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

会计学1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Whenthematerialishomogeneous,thestiffnessofsectionisconstant.Curvatureislineartothebendingmoment.MFeatureofstiffnessofsection第1页/共21页MyMuMcrEcI0MStageⅠStageⅡStageⅢReinforcedconcreteisnothomogeneous,sothestiffnessofsectionchangeswiththeincreaseinbendingmoment.Featureofstiffnessofsection第2页/共21页1、Curvatureofsection8.1.2Short-timestiffnessofflexuralmemberBs第3页/共21页2、Strainofsteelandconcreteatthecrackedsection

CompressiveareaofconcreteStrainatcrackedsection第4页/共21页3、Averagestrain4、Expressionforshort-timestiffnessStiffnessformula第5页/共21页5.Expressionofparameters(1).Leverarmcoefficient(3)第6页/共21页8.1.3FlexuralstiffnessofcrosssectioninflexuralmemberStiffnesswilldecreaseundersustainedloadCreepofconcreteSlipbetweenconcreteandsteelStiffnessformulaStrainofconcreteatcrackedsectionincreasesStrainofsteelatcrackedsectionincreases第7页/共21页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第8页/共21页8.2CrackcontrolofreinforcedconcretememberCausesofcracksandtheircontrolCrackscausedbyloadsOthercracksTensileforceBendingmomentshearTorqueVerticalanddiagonalcracksMaincauses第9页/共21页8.2.1mechanismofcrackingDevelopmentlength1.OccurrenceofcrackingDevelopmentlength第10页/共21页2、Numberofcracksisstable（裂缝出齐）IfthedevelopmentlengthisL，themaximumspacingbetweencracksis2L，andtheminimumspacingL，averagespacingLm＝1.5L第11页/共21页8.2.2AveragespacingbetweencracksLmisrelatedtothediameterofsteelbar,effectivereinforcementratioandtheconcretecover第12页/共21页8.2.3Averagecrackwidth第13页/共21页ChineseCode:8.2.4maximumcrackwidthSustainedloadeffectRandomicityofcracks第14页/共21页8.3Ductilityofsectionforreinforcedconcretemember8.3.1conceptofductility1、Definition：Thedeformationabilityofsectionthroughoutitsyieldingtoreachingultimateload2、Functionofductility：AbsorbandconsumeseismicenergyAvoidsuddenfailurewithoutwarningAdjustthestructuraldeformationmakingitaccommodatedtothedifferentsettlementandthermaldeformation.Achievethefullredistributionofinternalforce.第15页/共21页8.3.2DuctilitycoefficientofsectioncurvatureinflexuralmemberDuctilitycoefficientofsectioncurvature第16页/共21页FactorsaffectingtheductilityofsectionUltimatecompressivestrainofconcrete、yieldstrengthofreinforcement、strengthofconcrete、reinforcementratio。Ductilityincreaseswiththeincreaseincuandareaofcompressivereinforcements.Ductilitydecreaseswiththeincreaseinareaoftensilereinforcements.第17页/共21页8.3.3Ductilityofsectioncurvatureineccentriccompressionmember1.FailuremodeofeccentriccompressionmemberSmalleccentriccompressionfailureBrittleLargeeccentriccompressionfailureDuctileAxialforceratioAvoidsmalleccentriccompressionfailure2.Axialforceratio第18页/共21页8.4Durabilityofconcretestructure8.4.1generalconceptsofdurabilityDefinitionThestructurecanmeettherequirementsofserviceandsafetythroughoutitslifetimewithoutmajorrepairundernormalmaintenance.FactorsaffectingdurabilityInternalfactorse