第8章钢筋混凝土构件的变形裂缝及混凝土结构的耐久性

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Chapter8DuctilityanddurabilityandcheckofcrackanddeflectionDurabilityserviceabilitySafetyServiceabilitylimitstatedeformationCrackwidthDetailingrequirementsFunctionalrequirementsofstructureUltimatelimitstateStrengthdesignofmembers 8.1Checkofdeflectionofreinforced-concreteflexuralmembersFlexuralstiffness-bendingmomentneededforunitcurvatureofsection.Itshowstheabilityofsectionagainstdeflection.8.1.1Conceptofflexuralstiffnessofsection Whenthematerialishomogeneous,thestiffnessofsectionisconstant.Curvatureislineartothebendingmoment.MFeatureofstiffnessofsection MyMuMcrEcI0MStageⅠStageⅡStageⅢReinforcedconcreteisnothomogeneous,sothestiffnessofsectionchangeswiththeincreaseinbendingmoment.Featureofstiffnessofsection 1、Curvatureofsection8.1.2Short-timestiffnessofflexuralmemberBs 2、StrainofsteelandconcreteatthecrackedsectionCompressiveareaofconcreteStrainatcrackedsection 3、Averagestrain4、Expressionforshort-timestiffnessStiffnessformula 5.Expressionofparameters(1).Leverarmcoefficient(3) 8.1.3FlexuralstiffnessofcrosssectioninflexuralmemberStiffnesswilldecreaseundersustainedloadCreepofconcreteSlipbetweenconcreteandsteelStiffnessformulaStrainofconcreteatcrackedsectionincreasesStrainofsteelatcrackedsectionincreases 8.1.4Principleofminimumstiffnessandcomputationofdeflection1.PrincipleofminimumstiffnessAlongthelengthofspaninsimplebeam,deflectioniscomputedapplyingthestiffnessatthemaximum-momentsection(maximumstiffness),basedonmechanicalformulanotconsideringthesheardeformation(在简支梁全跨范围内，按弯矩最大处的截面弯曲刚度，即最小的截面弯曲刚度，用材料力学方法中不考虑剪切变形影响的公式来计算挠度。)2.Computationofdeflection3.DiscussionHowtocontrolthedeflection 8.2CrackcontrolofreinforcedconcretememberCausesofcracksandtheircontrolCrackscausedbyloadsOthercracksTensileforceBendingmomentshearTorqueVerticalanddiagonalcracksMaincauses 8.2.1mechanismofcrackingDevelopmentlength1.OccurrenceofcrackingDevelopmentlength 2、Numberofcracksisstable（裂缝出齐）IfthedevelopmentlengthisL，themaximumspacingbetweencracksis2L，andtheminimumspacingL，averagespacingLm＝1.5L 8.2.2AveragespacingbetweencracksLmisrelatedtothediameterofsteelbar,effectivereinforcementratioandtheconcretecover 8.2.3Averagecrackwidth ChineseCode:8.2.4maximumcrackwidthSustainedloadeffectRandomicityofcracks 8.3Ductilityofsectionforreinforcedconcretemember8.3.1conceptofductility1、Definition：Thedeformationabilityofsectionthroughoutitsyieldingtoreachingultimateload2、Functionofductility：AbsorbandconsumeseismicenergyAvoidsuddenfailurewithoutwarningAdjustthestructuraldeformationmakingitaccommodatedtothedifferentsettlementandthermaldeformation.Achievethefullredistributionofinternalforce. 8.3.2DuctilitycoefficientofsectioncurvatureinflexuralmemberDuctilitycoefficientofsectioncurvature FactorsaffectingtheductilityofsectionUltimatecompressivestrainofconcrete、yieldstrengthofreinforcement、strengthofconcrete、reinforcementratio。Ductilityincreaseswiththeincreaseincuandareaofcompressivereinforcements.Ductilitydecreaseswiththeincreaseinareaoftensilereinforcements. 8.3.3Ductilityofsectioncurvatureineccentriccompressionmember1.FailuremodeofeccentriccompressionmemberSmalleccentriccompressionfailureBrittleLargeeccentriccompressionfailureDuctileAxialforceratioAvoidsmalleccentriccompressionfailure2.Axialforceratio 8.4Durabilityofconcretestructure8.4.1generalconceptsofdurabilityDefinitionThestructurecanmeettherequirementsofserviceandsafetythroughoutitslifetimewithoutmajorrepairundernormalmaintenance.FactorsaffectingdurabilityInternalfactorsexternalfactorsStrengthofconcrete,water-cementratio,chemicalgradientcontentEnvironmentalconditionsConcretecarbonationandsteelcorrosion Short-termstiffnessBs