Nonlinear finite element analysis of circular conc

StructuralEngineeringandMechanics,Vol.35,No.3(2010)000-0001Nonlinearfiniteelementanalysisofcircularconcrete-filledsteeltubestructuresTengfeiXu,TianyuXiang*,RendaZhaoandYulinZhanDepartmentofBridgeEngineering,SouthwestJiaotongUniversity,Chengdu610031,P.R.China(ReceivedApril15,2009,AcceptedJanuary27,2010)Abstract.Thestructuralbehaviorsofcircularconcretefilledsteeltube(CFT)structuresareinvestigatedbynonlinearfiniteelementmethod.Anefficientthree-dimensional(3D)degeneratedbeamelementisadopted.Basedonthosepreviousstudies,amodifiedstress-strainrelationshipforconfinedconcretewhichintroducestheinfluenceofeccentricityonconfiningstressispresented.UpdatedLagrangeformulationisusedtoconsiderthegeometricalnonlinearityinducedbylargedeformationeffect.ThenonlinearbehaviorsofCFTstructuresareinvestigated,andtheaccuracyoftheproposedconstitutivemodelforconfinedconcreteismainlyconcerned.TheresultsdemonstratethattheconfiningeffectinCFTelementssubjectedtocombiningactionofaxialforceandbendingmomentisfarsophisticatedthanthatinaxialloadedcolumns,andanappropriateevaluationaboutthiseffectmaybeimportantfornonlinearnumericalsimulationofCFTstructures.Keywords:concrete-filledsteeltube;nonlinearfiniteelementanalysis;degeneratedbeamelement;updatedlagrangeformulation.1.IntroductionConcrete-filledsteeltube,whichcombinestheadvantagesofsteelandconcrete,hasattractedmoreandmoreattentionsincivilengineeringduringthepastfourdecades.First,thesteeltubeservesasaformforcastingofcoreconcrete,whichresultsinreductionofcostandimprovementofconstructionefficiency.Moreover,comparedwithhollowsteeltube,concretefillingmaypreventbulkingofsteeltube,andthestabilityofcompressiveelementcanbeimprovedenormously.Amongalltheseadvantages,themostattractivemeritofCFTisthattheelementexhibitsexcellentcompressiveresistancecapacity,ductility,andenergydissipationabilityduetotheconfiningpressureprovidedbysteeltube.Owingtothosedistinctivemerits,CFThasbeendemonstratedasasuitablestructuralelementforbuildingcolumns,bridgepiers,andarchribsthroughworldwideengineeringpractices.Agreatamountofsignificantresearchworkshavebeencontinuouslycontributedtothissubject.Inthosepreviousresearches,experimentalstudieshaveplayedakeyroleinestablishmentofthegeneraltheoryofCFT.Comparedwithexperimentalmethod,numericalapproachismoreeconomicalandeffective,whichmaybecapabletosimulatemanyexperimentallyunmanageable*Correspondingauthor,Professor,E-mail:[e-mail]2TengfeiXu,TianyuXiang,RendaZhaoandYulinZhancases.Moreover,throughcombingexperimentalandnumericalapproach,amoreclearinspectionaboutmechanicalnaturecanbeexpected.StripmethodisthemostcommonlyadoptednumericalmethodtoanalysistheresistanceandductilityofCFTmembers(Fujimotoetal.2004,Han2004).One-dimensionalstress-strainrelationshipisgenerallyusedinthestripmethod,andtheconfiningeffectisconsideredthroughmodifyingthepeakstressandstrainofconstitutivecurveofcommonconcrete.Anotherchoiceofnumericalmethodisnonlinearfiniteelementanalysis.Inrecentpublishedliteratures,nonlinearfiniteelementanalysisofCFTstructureswiththree-dimensionalsolidelementhasbeenwidelycarriedwithcommercialFEMpackage(Huangetal.2002,Huetal.2003,Huetal.2005,Guptaetal.2007).Withthree-dimensionalsolidelement,thenonlinearFEManalysiscansimulatesuchcomplexstructuralbehaviorsasbond-slipbetweensteelandconcrete,confiningeffectundersophisticatedstressstates,bucklingofsteeltubeandsoon,whichcanobtainacomprehensiveinsightaboutthebasicmechanicalnatureofCFTstructures.However,thisapproachisgenerallytime-consumingespeciallyforlargecomplexstructures.Moreover,withincrementofscaleofFEMmodel,strongnonlinearitycausedbycrackingandcrushingofconcrete,plasticityflowofsteel,andsoon,maybringsomedilemmasforiterationconvergence.TomeettherequirementofnonlinearanalysisforpracticallargeCFTstructures,three-dimensionalbeam-columnfiniteelementapproachmaybeacomprisedandeffectiveway.Inthisapproach,theelastic-plasticitybehaviorsofconcreteandsteel,whicharethemajornonlinearityfactorsofCFTstructures,areemphasized.Suchnonlinearresourcesasbond-slipbetweenconcreteandsteel,bucklingoftube,andsoonarenotconsidered.SomesuccessfulapplicationsofthismethodcanbefoundinthestudiesofMarques(2005)andMirzaetal.(1996).Inthepresentstudy,withthree-dimensionaldegeneratedbeamelementthenonlinearbehaviorsofCFTstructuresareinvestigated.Thetheorybehindthedegenerationtechniqueisnotnew,andhasbeenwidelyusedintheformulationofshellandplateelement(Ahmadetal.1970,Worsak1978,Parisch1981).Toformulatedegeneratedbeamelement,thehypothesisofplanesectionisadopted,andbasedonthishypothesisthethree-dimensionaldisplacementfieldcanbedescribedbynodalvariables.Meanwhile,thestrain-displacementequationsofthree-dimensionalstructureareretained.AnonlinearanalysisprogramforconcretestructureswithdegeneratedbeamelementdevelopedbyXiangetal.(2005)isadoptedinthepresentstudy.One-dimensionalstress-strainrelationshipofconcreteandsteelisadoptedinformulationofdegeneratedbeamelement.Asaforementioned,toinvolvetheconfiningeffect,somemodificationsaboutstress-strainrelationshipofcommonconcreteshoul