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土木工程毕业设计外文翻译外文原文I：AfundamentalexplanationofthebehaviourofreinforcedconcretebeamsinflexurebasedonthepropertiesofconcreteundermultiaxialstressM.D.KotsovosDepartmentofCivilEngineering,ImperialCollegeofScienceandTechnology,London(U.K.)Thepaperquestionsthevalidityofthegenerallyacceptedviewthatforareinforcedconcretestructuretoexhibitductilebehaviourunderincreasingloaditisnecessaryforthestressstrainrelationshipsofconcretetohaveagraduallydescendingpost-ultimatebranch.Experimentaldataarepresentedforreinforcedconcretebeamsinbendingwhichindicatethepresenceoflongitudinalcompressivestrainsonthecompressivefaceinexcessof0.0035.Itisshownthatthesestrains,whichareessentialforductilebehaviour,arecausedbyacomplexmultiaxialcompressivestateofstressbelowultimatestrengthratherthanpostultimatematerialcharacteristics.Thepresenceofacomplexstresssystemprovidesafundamentalexplanationforbeambehaviourwhichdoesnotaffectexistingdesignprocedures.1.INTRODUCTIONTheplanesectionstheorynot,onlyisgenerallyconsideredtodescriberealisticallythedeformationresponseofreinforcedandprestressedconcretebeamsunderflexureandaxialload,butisalsoformulatedsothatitprovidesadesigntoolnotedforbothitseffectivenessandsimplicity[1].Thetheorydescribesanalyticallytherelationshipbetweenload-carryingcapacityandgeometriccharacteristicsofabeambyconsideringtheequilibriumconditionsatcriticalcross-sections.Compatibilityofdeformationissatisfiedbytheplanecross-sectionsremainplaneassumptionandthelongitudinalconcreteandsteelstressesareevaluatedbythematerialstress-straincharacteristics.Transversestressesandstrainsareignoredforthepurposesofsimplicity.Thestress-straincharacteristicsofconcreteincompressionareconsideredtobeadequatelydescribedbythedeformationalresponseofconcretespecimenssuchasprismsorcylindersunderuniaxialcompressionandthestressdistributioninthecompressionzoneofacross-sectionattheultimatelimitstate,asproposedbycurrentcodesofpracticesuchasCP110[1],exhibitsashapesimilartothatshowninfigure1.Thefigureindicatesthatthelongitudinalstressincreaseswiththedistancefromtheneutralaxisuptoamaximumvalueandthenremainsconstant.Suchashapeofstressdistributionhasbeenarrivedatonthebasisofbothsafetyconsiderationsandthewidelyheldviewthatthestress-strainrelationshipofconcreteincompressionconsistsofbothanascendingandagraduallydescendingportion(seefig.2).Theportionbeyondultimatedefinesthepost-ultimatestresscapacityofthematerialwhich,Typicalstress-strainrelationshipforconcreteincompression.asindicatedinfigure1,isgenerallyconsideredtomakeamajorcontributiontothemaximumload-carryingcapacityofthebeam.However,arecentanalyticalinvestigationofthebehaviourofconcreteunderconcentrationsofloadhasindicatedthatthepost-ultimatestrengthdeformationalresponseofconcreteundercompressivestatesofstresshasnoapparenteffectontheoverallbehaviourofthestructuralformsinvestigated([2],[3]).Ifsuchbehaviouristypicalforanystructure,thenthelargecompressivestrains(inexcessof0.0035)measuredonthetopsurfaceofareinforcedconcretebeamatitsultimatelimitstate(seefig.1),cannotbeattributedtopost-ultimateuniaxialstress-straincharacteristics.Furthermore,sincethecompressivestrainattheultimatestrengthlevelofanyconcreteunderuniaxialcompressionisoftheorderof0.002(seefig.2),itwouldappearthatarealisticpredictionofthebeamresponseunderloadcannotbebasedsolelyontheascendingportionoftheuniaxialstress-strainrelationshipofconcrete.Inviewoftheabove,theworkdescribedinthefollowingappraisesthewidelyheldviewthatauniaxialstress-strainrelationshipconsistingofanascendingandagraduallydescendingportionisessentialfortherealisticdescriptionofthebehaviourofareinforcedconcretebeaminflexure.Resultsobtainedfrombeamssubjectedtoflexureundertwo-pointloadingindicatethatthelargestrainsexhibitedbyconcreteinthecompressionzoneofthebeamsareduetoatriaxialstateofstressratherthantheuniaxialpost-ultimatestress-straincharacteristicsofconcrete.Itisshownthattheassumptionthatthematerialitselfsuffersacompleteandimmediatelossofload-carryingcapacitywhenultimatestrengthisexceedediscompatiblewiththeobservedductilestructuralbehaviourasindicatedbyload-deflexionormoment-rotationrelationships.2.EXPERIMENTALDETAILS2.1.SpecimensThreerectangularreinforcedconcretebeamsof915mmspanand102mmheightx51mmwidthcross-sectionweresubjectedtotwo-pointloadwithshearspansof305mm(seefig.3).Thetensionreinforcementconsistedoftwo6mmdiameterbarswithayieldloadof11.8kN.Thebarswerebentbackattheendsofthebeamssoastoprovidecompressionreinforcementalongthewholelengthoftheshearspans.Compressionandtensionreinforcementalongeachshearspanwerelinkedbyseven3.2mmdiameterstirrups.Neithercompressionreinforcementnorstirrupswereprovidedinthecentralportionofthebeams.Duetotheabovereinforcementarrangementallbeamsfailedinflexureratherthanshear,althoughtheshearspantoeffectivedepthratiowas3.Thebeams,togetherwithcontrolspecimens,werecuredunderdamphessianat20~forsevendaysandthenstoredinthelaboratoryatmosphere(20oC~and40%R.H.)forabout2months,untiltested.FulldetailsoftheconcretemixusedaregivenintableI.2.2.TestingLoadwasappliedthroughahydraulicram