Structural behaviour of continuous composite beams with deformable shear connectors

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22 卷第 烟台大学学报自然科学与工程版Vol22 No2009 Journal of Yantai University Natural Science and Engineering EditionJul.200 文章编号:100 4 8820(2009)0 3 022 90 Received date: 200 90 3 30 BiographyChung Kwok Fai(196 0 ), maleborn in Hong Kongprofessor of Hong Kong Polytechnic University of ChinaDeputy Director of the Research Centre for Advanced Technology in Structural EngineeringEditor of the Inte- rnational Journal on Advances in Structural Engineeringof The Hong Kong Polytechnic UniversityVice Pr- esident of The Hong Kong Institute of Steel Constructionpursuing the field of steel and composite structuresStructural Behaviour of Continuous Composite Beams with Deformable Shear Connectors Chung Kwok FaiChan Chung Kei Department of Civil and Structural EngineeringThe Hong Kong Polytechnic UniversityHong Kong SARChinaAbstractThis paper presents a detailed finite element study on the structural behaviour of double span com- posite beams of practical configurationsAs it is essential to acknowledge that shear connectors are not rigidtheir non- linear load- slippage characteristics are incorporated into advanced finite element models through the use of spring elementsA parametric study on various double span composite beams of practical configurations is carried outand key findings of the study are presented in this paperThe key structural parameters include the deformation ductility of shear connectors as well as the degrees of shear connection provided at both the sag- ging and the hogging moment regionsThe structural behaviour of a total of composite beams with different types and numbers of shear connectors is compared and discussed while current design deficiency is identified for possible developmentAs the proposed finite element models are reckoned to be highly effective for detailed investigation into the structural behaviour of composite beamsthe effective use of the models will be instrumen- tal to the design development of more refined design rules for improved structural accuracy and efficiencyKey wordsfinite element modellingcomposite beamshear connectionnon- linear analysis CLC numberTU398 Document codeComposite Beams in Building Struc- tures Composite structures are widely used in building con- struction for many years owing to their high constructabilityIn generalcomposite beams are strong and stiff flexural members with long spanning capacitiesThe structural form of a composite beam is essentially a thin wide concrete slab con- nected with a steel section where the concrete slab is in com- pression while the steel section is largely in tensionShear connectorsusually headed studsare welded to the top flange of the steel section and fully embedded into the con- crete flangeDepending on the number of shear connectors provided along the interface between the steel section and the concrete slab which is either a solid concrete slab or a com- posite slabthe composite beam may operate in either full or partial shear connectionand henceexhibit a wide range of deformation characteristics according to the load- slippage characteristics of the shear connectorsIn many codes of practice for composite structures world- wideprescriptive design methods are adoptedFor examplein Eurocode [1] plastic design principles are adopted in de- signing composite beamsand their moment resistances under sagging and hogging moments are determined according to plastic stress blocksThe total load carrying capacities of continuous composite beams may be readily obtained from global plastic analyses provided that there is sufficient rota- tional ductility in the composite cross- sections under hogging momentsThese methods are developed from proven under- standings and knowledge on the structural behaviour of com- posite structures acquired through physical testingdesign

Transcript of Structural behaviour of continuous composite beams with deformable shear connectors

第 22卷第 3期 烟台大学学报(自然科学与工程版) Vol.22 No.32009年 7月 Journal of Yantai University (Natural Science and Engineering Edition) Jul.2009

  文章编号:1004 8820(2009)03 0229 09

  Received date: 2009 03 30  Biography: Chung Kwok Fai(1960 ), male, born in Hong Kong, professor of Hong Kong Polytechnic University of China,

Deputy Director of the Research Centre for Advanced Technology in Structural Engineering, Editor of the Inte-rnational Journal on “Advances in Structural Engineering” of The Hong Kong Polytechnic University, Vice Pr-esident of The Hong Kong Institute of Steel Construction, pursuing the field of steel and composite structures.

Structural Behaviour of Continuous Composite Beamswith Deformable Shear Connectors

Chung Kwok Fai, Chan Chung Kei(Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hong Kong SAR,China)

Abstract: This paper presents a detailed finite element study on the structural behaviour of double span com-posite beams of practical configurations.As it is essential to acknowledge that shear connectors are not rigid,their non-linear load-slippage characteristics are incorporated into advanced finite element models through theuse of spring elements.A parametric study on various double span composite beams of practical configurationsis carried out, and key findings of the study are presented in this paper.The key structural parameters includethe deformation ductility of shear connectors as well as the degrees of shear connection provided at both the sag-ging and the hogging moment regions.The structural behaviour of a total of 9 composite beams with differenttypes and numbers of shear connectors is compared and discussed while current design deficiency is identifiedfor possible development.As the proposed finite element models are reckoned to be highly effective for detailedinvestigation into the structural behaviour of composite beams, the effective use of the models will be instrumen-tal to the design development of more refined design rules for improved structural accuracy and efficiency.Key words: finite element modelling; composite beam; shear connection; non-linear analysisCLC number: TU398    Document code: A

1 Composite Beams in Building Struc-turesComposite structures are widely used in building con-

struction for many years owing to their high constructability.In general, composite beams are strong and stiff flexuralmembers with long spanning capacities.The structural form ofa composite beam is essentially a thin wide concrete slab con-nected with a steel section where the concrete slab is in com-pression while the steel section is largely in tension.Shearconnectors, usually headed studs, are welded to the topflange of the steel section and fully embedded into the con-crete flange.Depending on the number of shear connectorsprovided along the interface between the steel section and theconcrete slab which is either a solid concrete slab or a com-posite slab, the composite beam may operate in either full or

partial shear connection, and hence, exhibit a wide range ofdeformation characteristics according to the load-slippagecharacteristics of the shear connectors.

In many codes of practice for composite structures world-wide, prescriptive design methods are adopted.For example,in Eurocode 4[1] , plastic design principles are adopted in de-signing composite beams, and their moment resistances undersagging and hogging moments are determined according toplastic stress blocks.The total load carrying capacities ofcontinuous composite beams may be readily obtained fromglobal plastic analyses provided that there is sufficient rota-tional ductility in the composite cross-sections under hoggingmoments.These methods are developed from proven under-standings and knowledge on the structural behaviour of com-posite structures acquired through physical testing, design

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practice and construction experiences.In general, there is al-ways a need to develop scientific means to analyze, designand appraise the structural performance of composite struc-tures, and detailed structural responses are essential informa-tion for design of composite structures.

2 Objectives and Scope of WorkThis paper presents a detailed finite element study on the

structural behaviour of double span composite beams of prac-tical configurations.Advanced two dimensional finite elementmodels with material, geometrical and interfacial non-linearityare established.As it is essential to acknowledge that shearconnectors are not rigid, their non-linear load-slippage char-acteristics are incorporated into the advanced finite elementmodels through the use of spring elements.It should be notedthat the models have been successfully calibrated against test

data[2,3].A parametric study on a total of 9 composite beamsis carried out, and the key structural parameters include thedeformation ductility of shear connectors as well as the de-grees of shear connection provided at both the sagging and thehogging moment regions.The structural behaviour of variouscomposite beams is compared and discussed while current de-sign deficiency is identified for possible development.

3 Finite Element ModellingFigure 1 illustrates the general arrangement of the 12 m

double span composite beam under consideration, and detailsof both the geometrical dimensions and the mechanical prop-erties of the steel section and the concrete slab are also pro-vided.It should be noted that a solid concrete slab with topreinforcements only is adopted in the present study.

Fig.1 General arrangement of a double span composite beam  The material models of steel and concrete are illustratedin Figure 2, and it should be noted that:

A bi-linear stress-strain curve is adopted for the steelmaterial with a yield strength of py

Failure of the steel material is assumed to follow the vonMises failure criteria under both direct and shear stresses.This applies to the steel sections and the steel reinforcements.The maximum elongation limits of the steel sections and thesteel reinforcements are specified to be 15 and 10% respec-tively.

A non-linear stress-strain curve is adopted for concrete

  The compressive strength of concrete, pc, is taken to beequal to its cylinder strength while its tensile strength, pt, istaken as only 10 % of its compressive value.The limitingcompressive strain of concrete against crushing is taken to be0.35% while failure of the concrete is assumed to follow theDrucker-Prager failure criteria[4].It should be noted that thelimiting tensile strain of concrete is specified to be10%[2,5,6].Although such a value is unrealistically high, itis found to be necessary in order to obtain solutions when thebeam undergoes large deformations.Furthermore, in order tosimulate controlled concrete cracking in the presence of steel

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reinforcements, smeared layers① is also adopted.

Fig.2 Material curves of steel and concrete materials  Each shear connector is modelled with one horizontalspring and one vertical spring in order to simulate the longitu-dinal shear force and the pull-out force of the shear connec-tor.The initial part of the load-slippage curves of the horizon-tal and the vertical springs are obtained from the normalizedload-slippage curve proposed by Ollgaard et al[7].As one ofthe key objectives of the study is to investigate the effect ofdeformation ductility of shear connectors onto the structuralbehaviour of double span composite beams, three different

types of shear connectors, namely, Types D, R and N, withdifferent deformation ductility at large deformations are in-cluded in the study.The complete load-slippage characteris-tics of these three types of shear connectors are illustrated inFigure 3.Hence, a total of 9 composite beams are analyzedof which each type of the shear connectors with 1 to 3 shearconnectors per row are adopted, and comprehensive data in-terpretation on their numerical results is reported.

Fig.3 Load slippage curves of various types of shear connectors  As the plate elements of the steel section are ratherstocky, no local buckling will occur.Hence, a two dimen-sional finite element model of the double span composite beamis established using the general purpose finite element pack-age ABAQUS, as shown in Figure 4, and plane strain ele-ments S8 are employed to model both the steel section and theconcrete slab.While both material and geometrical non-line-arity is incorporated into the models, no residual stress in thesteel section is considered.

It is worth noting that while both the steel section and

the steel reinforcement are able to develop large strains, theconcrete is not as its maximum tensile strain is merely 0.1%.Numerical instability often occurs when the composite beam isanalyzed at large deformations, and hence, no solution isfound.One simple way to overcome this situation is to declarethe maximum tensile strain of the concrete to be 10% al-though this creates another problem of some residual tensilestresses in the concrete.In order to rectify the problem, anumber of those elements directly over the internal supportare removed from the regular finite element mesh, and thus,

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① ABAQUS.User’ s Manual, Version 6.4, Hibbitt, Karlsson and Sorensen, Inc.2004.

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the revised model effectively simulates concrete cracking inthe region of high tensile strains.Furthermore, the effectivewidths of the composite cross-sections at the sagging and thehogging moment regions, Bes and Beh, are taken to be 3000and 500 mm respectively.The value of Beh is assigned to beonly one sixth of that of Bes in order to minimize the error in-

duced in the hogging moment capacity owing to the assumedtensile strength of the concrete slab at large deformations.Nevertheless, a sensitivity study shows that the error in thehogging moment capacity is found to be less than 2 %, andthus, the hogging moment capacity of the composite beam isconsidered to be evaluated with acceptable accuracy.

Fig.4 Undeformed shape of Beam DS-D3

Fig.5 Deformed shapes of Beam DS-D3 at various deformation stages  In order to avoid local intrusion between the finite ele-ments of the concrete slab and the steel section during non-linear analyses, axial spring elements with extremely highcompressive stiffness but zero tensile stiffness are provided a-long the interfaces between the concrete slab and the steelsection.

4 Numerical ResultsFigure 5 presents typical deformed shapes of the models

at various critical deformations.It should be noted that there

are various check points along the entire deformation historyof the composite beam which are defined by the followingstrain levels in the steel and the concrete materials:  瞯Monitoring tensile or compressive strain in steel sec-tion and reinforcement, εsmax, which is defined as follows:

εsmax =cO ×psEs

×py275 . (1)

  Where co is the coefficient for the level of deformationpermitted in the limit state under consideration, and its rec-ommended value is 1.0 for serviceability limit state, and 6.0

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for ultimate limit state; py is the yield strength of the steel;and Es is the Young’s modulus of the steel.As shown in Figure 5, moment re-distribution always

takes place in the composite beams with various types andnumbers of shear connectors, and failure of the beams ofteninvolves a two-stage mechanism as follows:Stage 1 Failure at internal support under combined hoggingbending and shear; andStage 2 Failure at mid-span under sagging moment.

Figure 6 presents the load-deflection curves of all thecomposite beams while the numerical results are summarizedin Table 1.

In general, for composite beams with shear connectorsTypes D, R and N, there is no difference whether 3 or 2shear connectors are provided per row as full shear connectionis achieved in both cases, and the shear connectors are onlyrequired to slip by 0畅64 and 4畅1 mm respectively, i.e.smal-

ler than 5 mm, and before any reduction in the shear resist-ance takes place.Hence, the moment capacities of the com-posite cross-sections under both sagging and hogging momentsaccording to plastic stress blocks are readily mobilized, asshown in Table 1.Curves are found to be fairly ductile, allo-wing large strains to be developed in those four check pointsin the beams.Owing to large ductility of the steel materials,there is no sudden failure in the steel section and the steel re-inforcement even if their strains exceed their respective limit-ing values.However, as concrete crushing is a brittle failure,the composite cross-section is considered to be failed whenthe strain of the concrete slab reaches the corresponding limit-ing value.It should be noted that as the concrete strains ap-proach their limiting strains, their compressive strengths re-duce steadily and the corresponding sagging moment capaci-ties diminish accordingly.

Fig.6 Load-deflection curves with different types of shear connectors

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Tab.1 Load carrying capacities of composite beams with various types and layout of shear connectors

Beam Layout ofshear connector

Numerical analysis Back analysisWFEM /

kNMsag /

kN· mMhog /

kN· mSmax /mm

SMA HMRPBA /

kNMsag /

kN· m ksMhog /

kN· m kh

D3 �3 per row 706 1.9 767 ゥ.2 623 .9 0 I.58 0 浇.64 714 u.1 771 殚.5 1 .57 620 蜒.2 1 .58D2 �2 per row 697 1.8 761 ゥ.8 605 .1 2 I.30 4 浇.10 714 u.1 771 殚.5 1 .05 620 蜒.2 1 .05D1 �1 per row 614 1.5 654 ゥ.3 572 .1 9 I.50 14 栽.70 612 u.0 659 殚.8 0 .52 535 蜒.3 0 .53R3 �3 per row 706 1.9 767 ゥ.2 623 .9 0 I.58 0 浇.64R2 �2 per row 697 1.8 761 ゥ.8 605 .1 2 I.30 4 浇.10R1 �1 per row 560 1.9 572 ゥ.0 566 .4 21 v.0 8 浇.20N3 �3 per row 706 1.9 767 ゥ.2 623 .9 0 I.58 0 浇.64N2 �2 per row 697 1.8 761 ゥ.8 605 .1 2 I.30 4 浇.10N1 �1 per row 449 1.9 423 ゥ.5 555 .1 41 v.5 9 浇.30

  Notes:SMR denotes the sagging moment region near mid-span,HMR denotes the hogging moment region over internal support.  The situation is found to be very different in those com-posite beams with only 1 shear connector per row.As the de-grees of shear connection for both sagging and hogging mo-ments are about 0畅5, the shear connectors are required to slipsignificantly larger than 5 mm.In Beam DS-D1, the requiredslippages of the shear connectors near the end support ( forsagging moment) and the internal support ( for hogging mo-ment) are 9畅5 and 14畅7 mm respectively, as shown in Table1.These values are increased to 21畅0 and 8畅2 mm respectivelyin Beam DS-R1, and to 41畅5 and 9畅3 mm respectively inBeam DS-N1.Owing to the limited ductility in shear connectorTypes R and N, there are considerable reductions in the totalload carrying capacities in Beams DS-R1 and DS-N1.It shouldbe noted that as many shear connectors in Beam DS-N1 nearthe end support (for mobilization of sagging moment capacity)and the internal support ( for mobilization of hogging momentcapacity) are required to slip beyond 7mm, they become inef-fective, and thus, the cross-sections behave in a way similar tosteel sections rather than composite sections at large deforma-tions, demonstrating a shake down behaviour.

Figure 7 presents the variations of both the slippage andthe shear forces of shear connectors along the beam span forBeams DS-D3, DS-D2 and DS-D1, and the variations at vari-ous deformation stages of the four check points are plotted inthe same graphs for direct comparison.It should be noted that  瞯Owing to the presence of 3 shear connectors per row inBeam DS-D3, the degrees of shear connection of the compositecross-sections near the mid-span (for sagging moment) and theinternal support ( for hogging moment) are both 1畅57 ( al-though the values are usually taken not to be larger than 1畅0).As the force acting onto each of the 3 shear connectors is rela-tively small, many of the shear connectors are working withinthe initial ascending part of the non-linear load slippage curve.It is shown that the required slippages of the shear connectors

near the end and the internal supports are merely 0.58 and0畅64 mm respectively.

瞯In Beam DS-D2, the degrees of shear connection in thecomposite cross-sections near the mid-span and the internalsupports are found to be 1畅05, and the force acting onto eachof the 2 shear connectors is increased by 50% when comparedwith the force in Beam DS-D3.Hence, the required slippagesof the shear connectors near the end and the internal supportsare increased to 2畅3 and 4畅1 mm respectively, and many of theshear connectors are working beyond the initial ascending partof the non-linear load slippage curve at the full values of theirshear resistances.

瞯However, as only 1 shear connector per row is providedin Beam DS-D1, the degrees of shear connection of the com-posite cross-sections near the mid-span and the internal sup-ports are found to be merely 0畅53.The forces acting onto theshear connectors are fairly large, causing considerably largeslippages, i.e.9畅5 and 14畅7 mm near the end and the inter-nal supports respectively.Almost all shear connectors areworking at their full shear resistances.

In order to assess more realistic situations, the numericalresults of Beams DS-D1, DS-R1 and DS-N1 are plotted ontothe same graphs for comparison as shown in Figure 7, and thevariations of both the slippage and the shear forces of shearconnectors along the beam span for Beams DS-D1 may be re-garded as the reference data.It should be noted that:

瞯In Beam DS-R1, although the degrees of shear conec-tion in the composite cross-sections near the mid-span and theinternal supports are found to be 1畅05, and the forces actingonto the shear connectors are very large when compared withthe force in Beam DS-D3.Hence, the required slippages ofthe shear connectors near the end and the internal supports areincreased to 21畅0 and 8畅2 mm respectively, and many of theshear connectors are working along the descending part of the

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non-linear load slippage curve at reduced values of their shear resistances.

Fig.7 Distributions of slippage and longitudinal shear force along Beams DS-D3/D2/D1( left column)and Beams DS-D1/R1/N1(right column)

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  It should be noted that the slippage at the end support islarger than that near the internal support by a factor of 2畅5,and this contradicts to the general anticipation.Owing to thevery large slippage deformation in the shear connectors, someof the shear connectors are working at significantly reducedshear resistances, and hence, it is not possible to assess themoment capacities of the composite cross-sections correctlywithout allowing for such strength reduction.

瞯In Beam DS-N1, the situation is found to be very simi-lar to that in Beam DS-N1 except that the degrees of shearconnection in the composite cross-sections near the mid-spanand the internal supports are merely 0畅53, and the steep un-loading part of the load-slippage curve causes very large slip-pages in the shear connectors: 41.5 and 9畅3 mm for thoseshear connectors near the end and the internal supports re-spectively.As shown in Figure 8, there are many shear con-nectors become ineffective at large deformations, and hence,there are significant reductions in the sagging and the hoggingmoment capacities of the composite cross-sections as well as inthe total load carrying capacities of the composite beams.

Hence, both the deformation ductility of shear connectorsat large deformations and the degrees of shear connection arevery important to the structural behaviour of continuous com-posite beams.

5 ConclusionsThis paper presents an in-depth study on the structural

behaviour composite beams in buildings using advanced finiteelement models.While the ductility requirements in shearconnectors have been well known for many years, this paperpresents quantitative information on the requirements as wellas the consequences when the requirements are not met.It isdemonstrated that:

i)For continuous composite beams with full shear con-nection in both composite cross-sections in the sagging and thehogging moment regions, the slippages required to fully mobi-lize the moment capacities of the composite cross-sections aretypically smaller than 5 mm.Such slippages are generallyconsidered to be readily achieved in most shear connectors.

ii)However, for continuous composite beams with onlypartial shear connection in both composite cross-sections inthe sagging and the hogging moment regions, the slippages re-quired to fully mobilize the moment capacities are often large,being typically between 10 to 20 mm.If then the shear con-nectors do not possess sufficient deformation ductility at suchdeformations, it is essential to acknowledge possible strengthreduction in the shear connectors at failure, and the moment

capacities of the composite cross-sections should be reducedaccordingly.

In practice, there is a strong tendency to construct com-posite beams with only the minimum numbers of shear connec-tors for economical benefits, and many composite beams aredesigned with partial shear connection.Hence, compositebeams as described in item i) above are considered to be noteconomical while composite beams as described in item ii) a-bove assume that every shear connector possesses sufficientductility at large deformations.While many codified methodallow composite beams with partial shear connectors, such anassumption is often not warranted in reality.In general, it isconceived that significant strength reduction in the shear con-nection, i.e.a shear connector fully embedded in reinforcedconcrete, is inevitable owing to local crushing and cracking ofconcrete under high localized forces.Obviously, the challen-ges are fundamental to all composite structures:

Challenge ATo design and construct a shear connection of a shear

connector fully embedded in reinforced concrete which posses-ses sufficient ductility at large deformations, i.e.in the orderof 20 to 40 mm.

Challenge BTo design composite beams with shear connectors having

non-uniform shear resistances mobilized along the beam spanat failure.

Challenge CTo reinforce locally a series of shear connections in order

to prevent or reduce concrete crushing and cracking.Hence, it is demonstrated that double span composite

beams with shear connectors of different load slippage charac-teristics and different degrees of shear connection behave dif-ferently.As the internal force distribution in continuous com-posite beams depends not only on the flexural rigidities of thecomposite cross-sections but also on the ductility of shear con-nectors at both small and large deformations, the failure loadsof the beams will be affected accordingly.The proposed mod-els are considered to be highly effective as a research tool fordetailed investigation into the structural behaviour of compos-ite beams as well as a specialist tool for analysis and design ofcomposite beams in buildings.Effective use of the models willbe instrumental to the design development of more refined de-sign rules for improved structural accuracy.References:

[1]British Standards Institution, BS EN 1994-1-1.Eurocode4: Design of composite steel and concrete structures.Part 1.1

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[S]//General rules and rules for buildings.European Com-mittee for Standardization.2002.[2]Wang A J, Chung K F.Integrated analysis and design ofcomposite beams with flexible shear connectors under saggingand hogging moments [ J].Steel and Composite Structures,2006,6 (6): 459-478.[3]Wang A J.Advanced Nonlinear Finite Element Investiga-tion into Structural Behaviour of Composite Beams[D].HongKong:The Hong Kong Polytechnic University,2007.[4]Baskar K, Shanmugam N E,Thevendran V .Finite-ele-ment analysis of steel—concrete composite plate girder[ J].Journal of Structural Engineering ,2002,128 (9): 1158 -1168.

[5]Pi Y L, Bradford M A, Uy B.Second order nonlinear in-elastic analysis of composite steelconcrete members.I: Theo-ry[ J].Journal of Structural Engineering, 2006,132 (5):751-761.[6]Queiroz F D, Vellasco P C G S, Nethercot D A.Finiteelement modelling of composite beams with full and partialshear connection [ J].Journal of Constructional Steel Re-search,2007,63: 505-521.[7]Ollgaard J G, Slutter R G , Fisher J W.Shear strength ofstud connectors in light weight and normal weight concrete[J].American Institute of Steel Construction EngineeringJournal, 1971,8(2): 55-62.

  作者简介: 钟国辉(1930 ),男,香港人,香港理工大学教授,博士,香港理工大学结构工程高等技术研究中心副主任,国际期刊枟Advances in Structural Engineering枠编委,香港钢结构建筑协会副主席,研究方向:钢及组合结构.

具有可变形剪切连接件的连续复合梁的结构行为

钟国辉,Chan Chung Kei(香港理工大学 土木与结构工程学院,香港 中国)

摘要: 对原型双跨复合梁的结构行为给出了详细的有限元研究.由于基本上认为剪切连接件是非刚性的,在高等有限元模型中采用弹簧单元可以将它们的非线性荷载-滑移性质考虑在内.对各种原型双跨复合梁进行了参数研究,并提出了主要的研究结果.主要的结构参数包括剪切连接件在正弯矩和负弯矩区域内的变形韧性和变形程度.总共比较和讨论了 9 个具有不同类型和剪切件数目的复合梁的结构行为,对目前的设计缺陷进行了鉴定.由于提出的有限元模型对详细调查复合梁的结构行为非常有效,有效地利用所提出的模型将对更具有改善结构精度和效率的细化设计准则的发展有帮助.

关键词: 有限元模型;复合梁;剪切连接;非线性分析中图分类号:TU398    文献标识码: A

(责任编辑 周雪莹)

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