15811571(a)Elevationview.(b)Top view.Fig.3.Geometry and dimensions of test specimens (units:mm).(a)SRC-XH1and SRC-XH1-TB.(b)SRC-XH2and SRC-XH2-A2.(c)S-XH2.Fig.4.Column sections used (units:mm).#4transverse hoops with a spacing of 120mm were used for all the longitudinal SRC beams.In the transverse SRC-XH1-TB beams,#4transverse hoops were placed at 120mm intervals along with four #8longitudinal steel bars.Complete joint penetration welds were used to connect the beam web and flanges to the column flange,as outlined in Fig.9.Diaphragm plates with a thickness of 32mm were inserted into thecolumn at the level of each beam flange weld to stiffen the joint.The mechanical properties of the steels used are listed in Table 2.2.2.Test setup and procedureThe test setup is delineated in Fig.10.The column of the specimen was clamped to a floor beam that was tied down to a1572 C.-C.Chen et al./Journal of Constructional Steel Research65(2009)1569–1581(a)SRC-XH1.(b)SRC-XH1-TB.(c)SRC-XH2.(d)SRC-XH2-A2.(e)S-XH2.Fig.5.Details of specimens in the jointarea.Fig.6.Positioning of four layers of#4corner stirrups.strong floor.The top end of the specimen beam was connected toa servo-controlled actuator with a capacity of1000kN which wasin turn mounted to a strong wall;it was equipped with a built-in load cell to measure the applied load,P.Axial load–definedin terms of the column cross-sectional area,A g,and the nominalconcrete compressive strengthfcn=27.4MPa–was appliedto the column by a jack affixed to horizontal supports,andwas(a)SRC-XH1and SRC-XH1-TB.(b)SRC-XH2and SRC-XH2-A2.Fig.7.Longitudinal beam sections used(units:mm).measured by another load cell.A load equal to0.1A gfcnwasapplied at the beginning of the load test and was held constantthroughout the test.A linear variable differential transformer(LVDT)was connectedto the beam at the level of the actuator to measure the displace-ment at the load point.The corner stirrups were equipped with asingle-axis strain gauge as shown in Fig.11(a),and the longitudinalweb and flange of steel column sections were equipped with3-axisrosettes as shown in Fig.11(b).All specimens were loaded underdisplacement control,thus uniformly generating the same driftC.-C.Chen et al./Journal of Constructional Steel Research65(2009)1569–15811573Fig.8.Transverse beam of SRC-XH1-TB(units:mm).Table2Mechanical properties of steels used.angle(θ)history presented in Fig.12.The target displacement atthe load point,∆,was computed using the following equation:∆=θ×L b+h c2.(1)3.Experimental results3.1.General behavior3.1.1.Load versus displacement hysteresis loopsA total of14cycles were applied to SRC-XH1,with a maximumdrift angle of8%.SRC-XH1-TB,SRC-XH2,and SRC-XH2-A2weresubjected to12cycles at a maximum drift angle of6%.S-XH2was subjected to10cycles at a maximum drift angle of4%.The load versus displacement hysteresis loops for all specimensare illustrated in Fig.13.The specimens were basically withintheir elastic limits across the0.25%to1%drift angle range,butexperienced stable yielding between2%and4%.The drift anglescorresponding to maximum loads in the positive and negativedirection,P+peakand P−peak,are denotedα+peak andα−peak,respectively.All the specimens demonstrated load maxima at drift anglesα+peakandα−peak in excess of3.0%,with the notable exception of SRC-XH1-TB.The relevant test results are summarized and listed in Table3.The maximum load for the tested specimen,P t,is defined as themaximum value between P+peakand P−peak.Thus,despite SRC-XH1-TBhaving failed somewhat prematurely,its P t is at least3.0%higherthan that of SRC-XH1;it seems that transverse beams provide somecontribution to the joint shear strength.The6%P t increase in SRC-XH2-A2relative to SRC-XH2is attributed to the increased steel bardevelopment length in the former specimen.For SRC-XH1,SRC-XH2,and SRC-XH2-A2,after the first4%of the drift angle cycle,thestrength of each specimen was observed to deteriorate graduallywith a sudden load drop observed during the positive sweep of thesecond cycle at+4%.3.1.2.Crack patternIn Fig.14,the cracks and flaking patterns that evolved in allthe specimens during testing are shown.For SRC-XH1,SRC-XH2,and SRC-XH2-A2,cracking of the concrete in the joint was firstobserved during the0.5%drift angle shift.The diagonalcracks(a)Elevation view.(b)Top view.Fig.9.Welding connection detail for the XH column with a wide flanged beam.Table31574 C.-C.Chen et al./Journal of Constructional Steel Research 65(2009)1569–1581Fig.10.Test setup for allspecimens.(a)Strain gauge on corner stirrups.(b)Strain gauge on steel shape.Fig.11.The position of strain gauges on corner stirrups and steel cross-sections.+ 1%+ 2%+ 4%+ 6%+ 8%+ 10%+ 0.25%+ 0.5%D i s p l a c e m e n t (m m )-8-6-4-202468-1010D r i f t a n g l e (%)-264-211-158-106-52.8052.81061582112642468101214No. of cycle16Fig.12.Loading history.became longer and wider as the drift angle swept from 1%to 4%;two major diagonal cracks were identified,each emanating from a corner of the joint and extending diagonally to the exterior column surface.The concrete crack patterns of these SRC beam–column joints are similar to those of the 3T3specimen RC beam–column joint documented by Hwang et al.[6].The anchorage position of beam longitudinal bars had an obvious influence on the diagonal joint crack pattern;its effect isclearly illustrated in Fig.15.Stressed beam–column joints with the anchorage position A2(i.e.longer development length)possessed steeper diagonal cracks than those of joints with the anchorage position A1(i.e.shorter development length).Fig.16shows the concrete crack pattern of an SRC Type II exterior joint specimen from Chen and Lin [2]where two major diagonal cracks were identified,each starting in one corner of the joint and extending diagonally to the opposing corner.The crack pattern of this SRC Type II exterior joint is similar to that found in SRC-XH2-A2.For SRC-XH1-TB,cracks at the end surface of the transverse beam were first observed during the 0.5%drift angle stages.The diagonal cracks became wider and longer through the 1%and 4%drift angles,but were not severe.This ameliorated diagonal cracking was considered to be a result of partial shear load transfer to the transverse beams.For the S-XH2specimen,a welding connection in the joint was observed to crack during the first +4%drift angle cycle.When the crack occurred,the load suddenly dropped from +184.1to +106.5kN and the column web was subsequently observed to crack in the joint area.3.1.3.Failure modeThe combination of thick beam and thin column flanges caused specimens to fail at the beam-flange-to-column-flange connection due to heat buildup in the column flange.However,except in the case of SRC-XH1-TB,the beam–column joint in the specimensC.-C.Chen et al./Journal of Constructional Steel Research65(2009)1569–15811575 Fig.13.Load versus displacement hysteresis loops.experienced significant plastic deformation before failure of the connection.The web cross-sections experienced substantial yield and shear deformation with the latter considered to be the main inducement to fracture in beam-flange-to-column-flange welds.For SRC-XH1,SRC-XH2,and SRC-XH2-A2,connection failure occurred after significant joint yielding and extensive diagonal crack development.In contrast,for SRC-XH1-TB,beam-to-column connection failure occurred after only moderate joint yielding,1576 C.-C.Chen et al./Journal of Constructional Steel Research65(2009)1569–1581(a)SRC-XH1specimen.(b)SRC-XH1-TB specimen.(c)SRC-XH2specimen.(d)SRC-XH2-A2specimen.(e)S-XH2specimen.Fig.14.Crack and flaking patterns of specimens after testing.(a)SRC-XH2specimen.(b)SRC-XH2-A2specimen.Fig.15.Effect of anchorage position on diagonal cracks at the joint.C.-C.Chen et al./Journal of Constructional Steel Research 65(2009)1569–15811577Fig.16.Crack pattern of Chen and Lin’s SRCspecimen.Fig.17.Fracture at welding connection and column web of S-XH2specimen.resulting in significant concrete spalling.The experimental joint strength of the SRC-XH1-TB specimen was designed to be somewhat lower than expected in real structures.Connection failure occurred for S-XH2specimen only after significant web yielding and was somewhat premature in the positive direction;the peak load value may therefore slightly underestimate the specimen strength.Fractures at the welding connection and column web of an S-XH2specimen are shown in Fig.17.3.1.4.Corner stirrupFig.18indicates the maximum strain readings on corner stirrups at peak displacements.Corner stirrups behaved elastically throughout the test and the maximum strain measured for the SRC-XH2specimen was 0.00177,clearly lower than the yield strain,εy =0.00196.Chen and Lin’s research [2]into SRC specimens which contained both interior and Type II exterior joints indicated that the corner stirrups work well without exhibiting any negative behavior.Results pertaining to SRC specimens containing Type II corner and Type I exterior joints have also suggested that the corner stirrups work well.Corner stirrups in tandem with the specified steel cross-sections are considered to provide sufficient lateral support to the longitudinal steel bars and adequate confinement to concrete in the joint.Corner stirrups can be used to replace closed hoops,eliminating the possible difficulties encountered in their placement.3.2.Beam–column joint shear strengthWith reference to Fig.19(a),the bending moment of the beam at the column face,M beam ,is defined by Eq.(2):M beam =P t ×L b .(2)The shear force,V b ,transferred through the beams to the joint can be obtained via flexural analysis of the beam section experiencing a bending moment.The XTRACT program [7]was used to carry out the analysis,and V b was obtained by integration of the tensile and compressive stresses in the section.The column shear force,V col ,can be obtained using Eq.(3):V col =P tL b +h c 2L c.(3)The joint shear strength,V t ,is defined by referring to Fig.19(b):V t =V b −V col .(4)The actual concrete compressive strength f c and defined para-meters M beam ,V col ,V b ,and V t are listed for all specimens in Table 4.4.Shear strength evaluationThe most convenient and commonly used shear strength evaluation method for a beam–column joint is the strength su-perposition method.The shear strength contributed by the RC and integrated steel cross-sections are first calculated indepen-dently.They are then superposed to obtain the combined SRC beam–column joint strength.4.1.Shear strength contributed by structural steel shapesFor columns incorporating wide flanged steel cross-sections,the shear strength of the beam–column joint along the plane of the web is primarily afforded by the web itself.Chen and Lin [2]showed that the web component of the Type II exterior joint for a pure steel specimen designated S-XH yielded significantly during load tests.Accordingly,Chen and Lin [2]suggested that the shear strength provided by the longitudinal web,V s w ,for interior and Type II exterior joints can be estimated to be equal to the yield strength of the web.According to AISC-LRFD specifications [8],the shear strength provided by the web is given as V s w =0.6F y w d c t w(5)where F y w is the yield stress of the column web,d c is the column depth,and t w is the column web thickness.In the pure steel specimen S-XH2,the web built into a Type II corner joint was also shown to yield significantly.Furthermore,the equivalent SRC specimen with a Type II corner joint,SRC-XH2,exhibits similar phenomena.Fig.20(a)presents the hysteresis loops of load P against web shear strain for SRC-XH2.It indicates that the maximum shear strain in the longitudinal web significantly exceeded the shear yield strain,γsy =0.00288.Therefore,in addition to interior and Type II exterior joints,Eq.(5)may also be applied to Type I exterior and Type II corner joints.It was revealed by Chen and Lin [2]that shearing forces applied to longitudinal flanges in the joint during load testing may reach the yield stress;Eq.(6)was proposed to evaluate the shear strength provided by the two parallel longitudinal flanges for interior and Type II exterior joints.V slf=2 230.6F yf b f t f(6)where F yf is the yield stress of the column flange.Fig.20(b)shows the load P versus flange shear strain hysteresis loops of SRC-XH2.It indicates that the maximum shear strain in the longitudinal flange exceeded the shear yield strain.Accordingly,Eq.(6)should be applicable,and is also suggested for Type I exterior joints and Type II corner joints.1578 C.-C.Chen et al./Journal of Constructional Steel Research 65(2009)1569–1581SC 1SC 2SC 3SC 4LegendSRC-XH1-TBSRC-XH1SRC-XH2-A2SRC-XH210050-50-1000.00030.00060.00090.00120.0015150-150D i s p l a c e m e n t (m m )10050-50-100-150D i s p l a c e m e n t (m m )0.0018Strain 0.00030.00060.00090.00120.00150.0018StrainFig.18.Displacement versus strain on cornerstirrups.(a)External force due to load P i .(b)Forces in the joint area.Fig.19.External and internal forces acting on beam–column joints.4.2.Shear strength contributed by reinforced concreteThe shear strength contribution of reinforced concrete iscalculated pursuant to ACI-318-05stipulations [9].A requirement of the ACI code is that transverse hoops should be provided within the beam–column joint.However,test observations indicate that a combination of reinforcing steel cross-sections and corner stirrups readily perform the function of transverse hoops without any adverse repercussions;similar conclusions ware drawn by Chen and Lin [2]for specimens with interior and Type II exterior joints.C.-C.Chen et al./Journal of Constructional Steel Research65(2009)1569–15811579 Table4400300200100-100-200-400-300-0.03-0.02-0.010.010.020.03-γsyγsy500-500LoadP(kN)-0.040.04Shear Strain400300200100-100-200-400-300γsy0.0030.0060.0090.012500-500LoadP(kN)Shear Strain-0.0030.015(a)Longitudinal web.(b)Longitudinal flange.Fig.20.Load versus shear strain hysteresis loops for the SRC-XH2specimen.It was therefore considered that the ACI specifications had beensatisfied and that application of the associated framework forthe calculation of the shear strength provided by the reinforcedconcrete sections was valid.The following expressions can be usedto calculate the joint shear strength,V rc:For joints confined on all four facesV rc=1.67fcA j.(7)For joints confined on three faces or on two opposing facesV rc=1.25fcA j.(8)For othersV rc=1.00fcA j(9)where fcis the specified compressive strength of the concrete andA j is the effective area of the joint.The effective area of the joint can be calculated by:A j=b j×h j(10)where b j is the joint width and h j is the joint depth.The value of b j for a concentric joint should satisfy Eq.(11):b j=min{b b+h c,b b+2x}(11)where x is the horizontal distance between the beam and columnedges.4.3.Shear strength prediction for beam–column jointsInaddition to the four SRC test specimens described in theprevious sections,three SRC specimens from Chen and Lin[2]Table5Mechanical properties of steels used by Chen and Lin[2].are included to evaluate the applicability and accuracy of thesuperposition method.The joint details and longitudinal beamsections of Chen and Lin’s specimens are illustrated in Figs.21and22,respectively.The mechanical properties of the steels used arelisted in Table5.The compressive strength of concretes and the testresults obtained for each specimen are documented in Table6.The shear strength for SRC beam–column joints is calculated asthe sum:V src=V rc+V s w+V slf.(12)The values of V src and the V t/V src ratio for all SRC specimens arelisted in Table7.The ratio V t/V src ranges from1.27to1.62,with anaverage value of1.37,and is higher in SRC-XH2than in SRC-XH1by12.0%;increased depth of cross-sectional steel throughout thecolumn of the former enables the confinement of more concreteexplaining the higher shear strength in the beam–column joint.The V t/V src ratio of SRC-XH2-A2is larger than that of SRC-XH2by9.5%;beam–column joints with deeper anchorage positions tendedto possess higher joint shear strengths than joints with shalloweranchorage.To augment the shear strength,engineers should layheaded bars deeper into the joint with heads behind the columncenter line.1580 C.-C.Chen et al./Journal of Constructional Steel Research65(2009)1569–1581(a)SRC-XH.(b)SRC-XH-TB.(c)SRC-H.Fig.21.Beam–column joint details for Chen and Lin’s specimens.Table6Test results for Chen and Lin’s[2]specimens.Table7Collated shear strength predictions for SRC specimens.A beam framed into a column face is considered to provideconfinement to the joint if at least three-quarters of the face ofthe joint is covered by the beam[9].The width of the column facewas500mm and the beam width was400mm.The stipulatedratio is thus exceeded and therefore joint confinement factor,β=1.67is used for SRC-XH-TB,β=1.25is used for the threeexterior joints,andβ=1.00is used for three corner joints.TheACI standards which exist to facilitate computations pertaining tocompliant specimens can then be implemented to calculate the RCshear strength component of SRC joints;since classifications of theRC parts have been verified,the ACI formulae are thus applicableto all joint types.The value of V t/V src for all specimens is greater than unity,withan average value of1.37.The superposition method was thereforeable to predict the SRC joint shear strength with reasonableaccuracy.While Type I corner joints were not included in therange of test specimens,they are considered to possess higher jointshear strength relative to the extensively scrutinized Type II cornerjoints.5.ConclusionsBased on the experimental and analytical results reportedherein,the following conclusions are drawn:1.The superposition method was able to estimate the SRCbeam–column joint shear strength with reasonable accuracy.The ratio of the tested shear strength over the predicted shearstrength ranges from 1.27to 1.62,with an average valueC.-C.Chen et al./Journal of Constructional Steel Research 65(2009)1569–15811581Fig.22.SRC longitudinal beam of Chen and Lin’s specimens.of 1.37.The joint classification and strength evaluation methods provided by the ACI code can be implemented to calculate the RC shear strength component in SRC joints.2.The anchorage position of beam longitudinal bars has an obvious influence on the joint shear strength and the crack pattern.Beam–column joints with deeper anchorage positions (i.e.longer development lengths)possess higher joint shear strength and presents steeper diagonal cracks than those with shallower anchorage positions (i.e.shorter development length).3.For SRC columns,an increased depth of cross-sectional steel permits the confinement of more concrete,leading to higher shear strength in the beam–column joint.4.A combination of the corner stirrups and shaped steel cross-sections was able to provide sufficient lateral support to the longitudinal steel bars and adequate confinement of concretein the joint.Corner stirrups can be used to replace closed hoops entirely and relieve the difficulties associated with hoop placement.5.The crack pattern of SRC beam–column joints is quite similar to that of RC beam–column joints.Significant yielding and shear deformation are experienced by the web of steel cross-sections and large web shear deformation is considered to be the main cause of fracture in beam-flange-to-column-flange welds.AcknowledgementThe research reported in this paper was sponsored by the National Science Council of the Republic of China at Taiwan under Project NSC92-2211-E011-040.References[1]ACI-ASCE 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TF,Wang KC,Tsai HH.Role of hoops on shear strength ofreinforced concrete beam–column joints.ACI Structural Journal 2005;102(3):450.[7]Imbsen CC.XTRACT Software,Cross section analysis program for structuralengineers.Single user v-2.6.2.Imbsen and Associates,Inc.;2002.[8]American Institute of Steel Construction.Specification for structural steelbuildings.Chicago (IL):AISC Inc.;2005.[9]ACI Committee 318.Building code requirements for structural concrete (ACI318-05)and commentary (ACI 318R-05).Farmington Hills (MI):American Concrete Institute;2005.。