土木工程专业英语 课件 3.2.Limit State Design

3.2LimitStateDesign

Limitstatedesignrequiresthestructuretosatisfytwoprincipalcriteria:ultimatelimitstate承载能力极限状态

serviceabilitylimit正常使用极限状态UltimateLimitStateAstructureisdeemedtosatisfytheultimatelimitstatecriteriaifalldesignvaluesof

loadeffectcombination荷载效应组合

arebelowthedesignvaluesofbearingcapacity承载能力calculatedforthesectionunderconsideration.UltimateLimitStatestructuralcollapseofallorpartofthestructure(verylowprobabilityofoccurrence)andlossoflifecanoccur.Lossofequilibriumofapartorallofastructureasarigidbody(tipping

使倾斜,推倒,使倾覆,slidingofstructure).UltimateLimitStatesRuptureofcriticalcomponentscausingpartialorcompletecollapse.(flexural,shearfailure).UltimateLimitStatesFormationofaplasticmechanism-yieldingofreinforcedformsplastichingesatenoughsectionstomakestructureunstable.Instabilitycasedbydeformationsofstructurecausingbucklingofmembers.Fatigue-memberscanfractureunderrepeatedstresscyclesofserviceloads(maycausecollapse).ServiceabilityLimitStateAstructureisdeemedtosatisfytheserviceabilitylimitstatewhentheconstituentelementsdonotdeflectbymorethancertainlimitslaiddowninthebuildingcodes,thefloorsfallwithinpredeterminedvibrationcriteria,andcrackwidthsinconcretearekeptbelowspecifieddimensions.SimplysupportedBeamCantileverbeamServiceabilityLimitStatesFunctionaluseofstructureisdisrupted,butcollapseisnotexpectedMoreoftentoleratedthanananultimatelimitstatesincelessdangeroflossoflife.Excessivecrackwidthleakagecorrosionofreinforcementgradualdeteriorationofstructure.ServiceabilityLimitStatesMoreoftentoleratedthanananultimatelimitstatesincelessdangeroflossoflife. ExcessivedeflectionsfornormalservicecausedbypossibleeffectsmalfunctionofmachineryvisuallyunacceptableServiceabilityLimitStatesMoreoftentoleratedthanananultimatelimitstatesincelessdangeroflossoflife. ExcessivedeflectionsfornormalservicecausedbypossibleeffectsdamageofnonstructuralelementschangesinforcedistributionspondingonroofscollapseofroofServiceabilityLimitStatesMoreoftentoleratedthananultimatelimitstatesincelessdangeroflossoflife.Undesirablevibrationsverticalfloors/bridgeslateral/torsionaltallbuildingsChangeintheloadingCharacteristicLoads荷载标准值Theexpectedmaximummagnitudeofloadisreferredtoasthecharacteristicload.deadloadSelfweightofthestructureandanypermanentfittingsandfinishes;Magnitudecanbeestimatedonthebasisofmaterialdensitiesandcomponentsizes.ConstantmagnitudeandfixedlocationDeadLoadsExamples:WeightoftheStructure (Walls,Floors,Roofs,Ceilings,Stairways)FixedServiceEquipment (HVAC,PipingWeights,CableTray, Etc.)CanBeUncertain….pavementthicknessearthfilloverundergroundstructureCharacteristicLoads荷载标准值liveloadassociatedwithoccupationanduseofthebuildingThemagnitudeofliveloadscannotbequantifiedwithanyprecisionanditisthereforenecessarytomakesomeallowance公差

baseduponthetypeofactivitytobeperformedwithinthebuilding.CharacteristicLoads荷载标准值windloadThemagnitudeofwindloadsiscloselyrelatedtowindspeeddesignwindspeedrepresentsthemaximumlikelyspeedataparticulargeographicallocationbaseduponstatisticaldata.shapeanddimensionsofthebuildinganditslocalenvironment.WindLoadsDesignwindpressure, p=qz*G*Cp

qz=Windvelocitypressure

G= GustResponseFactor风振系数Cp= Externalpressurecoefficients(accountsfor pressuredirectionsonbuilding)earthquakespressuresPartialSafetyFactors荷载分项系数The

characteristic/normalvaluesofloadsItisassumedthatinninety-fivepercentcasesthecharacteristicloadswillnotbeexceededduringthelifeofthestructuresSm

Sm+1.645σPartialSafetyFactors荷载分项系数Preventstructuralfailurewhenstructuresarealsosubjectedto

overloading.dependingonthenatureofloadsortheircombinations,andthelimitstatebeingconsidered.Designload=Characteristicload

x

Partialsafetyfactorforload

characteristicstrengthofamaterial

材料强度标准值Characteristicstrengthofamaterialobtainedisthestrengthofthatmaterialbelowwhichnotmorethanfivepercentofthetestresultsareexpectedtofallfm-1.645σfm

partialsafetyfactorforthematerialGB50010-2002statesthatpartialsafetyfactorforconcrete=1.4partialsafetyfactorforsteelreinforcement=1.1whenassessingthedeflection,thematerialpropertiessuchascompressivestrengthshouldbetakenasthoseassociatedwiththecharacteristicstrengthofthematerial.正常使用极限状态--材料强度标准值LimitStateDesignUltimatelimitstatesresistancestobendingmoment,shearforce,axialthrust,torsionalmomentateverysectionshallnotbelessthantheirappropriatevaluesatthatsectionduetothemostunfavourable

combinationofthedesignloadsonthestructure.Designresistance≤Designloadco