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ELSEVIER0141-0296(95)00038-0EngineeringStructures,Vol.18,No.4,pp.311-320,1996Copyright©1996ElsevierScienceLtdPrintedinGreatBritain.Allrightsreserved0141~)296/96$15.00+0.00CyclicloadtestsconcretemodelY.L.MoandR.H.HanonprestressedflamesDepartmentofCivilEngineering,NationalChengKungUniversity,Tainan701,Taiwan(ReceivedSeptember1994;revisedversionacceptedDecember1994)Prestressedconcreteframesarecommonlyusedinbridgedesign.However,verylittleisknownabouttheirbehaviourunderreversedcyclicloads,particularlywhensubjectedtosevereearthquakes,andmostbridgecodesdonotprovidetherequireddesignguide-lines.Resultsfromsmall-scalemodelsofeightprestressedcon-creteframes,testedundervariousloadhistoriessimulatingearth-quakeforcesarepresented.Theexperimentalprimarycurves(horizontalforce-displacementrelationships)arecomparedwiththetheoreticalresults,andtheexperimentalhystereticloopsarecomparedwiththosecalculatedfromTadeka'srules.Theconcretestrengthisapproximately40MPa,andtheeffectiveprestressvariedbetween36%and51%oftheultimatestrengthofthepre-stressingsteel.Itwasfoundthattheductilityandenergydissi-pationincreasedwithdecreasingeffectiveprestress.Theeffectoftheloadhistoryonthereversedcyclicbehaviourisalsosignificant.Keywords:prestressedconcreteframes,bridgedesign,cyclicloadtesting1.IntroductionTherigidframebridgeisoneofthemostwidelyusedhigh-waybridgesworldwideI.Engineerstreatthesuper-andsubstructuresasoneunitbyconstructingacontinuouspre-stressedconcreterigidframewithsupportinglegs.Thistypeofconstructiondoesnotneedconcretepiersandpos-itionsthesupportsawayfromthelowerroadway,thusgiv-ingamoreeconomicalstructure.Generally,thistypeofbridgecanreducethedepthofthemaingirderandsavematerialsinthesuperstructure.Inseismicregionstheinves-tigationoftheresponseoftheprestressedconcreteframebridgestocyclicloadsisveryimportantandpracticallysignificant.Unfortunately,mostpreviousresearchworkonprestressedconcretebridgesconcentratedontheinelasticstaticbehaviour~-7.Ontheotherhand,itisgenerallyknownthatprestressedconcretedisplaysmorebrittlebehaviourthanreinforcedconcrete.Foruseinearthquake-resistantstructuresandtotakeadvantageofeconomicsavingsthroughprestress,itisimperativethatthevariousstructuralcomponentsbehaveinaductilemanner.Dependingonthestateofstressataparticularsection,theconcretemayalsoberequiredtodis-playlargeductilityandenergy-absorptioncapacity.Resultsfromseveralexperimentalstudieshavebeenreported8-13dealingwithreinforcedconcretestructuressubjectedtoreversedcyclicloads.Datafromtestsonprestressedcon-cretestructuresare,however,verylimited14,particularlyunderdifferentreversedcyclicloadhistories.Sincethedis-sipationofseismicenergythroughtheinelasticbehaviourofflexuralmembersispreferable,hingingofbeamsorcol-umnsduringasevereearthquakemaybeexpectedinmanycases.Appropriatedetailingofcriticalregionsinbeamsorcolumnsisthereforeneededtopreventbrittlestructuralfailure.Toapplythereversedcyclicloadtestdataforpracticalseismicdesignofprestressedconcreteframes,itisneces-sarytostudytheeffectofvariouscyclicloadhistories.Thispaperpresentsaseriesofreversedcyclicloadtestresultsfromsmall-scalemodelsofeightprestressedconcreteframesandexaminestheeffectsofprestressandreversedcyclicloadhistoryonprestressedconcreteframes.2.TestprogrammeTheframesweredividedintotwogroups.Thetestsstudytheeffectsoftheeffectiveprestressandthereversedcyclicloadhistoryonthebehaviouroftheprestressedconcreteframes.2.1.ConcreteThetargetcompressivestrengthfortheready-mixconcretewas5000psi(35MPa)inallthespecimens.24standardcylinderswerecastwitheachpourandtestedfrequentlyto311312Cyclicloadtestsonprestressedconcretemode/frames:Y.L.MoandR.H.HanFigureIFramecageFigure22-#3tendon-~'~--~2-#5/7~2em~3PVCtube15t|8__.~-_25~b]Bolt(typ.)11111-31=.5(typ.)H18I18IT~Z2.5(typ.)•I1II12.7==¢15201_~11~tendonj/9cm(typ.)Steelplate(typ.)100Ig--N12-#5-2-#SeetionC-C#2@8cmTestspecimen(incm)monitorconcretestrengthwithage.Thenominalmaximumsizeofthecoarseaggregatewas10mm.2.2.RebarsandprestressingsteelDeformedNo.3Grade40andNo.5Grade60barswereusedinthespecimens.Inthebeams,longitudinalsteelcon-sistedoffourNo.3barsineachspecimen,providingasteelratioof0.79%ofthegrosscross-sectionalareaofthebeams.SimilarlylongitudinalsteelinthecolumnsconsistedoffourNo.5barsineachspecimen,providing2.0%steelTable1PropertiesofstrandandrebarsYieldingUltimatestressYieldingstressUltimate(N/mm2)strain(N/mma)strainStrand(average)1807.80.008612086.40.0160No.5(average)526.60.00251746.10.3037No.3(average)312.80.00149454.00.2637No.2(average)351.30.00167455.70.2605ratio.TiesweremadeofNo.2steelbars.TypicalGrade250strandswithadiameterof12.7mmwereusedinthebottomofthebeamsforallthespecimens,providing0.28%prestressingsteelratio.ThemechanicalpropertiesofstrandsandrebarsarealsoshowninTable1.2.3.CagesFigure1indicatesaframecage.Thebeamandcolumnlengthsofeachframecagewerell70mmand475ram,respectively.ThetiesweredeterminedaccordingtothesheardesignrequirementsspecifiedintheACIcode~5.Twohigh-strengthall-threaded25mm-diameterbarswereplacedatthebottomofeachcolumn.Thesebarswereinsertedint