荷载、强度和结构安全

河北建筑工程学院毕业设计（论文）外文资料翻译系别：土木工程系专业：土木工程专业班级：工043班姓名：臧海月学号：19EnglishCivilEngineering（用外文写）指导教师评语：签字：年月日注：请将该封面与附件装订成册。外文原文Loads,Strength,andStructuralSafetyA.LoadsLoadsthatactonstructuresareusuallyclassifiedasdeadloadsorliveloads.Deadloadsarefixedinlocationandconstantinmagnitudethroughoutthelifeofthestructure.Usuallytheself-weightofastructureisthemostimportantpartofthedeadload.Thiscanbecalculatedclosely,basedonthedimensionsofthestructureandtheunitweightofthematerial.Concretedensityvariesfromabout90to120pcf(14to19kN/m3)forlightweightconcrete,andisabout145pcf(23kN/m3)fornormalconcrete.Incalculatingthedeadloadofstructuralconcrete,usuallya5pcf(1kN/m3)incrementisincludedwiththeweightoftheconcretetoaccountforthepresenceofthereinforcement.Liveloadsareloadssuchasoccupancy,snow,wind,ortrafficloads,orseismicforces.Theymaybeeitherfullyorpartiallyinplace,ornotpresentatall.Theymayalsochangeinlocation.Althoughitistheresponsibilityoftheengineertocalculatedeadloads,liveloadsareusuallyspecifiedbylocal,regional,ornationalcodesandspecifications.TypicalsourcesarethepublicationsoftheAmericanNationalStandardsInstitute,theAmericanAssociationoftheStateHighwayandTransportationOfficialsand,forwendloads,therecommendationsoftheASCETaskCommitteeonWindForce.Specifiedliveloadsusuallyincludesomeallowanceforoverload,andmayincludedynamiceffects,explicitlyorimplicitly.Liveloadscanbecontrolledtosomeextentbymeasuressuchaspostingofmaximumloadsforfloorsorbridges,buttherecanbenocertaintythatsuchloadswillnotbeexceeded.Itisoftenimportanttodistinguishbetweenthespecifiedload,andwhatistermedthecharacteristicload,thatis,theloadthatactuallyisineffectundernormalconditionsofservice,whichmaybesignificantlyless.Inestimatingthelong-termdeflectionofastructure,forexample,itisthecharacteristicloadthatisimportant,notthespecifiedload.Thesumofthecalculateddeadloadandthespecifiedliveloadiscalledtheserviceload,becausethisisthemaximumloadwhichmayreasonablybeexpectedtoactduringtheservicelifeofthestructure.Thefactoredload,orfailureloadwhichastructuremustjustbecapableofresistingisamultipleoftheserviceload.Loadsarerandomprocesses;moreprecisely,theyarestochasticprocesses.However,inordertomatchtherequirementsofthemethodsofcalculationactuallyusedinmoststructuralspecification(allowablestressesandsemi-probabilisticmethods),eachloadisalsocharacterizedbytheparametersrepresentativeofthedifferentcomputationalmethods.Theloadscanbeclassifiedwithrespecttotheireffectonthestructure(staticordynamic)orwithrespecttotheirvariationofintensity.Loadscanalsobeclassifiedwithrespecttosomeparticularaspect,suchaslimitedornotlimited,havinglongorshortduration,dependentornotonhumanactivitiesetc.Loadsconsistof:(1)concentratedanddistributedforces(directactions),(2)imposeddeformations(indirectactions).Aloadisassumedasasingleloadifitisnotrelatedtoanyotherloadorimposeddeformationactingonthestructure.Inpracticemorethanonesingleloadactsonthestructure,althoughitisconvenienttoconsidereachloadseparately.Loadsarerandomprocesses;moreprecisely,theyarestochasticprocesses.However,inordertomatchtherequirementsofthe:methodsofcalculationactuallyusedinmoststructuralspecifications('allowablestressesandsemi-probabilisticmethods),eachloadisalsocharacterizedbytheparametersrepresentativeofthedifferentcomputationalmethods',Theloadscanbeclassifiedwithrespecttotheireffectonthestructure(staticordynamic)orwithrespecttotheirvariationofintensity.Loadscanalsobeclassifiedwithrespecttosomeparticularaspect,suchaslimitedornotlimited,havinglongorshortduration,dependentornotonhumanactivitiesetc.B.StrengthThestrengthofastructuredependsonthestrengthofthematerialsfromwhichitismade.Thepropertiesofsteelthathavebeendescribedsofarareapplicableonlyiftheambienttemperaturestayswithinreasonableproximityof70。F,say,from30toll0。F.Thepropertiesofsteeldonotchangeappreciablyfortemperaturesuptoapproximately300。Fto400。F,althoughthestress-straincurveshowsincreasingnonlinearitywhenthetemperatureexceeds250。F.Itisthereforefortunatethatmoststructureswillneverexperienceheatlevelsthatgopastthesepoints,andeveninthosethatdo,thehightemperaturesarenormallyofveryshortdurationandappearonlyoverasmallportionofthestructure.Atypicalexampleiswhatmaytakeplaceinastructureduringafire:Thetemperaturesmayreachhighlevels,butonlyforaveryshorttime,andnormallyonlyinhighlylocalizedspots.Exceptionsdoappear,ashasbeenevidencedbysomeofthemorespectacularfire-relatedcollapses(McCormickPlace,inChicago,Illinois,forexample),butthesearefortunatelyfewandfarbetween.Realisticconditionsarebetterrepresentedbywhattookplaceduringthefull-scalefiretestthatwasconductedinaparkinggarageinScranton,Pennsylvania,in1972:Damagewaslocalized,andmostofitwaseasilyrepaired,forexample,bycleaningblackenedareasandreplacingdamagedtiles.Nevertheless,itisimportanttoknowhowheataffectsthematerial,and,ifnecessary,totakeheatintoaccountinthedesignprocess.TherelationshipsbetweenthetemperatureandtheprimarystrengthandstiffnesscharacteristicsofsteelareshowninFig.5.6.Forallpracticalpurposes,Fy,Fu,andEshowdecreasingvaluesasthetemperatureincreases,althoughtherateofdecreasebecomessignificantonlyafterthetemperaturehasreachedapproximately1,000*F.Fuactuallyexhibitsaslightincreasebetween250*and600*F,whichisduetothephenomenonofstrainaging.Theyieldandultimatestresseshavedroppedtoaboutone-halfoftheirroomtemperaturevalueat1,100--1,200'F;atthislevelEhasreachedabout60percentofitsoriginalvalue.ThelevelofEisactuallymoreimportantforthestructure,sincedeflectionsaredirectlyproportionaltothemodulusofelasticity.Thephenomenonofcreepwillalsocomeintoplayiftheloadedstructureissubjectedtoincreasedtemperaturesforanextendedperiod.Temperingofsteelisnormallydoneinconjunctionwithquenching,suchaswhenthehigh-strengthquenchedandtemperedsteelsareproduced.Therapidcoolingofthesteelduetoquenchingwillproducethehard,fine-grainedstructurecalledmartensite.Althoughverystrong,martensiteisalsoverybrittle,andthetemperingisdonetoreshapethecrystallingstructureonlytotheextentthatductilityandtoughnessareincreased,butstrengthismaintained.Anyamountofheatinputandsubsequentcoolingwillproduceacertainlevelofbuilt-inorYii'~stresses,duetotherestraintsofthematerialandthestructuretothecontractionsthat3musttakeplace.Thisoccursveryprominentlyinweldedjoints;itwillalsooccurthroughoutanystructureorpartofitthathasbeenheated.Iftheheathasbeenappliedunevenly,andthecontractionsarenotrestrained,acertainamountofdistortionisboundtoappear.Thismaymakestructuralfitupdifficult,butappropriateapplicationofheatandcontrolledcoolingmayrelievesuchproblems.Thedesignerthatisconcernedaboutweldingandotherresidualstressesinjointswithhighdegreesofrestraintmaychoosetoredesigntheconnectiongeometrytoavoidproblems.Referencedetailsthesolutiontosuchproblemsastheypertaintolamellartearing,buttherecommendationsareexcellentadviceforthedesignandfabricationofweldedconnectionsingeneral.Itisalsopossibletousestressrelievinginsomeform,aswasdoneforanumberofthebeam-column-diagonalconnectionsintheexteriorframesoftheJohnHancockCenterinChicago,IllinoisLocalquenchingeffectsmayappearasafireisextinguishedinastructure,andwaterfromthefirehoseshitsheatedsteel.However,itisrareforlocalquenchingeffectstooccuroveranythingbutaminorarea.Thestructuraleffectisthereforeminimal,butheattreatmentcanbeappliedtoremoveanyproblemspots,iftheownerofthestructureisleeryofdoingnothing.Naturally,ifthemembershavebeendeformedbadlytheymayrequirereplacementanyway.However,thematerialinitselfdoesnotusuallysufferirreparabledamageduetoafire.C.StructuralSafetyandReliabilitySafetyrequiresthatthestrengthofastructurebeadequateforallloadsthatmayconceivablyactonit.Ifstrengthcouldbepredictedaccuratelyandifloadswereknownwithequalcertainty,thensafetycouldbeassuredbyprovidingstrengthjustbarelyinexcessoftherequirementsoftheloads.Buttherearemanysourcesofuncertaintyintheestimationofloadsaswellasinanalysis,design,andconstruction.Theseuncertaintiesrequireasafetymargin.Inrecentyearsengineershavecometorealizethatthematterofstructuralsafetyisprobabilisticinnature,andthesafetyprovisionsofmanycurrentspecificationsreflectthisview.Separateconsiderationisgiventoloadsandstrength.Loadfactors,largerthanunity,ateappliedtothecalculateddeadloadsandestimatedorspecifiedserviceliveloads,toobtainfactoredloadsthatthemembermustjustbecapableofsustainingatincipientfailure.Loadfactorspertainingtodifferenttypesofloadsvary,dependingonthedegreeofuncertaintyassociatedwithloadsofvarioustypes,andwiththelikelihoodofsimultaneousoccurrenceofdifferentloads.Morestructuralreliabilitytheoryisconcernedwiththerationaltreatmentofuncertaintiesinstructuralengineeringandwiththemethodsforassessingthesafetyandserviceabilityofcivilengineeringandotherstructures.Itisasubjectwhichhasgrownrapidlyduringthelastdecadeandhasevolvedfrombeingatopicforacademicresearchtoasetofwell-developedordevelopingmethodologieswithawiderangeofpracticalapplications.Uncertaintiesexistinmostareasofcivilandstructuralengineeringandrationaldesigndecisionscannotbemadewithoutmodellingthemandtakingthemintoaccount.Manystructuralengineersareshieldedfromhavingtothinkaboutsuchproblems,atleastwhendesigningsimplestructures,becauseoftheprescriptiveandessentially-deterministicnatureofmostcodesofpractice.Thisisanundesirablesituation.Mostloadsandotherstructuraldesignparametersarerarelyknownwithcertaintyandshouldberegardedasrandomvariablesorstochasticprocesses,evenifindesigncalculationstheyareeventuallytreatedasdeterministic.Someproblemssuchastheanalysisofloadcombinationscannotevenbeformulatedwithoutrecoursetoprobabilisticreasoning.Untilfairlyrecentlytherehasbeenatendencyforstructuralengineeringtobedominatedbydeterministicthinking,characterizedindesigncalculationsbytheuseofspecifiedminimummaterialproperties,specifiedloadintensitiesandbyprescribedproceduresforcomputingStressesanddeflections.Thisdeterministicapproachhasalmostcertainlybeenreinforcedbytheverylargeextenttowhichstructuralengineeringdesigniscodifiedandthelackoffeedbackabouttheactualperformanceofstructures.Forexample,actualstressesarerarelyknown,deflectionsarerarelyobservedormonitored,andsincemoststructuresdonotcollapsetherealreservesofstrengthsaregenerallynotknown.Incontrast,inthefieldofhydraulicsystems,muchmoreisknownabouttheactualperformanceof,say,pipenetworks,weirs,spillwaysetc.,astheirperformanceinservicecanberelativelyeasilyobservedordetermined.Moststructuraldesignisundertakeninaccordancewithcodesofpractice,whichinmanycountrieshavelegalstatus,meaningthatcompliancewiththecodeautomaticallyensurescompliancewiththerelevantclausesofthebuildinglaws.Structuralcodestypicallyandproperlyhaveadeterministicformatanddescribewhatareconsideredtobetheminimumstandardsfordesign,constructionandworkmanshipforeachtypeofstructure.Mostcodescanbeseentobeevolutionaryinnature,withchangesbeingintroducedormajorrevisionsmadeatintervalsof3-10yearstoallowfor:newtypesofstructuralform,theeffectsofimprovedunderstandingofstructuralbehaviour,theeffectsofchangesinmanufacturingtolerancesorqualitycontrolprocedures,abetterknowledgeofloads,etc.Thelackofinformationabouttheactualbehaviourofstructurescombinedwiththeuseofcedesembodyingrelativelyhighsafetyfactorscanleadtotheview,stillheldbysomeengineersaswellasbysomemembersofthegeneralpublic,thatabsolutesafetycanbeachieved.Absolutesafetyisofcourseunobtainable;andsuchagoalisalsoundesirable,sinceabsolutesafetycouldbeachievedonlybydeployinginfiniteresources.Itisnowwidelyrecognized,however,thatsomeriskofunacceptablestructuralperformancemustbetolerated.Themainobjectofstructuraldesignisthereforetoensure,atanacceptablelevelofprobability,thateachstructurewillnotbecomeunfitforitsintendedpurposeatanytimeduringitsspecifieddesignlife.Moststructures,however,havemultipleperformancerequirements,commonlyexpressedintermsofasetofserviceabilityandultimatelimitstates,mostofwhicharenotindependent;andthustheproblemismuchmorecomplexthanthespecificationofjustasingleprobability.Thereisaneedforallstructuralengineerstodevelopanunderstandingofstructuralreliabilitytheoryandforthistobeappliedindesignandconstruction,eitherindirectlythroughcodesorbydirectapplicationinthecaseofspecialstructureshavinglargefailureconsequences,theaiminbothcasesbeingtoachieveeconomytogetherwithallappropriatedegreeofsafety.Thesubjectisnowsufficientlywelldevelopedforittobeincludedasaformalpartofthetrainingofallcivilandstructuralengineers,bothatundergraduateandpost-graduatelevels.Coursesonstructuralsafetyhavebeengivenatsomeuniversitiesforanumberofyears.2、外文资料翻译译文荷载、强度和结构安全一、荷载作用在结构上的荷载通常分为恒载或活载。恒载在结构整个使用寿命期间的位置是固定的，其大小是不变的。通常，结构的自重是恒载是最重要部分。它可以根据结构的尺寸和材料的单位重量进行精确计算。混凝土的密度是变化的，对于轻质混凝土大约从90至120pcf(14至19KN/M=3\*Arabic3)的增加量。活载就是如人群、雪、风和车辆荷载或地震力等荷载。他们可能全部或部分地出现，或者根本不出现。这些荷载的位置是变化的。计算恒荷载是工程师的职责，然而活何载通常由当地的地区的或国家的规范和准则所规定。标准的来源是美国国家标准学会，美国州际公路与运输工作者协会主办的刊物，对于风荷载采用美国土木工程学会风力专题委员会的建议。规定活荷载一般包含某些容许的超载，并可以明显地或隐含地计入冲击作用。活载可以采用在楼板或桥梁标明最大荷载那牙膏的措施在某种程度上加以控制，但是也不能肯定这些荷载不会被超过。将规定荷载所谓特征荷载区别开来往往是很重要的，也就是说，后者是正常使用情况下实际起作用的荷载，它是很小的。例如在计算结构的长期挠度时，重要的是特征荷载，而不是规定荷载。计算得到的恒载和规定的活载的总和称为使用荷载，因为这是在结构使用寿命期间可预料到要作用的最大荷载。使用荷载乘以一个系数就是计算荷载，即使破坏荷载，它就是结构必须恰好能抵抗的荷载。二、强度结构的强度取决于建造它的材料的强度。到目前为止所知道的钢材的强度只适用于环境温度保持在70度上下的情况，大约从30度到110度。当温度超250度时，虽然应力应变曲线呈现出非线形增长，但温度达到300度到400度时钢材的性质仍没有明显改变。幸亏大多数结构从不处于超出这些数字的热度，即使处于那种温度下，高温持续时间都很短，并且只是结构的一小部分处于此温度下，一个典型的例子就是当火灾发生时所发生的一切：温度可能达到很高值，但持续时间很短，并且高温只出现在结构的局部。就象被一些比较令人震惊的与火灾有关的倒塌事件（如美国芝加哥市的MCCORMICK广场）所证明的那样，确实有例外，幸亏此类事件极少发生。1972年，在宾夕法尼亚洲市斯科兰顿市一个停车场的汽车修理厂内所进行的原型火灾实验中所发生的一切较好的描述了火灾发生时的真实情况，损坏只发生在局部，并且大部分都容易修补，如清理掉发黑的区域并替换下损坏的面砖。尽管如此了解温度对钢材料性质影响仍然很重要，并且如果有必要的话在结构设计过程中考虑温度的影响。钢材的初始强度和韧性于温度的关系如图所示。实际上，屈服强度、极限强度和弹性模量随着温度的升高而降低。只有当温度达到约1000度时降低的速度才加剧。由于应变硬化现象的影响在250度到600度之间，极限强度随温度的升高略呈上升的趋势。在1100到1200度时，屈服应力和极限应力值已经降低到室温下的一半，此时弹性模量E已达到初始值的60%。对结构来说E的水平更为重要，因为结构的变形与弹性模量成正比。如果承载结构长期处于升温状态下蠕变现象也会发生。如同在生产高强度淬火和回火钢材过程中一样，钢材的回火和淬火同时发生。因淬火而使钢材冷却将产生坚硬的被称作马氏体的细粒状结构物。马氏体强度虽然很高但非常脆，而且回火使晶状体结构的形状仅在某种程度上发生改变，即延性和韧性增加了，但强度仍保持不变。适当升温仅接着再降温会产生一定的内在或残余应力，产生的原因是降温时产生的收缩变形受到了材料和结构的限制。这种情况在焊接节点中经常出现，结构或结构的一部分在加热的整个过程中也会出现残余应力。如果结构受热不均匀而且收缩变形也不受限制，则在结构中将会产生一定的扭曲变形，此变形使结构组装困难，适当的升温并控制降温可以使此问题得到缓解。关心高度约束节点中，残余应力和其它类型残余应力的设计人员可能采用重新设计连接的几何形状来避免残余应力的问题。文献详细论述了关于层状