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AxialLoadBehaviorofStiffenedConcrete-FilledSteelColumns12345678C.S.Huang;Y.-K.Yeh;G.-Y.Liu;H.-T.Hu;K.C.Tsai;Y.T.Weng;S.H.Wang;andM.-H.WuAbstract:Thisstudyinvestigatestheaxialloadbehaviorofconcrete-®lledsteeltubular~CFT!columnswiththewidth-to-thicknessratiosbetween40and150,andproposesaneffectivestiffeningschemetoimprovethemechanicalpropertiesofsquarecross-sectionalCFTcolumns.Seventeenspecimensweretestedtoexaminetheeffectsofcross-sectionalshapes,width-to-thicknessratios,andstiffeningarrangementsontheultimatestrength,stiffness,andductilityofCFTcolumns.Moreover,nonlinear®niteelementanalysiswasalsoconductedtoinvestigatecross-sectionalaxialstressdistributionattheultimatestrength.Comparingthemeasuredultimatestrengthwithestimatesbyusingsomecurrentspeci®cationssuggestedthatcurrentspeci®cationsmayconsiderablyunderestimatetheultimatestrengthofcircularCFTcolumns,particularlyforcolumnswithasmallwidth-to-thicknessratio.Resultsinthisstudydemonstratethattheproposedstiffeningschemecansigni®cantlyenhancetheultimatestrengthandductilityofsquareCFTcolumns.DOI:10.1061/~ASCE!0733-9445~2002!128:9~1222!CEDatabasekeywords:Tubes;Axialloads;Finiteelementmethod;Stiffening;Steelcolumns.IntroductionAccordingtoZhong'sbook~Zhong1995!,thereviewbyUy~1998!andthereportbyHuangetal.~1998!,numerousresearch-Inmodernstructuralconstructions,concrete-®lledsteeltubularershaveinvestigatedtheaxialloadbehaviorofCFTcolumns~CFT!columnshavegraduallybecomeacentralelementinstruc-throughlaboratorytestsand®niteelementanalyses~i.e.,Furlongturalsystemslikebuildings,bridgesandsoforth.CFTcolumns1967;GardnerandJocobson1967;KnowlesandPark1969;havebecomesowidespreadowingtotheiraxiallycompressedTomiietal.1977;Sakinoetal.1985;LinandHuang1989;KwonnaturemakingthemsuperiortoconventionalreinforcedconcreteandSong1998;andSchneider1998!.Thesestudiescon®rmedandsteelstructuralsystemsintermsofstiffness,strength,ductil-thatthebehaviorofCFTcolumnsisheavilyin¯uencedbytheity,andenergyabsorptioncapacity.Thesteeltubenotonlytakeswidth-to-thicknessratio~B/torD/t,Dfordiameterofcircularaxialload,butalsoprovidescon®ningpressuretotheconcretecrosssection!,theheight-to-widthratio~L/BorL/D!,thecross-core,whiletheconcretecoretakesaxialloadandpreventsorsectionalshapeofthesteeltube,andthestrengthratioofconcretedelayslocalbucklingofthesteeltube.Furthermore,concrete-tosteel.Forexample,Furlong~1967!experimentallyinvestigated®lledcompositecolumnsalsohavetheadvantageofrequiringnotheeffectsofB/t~between30and100!,andthestrengthratioofformworkduringconstruction,thusreducingconstructioncosts.concretetosteelontheaxialstrengthofCFTcolumns.Mean-while,KnowlesandPark~1969!experimentallystudiedhowD/t1AssociateProfessor,Dept.ofCivilEngineering,NationalChiao~between15and60!andL/D~between221!affectthebehaviorTungUniv.,1001Ta-HsuehRd.,Hsinchu,Taiwan30050.Email:ofcircularCFTcolumns.Tomiietal.~1977!collectedandana-cshuang@cc.nctu.edu.twlyzedabout270experimentalresultsforCFTcolumnswithcir-2AssociateResearchFellow,NationalCenterforResearchonEarth-cular,octagonal,andsquarecrosssections.Schneider~1998!alsoquakeEngineering,Taipei,Taiwan10617.3AssociateResearchFellow,NationalCenterforResearchonEarth-experimentallyinvestigatedtheaxialloadbehaviorofCFTcol-quakeEngineering,Taipei,Taiwan10617.umnswithcircularandrectangularcrosssections,andwithD/t4Professor,Dept.ofCivilEngineering,NationalChengKungUniv.,between17and50andL/Dbetween4and5.Additionally,aTainan,Taiwan70101.nonlinear®niteelementanalysiswascarriedouttoinvestigatethe5Professor,Dept.ofCivilEngineering,NationalTaiwanUniv.,Taipei,adequacyofthedesignprovisions.Sincetheearly1960s,re-Taiwan10617.searchersinChinahavesystematicallyandcomprehensivelyin-6GraduateStudent,Dept.ofCivilEngineering,NationalTaiwanvestigatedthebehaviorofcircularCFTcolumnsthroughlabora-Univ.,Taipei,Taiwan10617.7torytestsandnumericalanalyses,andZhong'sbooksummarizesFormerGraduateStudent,Dept.ofCivilEngineering,NationalTai-wanUniv.,Taipei,Taiwan10617.someoftheresults~Zhong1995!.Theseearlyworksrevealedthat8FormerGraduateStudent,Dept.ofCivilEngineering,Nationalasquareconcrete-®lledsteeltubedoesnotperformaswellasaChengKungUniv.,Tainan,Taiwan70101.circularonebecauseasquaresteeltubeprovideslesscon®ningNote.AssociateEditor:AmirMirmiran.DiscussionopenuntilFebru-pressure,makinglocalbucklingmorelikely.Thisfactisre¯ectedary1,2003.Separatediscussionsmustbesubmittedforindividualpa-inmoderndesigncodessuchasManualofSteelConstruction:pers.Toextendtheclosingdatebyonemonth,awrittenrequestmustbeLoadandResistanceFactorDesign(LRFD)byAmericanInsti-®ledwiththeASCEManagingEditor.ThemanuscriptforthispaperwastuteofSteelConstruction~AISC1994;1997!andRecommenda-submittedforreviewandpossiblepublicationonMay9,2001;approvedonJanuary7,2002.ThispaperispartoftheJournalofStructuraltionsforDesignandConstructionofConcrete-FilledSteelTubu-Engineering,Vol.128,No.9,September1,2002.©ASCE,ISSN0733-larStructuresbyArchitecturalInstituteofJapan~AIJ1997!,in9445/2002/9-1222±1230/$8.001$.50perpage.whichthelimitingwidth-to-thicknessratioofasteel1222/JOURNALOFSTRUCTURALENGINEERING/SEPTEMBER2002 foundthatalthoughthesecondstiffeningschemedoesnotin¯u-encestrength,itdoesenhancetheductilityofsquareCFTcol-umns.ThisworkproposesanewstiffeningschemeasanalternativeforenhancingthebehaviorofsquareCFTcolumnsintermsofstrengthandductility.Theproposedstiffeningschemeinvolvesweldingasetoffoursteelbars~calledtiebarsbelow!atregularspacingalongthetubeaxis.ThestiffenersaretypicallyarrangedasdisplayedinFig.1.Toinvestigatetheeffectivenessoftheproposedstiffeningscheme,laboratorytestsof14specimensunderconcentriccompressionwerecarriedoutforshortsquareCFTcolumnswithorwithoutstiffening.Forcomparison,experi-mentswerealsoconductedonthreeshortCFTcolumnswithcir-cularcrosssections.TheprimaryparametersconsideredinthistestprogramincludeB/t,thespacingofthetiebars(Ls),andthediametersofthetiebars.Thesefactorswereexperimentallyin-vestigatedtoassesstheirin¯uenceonultimatestrength,initialstiffness,andductility.Finally,tofurtherunderstandtheeffectsofcross-sectionalshapesandstiffeningincon®ningtheconcretecore,nonlinear®niteelementanalyseswereconductedtodeter-minethedistributionofaxialstressalongatypicalcrosssectionwhenaCFTcolumnreachedmaximumstrength.Fig.1.~a!Photofortypicalarrangementofstiffness,and~b!layoutExperimentalProgramoftypicalstiffenedcrosssectionSeventeentestspecimenswithL/B53weretestedunderconcen-tricaxialcompression.Ofthespecimens,®vewereforB/ttubewitharectangularcrosssectionissmallerthanthatwitha(orD/t)540,eightwereforB/t(orD/t)570,andfourwereforcircularcrosssection.B/t(orD/t)5150.ThreeofthespecimenswereforcircularToimprovethebehaviorofrectangularCFTcolumns,twocrosssectionandtherestwereforsquarecrosssection.Table1stiffeningmeasuresarefrequentlyadopted:weldinglongitudinalsummarizesthedimensionsandmaterialpropertiesofthespeci-steelstripsontheinternalsurfaceofasteeltubeandweldingmens,withAsandfy,trepresentingthecross-sectionalareaandshearstuds.GeandUsami~1992!andKwonandSong~1998!theyieldstrengthofthesteeltube,respectively;f8cdenotingtheexperimentallystudiedtheeffectsoftheformermeasureonthestrengthoftheconcretecore;AtotalbeingthetotalcompositebehaviorofsquareCFTcolumns.Accordingtotheirresults,thiscross-sectionalarea;anddandfy,brepresentingthediameterandapproachincreasesultimatestrengthbecausethelongitudinalyieldstrengthofthetiebars,respectively.Forconvenience,eachstiffenerssharetheaxialload.However,thismethodalsocausesaspecimenhasanindividualdesignation,involvingtwoEnglishseverestrengthlossinthepostbucklingrange.Linetal.~1993!lettersfollowedbyaseriesofnumbers~seeTable1!.The®rstTable1.PropertiesforConcrete-FilledSteelTubeComponentsTieBarSpecimenBorD~mm!t~mm!B/torD/tAs/AtotalSteeltubefy,t~MPa!Concretefc8~MPa!d~mm!fy,b~MPa!SU-0402005400.098265.827.15ÐÐSS-040-050~3!2005400.098265.827.159.52410.9SS-040-050~4!2005400.098265.827.1512.7386.0SS-040-100~4!2005400.098265.827.1512.7386.0CU-0402005400.098265.827.15ÐÐSU-0702804700.056272.631.15ÐÐSS-070-050~3!2243.2700.056323.823.949.52395.7SS-070-050~3!R2243.2700.056323.825.739.52395.7SS-070-093~2!2804700.056272.630.497588.6SS-070-093~3!2804700.056272.630.499.52615.3SS-070-140~3!2804700.056272.629.849.5615.3SS-070-187~3!2804700.056272.629.189.52615.3CU-0702804700.056272.631.15ÐÐSU-15030021500.026341.727.20ÐÐSS-150-050~2!30021500.026341.724.007735.8SS-150-100~2!30021500.026341.725.217735.8CU-15030021500.026341.727.23ÐÐJOURNALOFSTRUCTURALENGINEERING/SEPTEMBER2002/1223 letterrepresentsthecross-sectionalshapeofthespecimen~CandSforcircularandsquare,respectively!.Thesecondletter,SorU,denotesaspecimenwithorwithoutstiffening,respectively.Meanwhile,thethreenumbersfollowingtheEnglishlettersde-notethewidth-to-thicknessratio.Forstiffenedspecimens,thelastthreenumbersbeforetheparenthesesrepresentthecenter-to-centerspacingbetweenthestiffenedcrosssectionsLsintermsofmillimeters.Finally,theparenthesizednumberisthereinforcingbarnumberofthetiebar.Tomoreaccuratelysimulatepracticalapplications,thesteeltubesofthesespecimenswerenotan-nealed.Additionally,toinvestigatethebehaviorofCFTcolumnsin-¯uencedbytheresidualstressesfromthewelding,onespecimenwasconstructedbyannealingitsstiffenedsteeltubebeforepour-ingconcreteintothetube.AnadditionalRfollowingtheparen-thesesinTable1isusedtodenotethisspecimenthathasbeenannealed.Allsteeltubeswerecold-formedcarbonsteelwithayieldFig.2.ExperimentalbehaviorofPversusÅ«forB/t(D/t)540strengthofbetween265and342MPa,dependingonthethick-nessofthesteeltube.TheyieldstrengthwasdeterminedfromtestsontensilecouponstakenfromthesteelplatesthatwerealsoB/t540forthesquarecrosssectionandD/t540and70fortheusedtoformthesteeltubes.ThesizeofcouponsandthetestcircularcrosssectionsatisfytherequirementsprescribedintheseproceduresfollowAmericanSocietyforTestingandMaterials,twodesigncodes.ASTME10-84~ASTM1991!requirements.ThesquaretubesFourLVDTswereusedtodeterminetheaveragelongitudinalwereconstructedbyseamweldingtwoU-shapedcold-formedstrain.Forcircularspecimens,theLVDTswereplaced90°apartsteelplates.Ifstiffeningwerespeci®ed,thetiebarswere®lletaroundtheperipheryofthespecimen,whileforsquarespeci-weldedtotheU-shapedcold-formedsteelplates~seeFig.1!be-mens,theLVDTswereplacedalongthecentrallineofeachoftheforemakingtheseamcompletepenetrationgroovewelds~seefoursides.Setsofstraingaugescontainingthree-elementrectan-Fig.1!.ThetypicallayoutofthetiebarsisalsoshowninFig.1.gularrosetteswerealsoplacedontheexteriorofeachCFTcol-Notably,aCFTconstructedbyseamweldingfourpiecesofsteelumntomeasurethelocalstrainsofthesteeltubewall.Finally,platesislikelytofailintheseamweldwhentestedbecauseofuniaxialstraingaugeswereplacedonsomeofthetiebarsinthestressconcentrationatthecorners~LinandHuang1989!.stiffenedspecimens.TypeIPortlandcement,sand,andamaximumaggregatesizeThecompressiontestswereconductedina4900kNuniversalof2cmweremixedtoobtainan18cmslumpwitha28-daytestingmachine.Axialloads,measuredbythetestmachineloadtargetstrengthof27.47MPa.Table1liststheconcretestrengthcell,wereslowlyappliedtothespecimensuchthattheaveragestrainratewasapproximately2.531025L/s.Thecompressionforeachspecimen,obtainedfromcylindertestsconductedatthesametestdayfortheCFTspecimen.testsstoppedatastrainlevelof5%.IfLVDTscouldpossiblybedamagedbythelargedeformationsofaspecimen,theywereThevaluesofAs/AtotallistedinTable1revealthatalltheremovedbeforethe5%strainlevel.specimenssatisfyAIJspeci®cationsonAs/Atotal>0.8%.How-ever,thespecimensofB/t(orD/t)5150violatetheLRFDre-quirementsofAs/Atotal>4%.ExperimentalResultsandDiscussionToconsiderthelimitationsofthewidth-to-thicknessratiospre-scribedinsomeofthedesigncodes,LRFDspeci®cations~AISCFigs.2±5showload±averagestraincurves~orP±Å«curves!for1997!requirethatforasquarecrosssectionallspecimens.ThesemeasuredresultsarepresentedaccordingtoB2Esthedifferentwidth-to-thicknessratiosofthespecimens.Gener-70,withtheFig.9.AxialstressdistributionatP5PpforB/t(D/t)570:~a!overestimationbeingunder5%.CU-70;~b!SU-70;and~c!SS-070-093~2!4.Thestiffnesscomputedbydirectlysuperposingthestiffnessofthesteeltubeandthecoreconcreteissigni®cantlyover-estimated,sometimesbyover40%.meansthatthebehavioroftheconcretecore,ifconsideredinthe5.Theproposedstiffeningschemecanenhancetheultimateuniaxialstressstate,isjustlikeanuncon®nedconcretecylinderstrengthandductilityofsquareCFTcolumnsbutmaysome-underaxialloading.whatdecreasethestiffnessforB/t<70.Theenhancement,Fig.9displaystheapproximatedistributionsofconcreteaxialespeciallyforductility,becomesmoresigni®cantasthelon-stressesatthecrosssectionlocatedB/6~orD/6!abovethemiddlegitudinalspacingoftiebarsdecreases~i.e.,Ls5B/4±B/6forcrosssectionundertheultimateload.Sincetheproblemissym-B/t540±150!.Thefailuremodesofthespecimensindicatemetrical,onlythestressdistributionof1/4thecrosssectionisthatthestiffeningschemeeffectivelydelayslocalbuckling.shown.Notably,thiscrosssectionisastiffenedcrosssectionfor6.Thenonlinear®niteelementanalysisrevealsthatthecross-SpecimenSS-070-093~2!.Thethreespecimenshavetotallydif-sectionalaxialstressdistributionforSpecimenCU-070isferentdistributionpatterns.SpecimenCU-070hasanalmostuni-quiteuniformattheultimatestrength.Theexistenceofstiff-formdistributionoverthecrosssection,andtheexistenceofstiff-enersaltersthepatternofaxialstressdistributionforsquareenersnotonlychangesthepatternofthestressdistributionbutcross-sectionalspecimens.SpecimenSS-070-093~2!showsalsoincreasesstresscapacity.Meanwhile,SpecimenSU-070hasstressconcentrationaroundthecornersandaroundthecon-stressconcentrationaroundthecorner,whileSpecimenSS-070-nectionsbetweenthetiebarsandthesteeltube,whileSpeci-093~2!hasstressconcentrationbotharoundthecornerandaroundmenSU-070onlyshowsthestressconcentrationaroundthetheconnectionsbetweenthetiebarsandthesteeltube.Theaxialcorners.Theaxialstressesatthestressconcentrationareasstressesaroundthestressconcentrationareasaremuchlargerthanindicatethattheproposedstiffeningschemeimprovesthef8c,meaningthatcon®nementisbetterintheseareas.Interest-con®nementofsteeltubeonthecoreconcrete.ingly,theaxialstressesatthestressconcentrationareasforsquareTheenhancedmechanicalpropertiesachievedbyapplyingthespecimensexceedthoseforcircularspecimens.TheaxialstressatproposedstiffeningschemetoasquareCFTcolumnshouldalsothecornerforSpecimenSU-070was50%greaterthanthecylin-bevalidforaCFTcolumnwithhighstrengthcoreconcrete,es-derstrength,whiletheaxialstressesatthestressconcentrationpeciallyinimprovingtheductilityofthehighstrengthconcrete.JOURNALOFSTRUCTURALENGINEERING/SEPTEMBER2002/1229 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