《土木工程专业英语 第2版》 翻译版 课件全套 鲁正 Unit 1 Introduction to Reinforced Concrete Design-Unit 5 Composite Construction

Unit1IntroductiontoReinforcedConcreteDesignEnglishforCivilEngineering——Unit1IntroductiontoReinforcedConcreteDesign1.1Concrete,ReinforcedConcrete,andPrestressedConcrete（混凝土，钢筋混凝土，以及预应力混凝土）1.2StructuralForms（结构形式）1.3Loads（荷载）1.4Serviceability,Strength,andStructuralSafety（可用性，强度以及结构安全性）1.4.1VariabilityofLoads（荷载变异性）1.4.2Strength（强度）1.4.3StructuralSafety（结构安全性）1.5DesignBasis（设计基础）1.1Concrete,ReinforcedConcrete,andPrestressedConcreteConcreteisastonelikematerialobtainedbypermittingacarefullyproportionedmixtureofcement,sandandgravelorotheraggregate,andwatertohardeninformsoftheshapeanddimensionsofthedesiredstructure.Thebulkofthematerialconsistsoffineandcoarseaggregate.Cementandwaterinteractchemicallytobindtheaggre­gateparticlesintoasolidmass.Additionalwater,overandabovethatneededforthischemicalreaction,isnecessarytogivethemixturetheworkabilitythatenablesittofilltheformsandsurroundtheembeddedreinforcingsteelpriortohardening.混凝土的组成stonelike像石头一样的

cement水泥

gravel砂砾aggregate骨料

coarse粗糙的workability和易性1.1Concrete,ReinforcedConcrete,andPrestressedConcreteConcreteisastonelikematerialobtainedbypermittingacarefullyproportionedmixtureofcement,sandandgravelorotheraggregate,andwatertohardeninformsoftheshapeanddimensionsofthedesiredstructure.Thebulkofthematerialconsistsoffineandcoarseaggregate.Cementandwaterinteractchemicallytobindtheaggre­gateparticlesintoasolidmass.Additionalwater,overandabovethatneededforthischemicalreaction,isnecessarytogivethemixturetheworkabilitythatenablesittofilltheformsandsurroundtheembeddedreinforcingsteelpriortohardening.混凝土是一种把水泥、砂和石砾或者其他骨料、以及水按比例混合并硬化成所需结构的形状和尺寸的石头状的材料。这种材料大部分由粗细骨料组成。水泥和水通过化学作用把骨料颗粒凝结为固体质量。超过化学作用所需的额外的水用来提供混合物的和易性，使之在硬化之前能够填入模板以及包围嵌入的钢筋。1.1Concrete,ReinforcedConcrete,andPrestressedConcreteConcreteswithawiderangeofpropertiescanbeobtainedbyappropriateadjustmentoftheproportionsoftheconstituentmaterials.Specialcements(suchashighearlystrengthcements),specialaggregates(suchasvariouslightweightorheavyweightaggregates),admixtures(suchasplasticizers,air-entrainingagents,silicafume,andflyash),andspecialcuringmethods(suchassteam-curing)permitanevenwidervari­etyofpropertiestobeobtained.特种混凝土property

特性

constituent构成的

admixture外加剂plasticizer塑化剂air-entrainingagent加气剂silicafume硅粉flyash粉煤灰

steam-curing蒸汽养护1.1Concrete,ReinforcedConcrete,andPrestressedConcreteConcreteswithawiderangeofpropertiescanbeobtainedbyappropriateadjustmentoftheproportionsoftheconstituentmaterials.Specialcements(suchashighearlystrengthcements),specialaggregates(suchasvariouslightweightorheavyweightaggregates),admixtures(suchasplasticizers,air-entrainingagents,silicafume,andflyash),andspecialcuringmethods(suchassteam-curing)permitanevenwidervari­etyofpropertiestobeobtained.通过调整组成部分的比例，可获得各种强度的混凝土。特殊的水泥（比如早强水泥），特殊的骨料（比如各种轻骨料或者重骨料），外加剂（比如增塑剂和加气剂）和特殊的养护方法（比如蒸汽养护）可以得到更多种类的特性。1.1Concrete,ReinforcedConcrete,andPrestressedConcreteThesepropertiesdependtoaverysubstantialdegreeontheproportionsofthemix,onthethoroughnesswithwhichthevariousconstituentsareintermixed,andontheconditionsofhumidityandtemperatureinwhichthemixismaintainedfromthemomentitisplacedintheformsuntilitisfullyhardened.Theprocessofcontrollingconditionsafterplacementisknownascuring.Toprotectagainsttheunintentionalproductionofsubstandardconcrete,ahighdegreeofskillfulcontrolandsupervisionisnecessarythroughouttheprocess,fromtheproportioningbyweightoftheindivid­ualcomponents,throughmixingandplacing,untilthecompletionofcuring.混凝土的制作与养护substantial实质的

thoroughness充分性

intermix混合humidity湿度

curing养护1.1Concrete,ReinforcedConcrete,andPrestressedConcreteThesepropertiesdependtoaverysubstantialdegreeontheproportionsofthemix,onthethoroughnesswithwhichthevariousconstituentsareintermixed,andontheconditionsofhumidityandtemperatureinwhichthemixismaintainedfromthemomentitisplacedintheformsuntilitisfullyhardened.Theprocessofcontrollingconditionsafterplacementisknownascuring.Toprotectagainsttheunintentionalproductionofsubstandardconcrete,ahighdegreeofskillfulcontrolandsupervisionisnecessarythroughouttheprocess,fromtheproportioningbyweightoftheindivid­ualcomponents,throughmixingandplacing,untilthecompletionofcuring.这些特性很大程度上依赖于混合比例，不同组成部分搅拌的充分性，以及混合物在浇注后直到硬化前的湿度和温度。控制这些条件的过程叫养护。为了避免次品混凝土，整个过程需要严格的技术控制和检查。这个过程从不同组分按重量配比，到混合、浇注，直到养护完成。1.1Concrete,ReinforcedConcrete,andPrestressedConcreteThefactorsthatmakeconcreteauniversalbuildingmaterialaresopronouncedthatithasbeenused,inmoreprimitivekindsandwaysthanatpresent,forthousandsofyears,startingwithlimemortarsfrom12,000to6000BCEinCrete,Cyprus,Greece,andtheMiddleEast.Thefacilitywithwhich,whileplastic,itcanbedepositedandmadetofillformsormoldsofalmostanypracticalshapeisoneofthesefactors.Itshighfireandweatherresistanceisanevidentadvantage.混凝土的历史与优点pronounced显著的

primitive原始的

limemortars水泥砂浆facility容易

plastic塑性的deposited放置mold模具1.1Concrete,ReinforcedConcrete,andPrestressedConcreteThefactorsthatmakeconcreteauniversalbuildingmaterialaresopronouncedthatithasbeenused,inmoreprimitivekindsandwaysthanatpresent,forthousandsofyears,startingwithlimemortarsfrom12,000to6000BCEinCrete,Cyprus,Greece,andtheMiddleEast.Thefacilitywithwhich,whileplastic,itcanbedepositedandmadetofillformsormoldsofalmostanypracticalshapeisoneofthesefactors.Itshighfireandweatherresistanceisanevidentadvantage.混凝土应用如此普遍，有一个显然的原因便是其已经使用了上千年了。早在公元前12000年到6000年间克里特岛、塞浦路斯、希腊以及中东便开始使用混凝土，尽管是以水泥砂浆这样一种更原始的方式在使用。当混凝土处于塑性状态时，它可以很容易地浇注并填充到任何形式的模板或磨具中。它具有高抗火性和耐候性的优点。1.1Concrete,ReinforcedConcrete,andPrestressedConcreteMostoftheconstituentmaterials,withtheexceptionofcementandadditives,areusuallyavailableatlowcostlocallyoratsmalldistancesfromtheconstructionsite.Itscompressivestrength,likethatofnaturalstones,ishigh,whichmakesitsuitableformembersprimarilysubjecttocompression,suchascolumnsandarches.Ontheotherhand,againasinnaturalstones,itisarelativelybrittlematerialwhosetensilestrengthissmallcomparedwithitscompressivestrength.Thispreventsitseconomicaluseinstructuralmembersthataresubjecttotensioneitherentirely(suchasintie-rods)oroverpartoftheircrosssections(suchasinbeamsorotherflexuralmembers).混凝土组成材料的特性additive添加剂

constructionsite施工现场

compressivestrength抗压强度tensilestrength抗拉强度

1.1Concrete,ReinforcedConcrete,andPrestressedConcreteMostoftheconstituentmaterials,withtheexceptionofcementandadditives,areusuallyavailableatlowcostlocallyoratsmalldistancesfromtheconstructionsite.Itscompressivestrength,likethatofnaturalstones,ishigh,whichmakesitsuitableformembersprimarilysubjecttocompression,suchascolumnsandarches.Ontheotherhand,againasinnaturalstones,itisarelativelybrittlematerialwhosetensilestrengthissmallcomparedwithitscompressivestrength.Thispreventsitseconomicaluseinstructuralmembersthataresubjecttotensioneitherentirely(suchasintie-rods)oroverpartoftheircrosssections(suchasinbeamsorotherflexuralmembers).除了水泥之外的组成材料，大部分都可在当地或施工现场就近以很低廉的价格获得。像天然的石头一样，它的抗压强度很高，所以适合于做受压构件，如柱和拱。另一方面，与石头一样，它是相对较脆的材料，其抗拉强度低于抗压强度。这使得它用在整体受拉（如系杆）或截面部分受拉（如梁或其他受弯构件）的结构构件时不经济。1.1Concrete,ReinforcedConcrete,andPrestressedConcreteTooffsetthislimitation,itwasfoundpossible,inthesecondhalfofthenineteenthcentury,tousesteelwithitshightensilestrengthtoreinforceconcrete,chieflyinthoseplaceswhereitslowtensilestrengthwouldlimitthecarryingcapacityofthemember.Thereinforcement,usuallyroundsteelrodswithappropriatesurfacedeformationstoprovideinterlocking,isplacedintheformsinadvanceoftheconcrete.将钢材引入到混凝土中（即钢筋混凝土）capacity承载力rod棒，杆interlocking咬合作用1.1Concrete,ReinforcedConcrete,andPrestressedConcreteTooffsetthislimitation,itwasfoundpossible,inthesecondhalfofthenineteenthcentury,tousesteelwithitshightensilestrengthtoreinforceconcrete,chieflyinthoseplaceswhereitslowtensilestrengthwouldlimitthecarryingcapacityofthemember.Thereinforcement,usuallyroundsteelrodswithappropriatesurfacedeformationstoprovideinterlocking,isplacedintheformsinadvanceoftheconcrete.为了弥补这种限制，19世纪中叶时，人们发现可以把高抗拉强度的钢筋应用于混凝土，尤其是用于因混凝土的低抗拉强度从而限制了构件承载力的部位。钢筋通常为圆形，外表通常带有适当变形以提供咬合力，并以利于混凝土的形式放置。1.1Concrete,ReinforcedConcrete,andPrestressedConcreteWhencompletelysurroundedbythehardenedconcretemass,itformsanintegralpartofthemember.Theresultingcombinationoftwomaterials,knownasreinforcedconcrete，combinesmanyoftheadvantagesofeach:therelativelylowcost,goodweatherandfireresistance,goodcompressivestrength,andexcellentformabilityofconcreteandthehightensilestrengthandmuchgreaterductilityandtoughnessofsteel.Itisthiscombinationthatallowsthealmostunlimitedrangeofusesandpossi­bilitiesofreinforcedconcreteintheconstructionofbuildings,bridges,dams,tanks,reservoirs,andahostofotherstructures.钢筋混凝土优点hardened变硬的formability可成形性toughness韧性reservoirs水库1.1Concrete,ReinforcedConcrete,andPrestressedConcreteWhencompletelysurroundedbythehardenedconcretemass,itformsanintegralpartofthemember.Theresultingcombinationoftwomaterials,knownasreinforcedconcrete，combinesmanyoftheadvantagesofeach:therelativelylowcost,goodweatherandfireresistance,goodcompressivestrength,andexcellentformabilityofconcreteandthehightensilestrengthandmuchgreaterductilityandtoughnessofsteel.Itisthiscombinationthatallowsthealmostunlimitedrangeofusesandpossi­bilitiesofreinforcedconcreteintheconstructionofbuildings,bridges,dams,tanks,reservoirs,andahostofotherstructures.当被硬化的混凝土完全包裹后，钢筋变成构件整体的一部分。两种材料的结合，被称为钢筋混凝土，它结合了各自的许多优点：混凝土的低成本，高耐候性和抗火性，高抗压强度和高成型性；钢筋的高抗拉强度，高延性和韧性。正是这种联合作用，使得钢筋混凝土有着近乎无限的应用范围和可能，可以用于房屋，桥梁，大坝，水池，水库和很多其他的结构。1.1Concrete,ReinforcedConcrete,andPrestressedConcreteAspecialwayhasbeenfound,however,tousesteelsandconcretesofveryhighstrengthincombination.Thistypeofconstructionisknownasprestressedconcrete.Thesteel,intheformofwires,strands,orbars,isembeddedintheconcreteunderhightensionthatisheldinequilibriumbycompressivestressesintheconcreteafterhard­ening.Becauseofthisprecompression,theconcreteinaflexuralmemberwillcrackonthetensionsideatamuchlargerloadthanwhennotsoprecompressed.Prestressinggreatlyreducesboththedeflectionsandthetensilecracksatordinaryloadsinsuchstructures,andtherebyenablesthesehigh-strengthmaterialstobeusedeffectively.预应力混凝土prestressed预应力的wire网strand串bar棒equilibrium平衡precompression预压力1.1Concrete,ReinforcedConcrete,andPrestressedConcreteAspecialwayhasbeenfound,however,tousesteelsandconcretesofveryhighstrengthincombination.Thistypeofconstructionisknownasprestressedconcrete.Thesteel,intheformofwires,strands,orbars,isembeddedintheconcreteunderhightensionthatisheldinequilibriumbycompressivestressesintheconcreteafterhard­ening.Becauseofthisprecompression,theconcreteinaflexuralmemberwillcrackonthetensionsideatamuchlargerloadthanwhennotsoprecompressed.Prestressinggreatlyreducesboththedeflectionsandthetensilecracksatordinaryloadsinsuchstructures,andtherebyenablesthesehigh-strengthmaterialstobeusedeffectively.然而，已经发现了一种特殊的方法来将钢筋与高强度混凝土组合起来，这种形式的结构就是预应力混凝土。钢筋的形式通常是钢线或钢绞线，有时是钢筋，在超张拉后埋入混凝土中，通过硬化后的混凝土的受压应力保持平衡。由于预先受压，受弯构件的混凝土在受拉部位的开裂荷载比未受预压的得以提高。预应力大大减小了这些结构在普通荷载下的变形和受拉开裂，使这些高强材料得以有效使用。1.2StructuralFormsFloorsupportsystemsforbuildingsincludethemonolithicslab-and-beamfloorshowninFig.1-1,theone-wayjoistsystemofFig.1-2,andtheflatplatefloor,withoutbeamsorgirders,showninFig.1-3.TheflatslabfloorofFig.1-4,frequentlyusedformoreheavilyloadedbuildingssuchaswarehouses,issimilartotheflatplatefloor,butmakesuseofincreasedslabthicknessinthevicinityofthecolumns,aswellasflaredcolumntops,toreducestressesandincreasestrengthinthesupportregion.Thechoiceamongtheseandothersystemsforfloorsandroofsdependsuponfunc­tionalrequirements,loads,spans,andpermissiblememberdepths,aswellasoncostandestheticfactors.楼层支撑体系monolithic整体的warehouse仓库vicinity临近flared向外展开的esthetic审美的1.2StructuralFormsFig.1-3Flatplatefloorslab,carrieddirectlybycolumnswithoutbeamsorgirders.(PortlandCementAssociation.)Fig.1-1One-wayreinforcedconcretefloorslabwithmonolithicsupportingbeams.(PortlandCementAssociation.)Fig.1-2One-wayjoistfloorsystem,withcloselyspacedribssupportedbymonolithicconcretebeams;transverseribsprovideforlateraldistributionoflocalizedloads.(PortlandCementAssociation.)Fig.1-4Flatslabfloor,withoutbeamsbutwithslabthicknessincreasedatthecolumnsandwithflaredcolumntopstoprovideforlocalstressconcentrationofforces.1.2StructuralFormsFloorsupportsystemsforbuildingsincludethemonolithicslab-and-beamfloorshowninFig.1-1,theone-wayjoistsystemofFig.1-2,andtheflatplatefloor,withoutbeamsorgirders,showninFig.1-3.TheflatslabfloorofFig.1-4,frequentlyusedformoreheavilyloadedbuildingssuchaswarehouses,issimilartotheflatplatefloor,butmakesuseofincreasedslabthicknessinthevicinityofthecolumns,aswellasflaredcolumntops,toreducestressesandincreasestrengthinthesupportregion.Thechoiceamongtheseandothersystemsforfloorsandroofsdependsuponfunc­tionalrequirements,loads,spans,andpermissiblememberdepths,aswellasoncostandestheticfactors.建筑楼盖支撑体系包括如图1-1所示的整体板梁楼盖，图1-2所示的单向托梁体系，图1-3的无梁楼盖。图1-4所示的平板楼盖经常在荷载较大的建筑中使用，如仓库。它与无梁楼盖相似，但增大了柱边缘楼板的厚度以及柱端部的承载面积，从而减小了应力，增大了支撑区域的强度。支撑体系的选择取决于功能要求，以及荷载，跨度，构件尺寸，成本和审美等因素。1.2StructuralFormsWherelongclearspansarerequiredforroofs,concreteshellspermituseofextremelythinsurfaces,oftenthinner,relatively,thananeggshell.ThefoldedplateroofofFig.1-5issimpletoformbecauseitiscomposedofflatsurfaces;suchroofshavebeenemployedforspansof200ftandmore.ThecylindricalshellofFig.1-6isalsorelativelyeasytoformbecauseithasonlyasinglecurvature;itissimilartothefoldedplateinitsstructuralbehaviorandrangeofspansandloads.ShellsofthistypewereoncequitepopularintheUnitedStateandremainpopularinotherpartsoftheworld.混凝土壳roof屋顶shell壳cylindrical圆柱形的curvature曲率；弯曲1.2StructuralFormsFig.1-5Foldedplateroofof125ftspan,inadditiontocarryingordinaryroofloads,carriesthesecondflooraswellfromasystemofcablehangers;thegroundflooriskeptfreeofcolumns.Fig.1-6Cylindricalshellroofprovidingcolumn-freeinteriorspace.1.2StructuralFormsWherelongclearspansarerequiredforroofs,concreteshellspermituseofextremelythinsurfaces,oftenthinner,relatively,thananeggshell.ThefoldedplateroofofFig.1-5issimpletoformbecauseitiscomposedofflatsurfaces;suchroofshavebeenemployedforspansof200ftandmore.ThecylindricalshellofFig.1-6isalsorelativelyeasytoformbecauseithasonlyasinglecurvature;itissimilartothefoldedplateinitsstructuralbehaviorandrangeofspansandloads.ShellsofthistypewereoncequitepopularintheUnitedStateandremainpopularinotherpartsoftheworld.当屋顶净跨度较大时，利用混凝土壳可以采用厚度极薄的材料。图1-5所示的折板屋顶比较容易建造，因为它是由平面构成；这样的屋顶已经被用在跨度200英尺或更大的结构上了。图1-6所示的圆柱形壳也比较容易建造，因为它只有一个曲率；它的结构性能以及可接受的跨度和荷载均类似于折板屋顶。这种类型的壳曾一度在美国非常流行，现在在世界其他地区也依旧流行。1.2StructuralFormsCylindricalconcretetanksarewidelyusedforstorageofwaterorinwastepurifi­cationplants.ThedesignshowninFig.1.11isproofthatasanitaryengineeringfacilitycanbeestheticallypleasingaswellasfunctional.Cylindricaltanksareoftenpre-stressedcircumferentiallytomaintaincompressionintheconcreteandeliminatethecrackingthatwouldotherwiseresultfrominternalpressure.圆柱形混凝土水箱storage储存purifi­cation净化sanitary卫生的esthetically审美地circumferentially圆周地internal内部的1.2StructuralFormsFig.1-11CircularconcretetanksusedasapartofthewastewaterpurificationfacilityatHowden,England.(NorthumbrianWaterAuthoritywithLuderandJones,Architects.)1.2StructuralFormsCylindricalconcretetanksarewidelyusedforstorageofwaterorinwastepurifi­cationplants.ThedesignshowninFig.1-11isproofthatasanitaryengineeringfacilitycanbeestheticallypleasingaswellasfunctional.Cylindricaltanksareoftenpre-stressedcircumferentiallytomaintaincompressionintheconcreteandeliminatethecrackingthatwouldotherwiseresultfrominternalpressure.圆柱形混凝土水箱广泛应用在储存水以及污水处理站方面。图1-11所示的设计展示了一个卫生工程设施既可以在审美上满足要求，也可以在功能上满足。圆柱体水箱通常是圆周地设置预应力，以使得混凝土一直保持为压力，防止内力产生裂缝。1.3LoadsDeadloadsarethosethatareconstantinmagnitudeandfixedinlocationthroughoutthelifetimeofthestructure.Usuallythemajorpartofthedeadloadistheweightofthestructureitself.Thiscanbecalculatedwithgoodaccuracyfromthedesigncon­figuration,dimensionsofthestructure,anddensityofthematerial.Forbuildings,floorfill,finishfloors,andplasteredceilingsareusuallyincludedasdeadloads,andanallowanceismadeforsuspendedloadssuchaspipingandlightingfixtures.Forbridges,deadloadsmayincludewearingsurfaces,sidewalks,andcurbs,andanallowanceismadeforpipingandothersuspendedloads.恒载deadloads恒载magnitude幅值plasteredceiling灰泥吊顶piping管道系统suspendedload悬挂荷载sidewalk人行道curb路边1.3LoadsDeadloadsarethosethatareconstantinmagnitudeandfixedinlocationthroughoutthelifetimeofthestructure.Usuallythemajorpartofthedeadloadistheweightofthestructureitself.Thiscanbecalculatedwithgoodaccuracyfromthedesigncon­figuration,dimensionsofthestructure,anddensityofthematerial.Forbuildings,floorfill,finishfloors,andplasteredceilingsareusuallyincludedasdeadloads,andanallowanceismadeforsuspendedloadssuchaspipingandlightingfixtures.Forbridges,deadloadsmayincludewearingsurfaces,sidewalks,andcurbs,andanallowanceismadeforpipingandothersuspendedloads.恒载是指那些在结构生命周期中大小和位置均不变的荷载。恒载通常主要为结构自重。这通常能通过结构的设计布置，结构尺寸，材料密度来较精确地计算得到。对于建筑来说，恒载通常还包括楼板填塞层，地板装修以及吊顶，还应对管道系统以及照明装置等悬挂荷载留余。对于桥梁，恒载包括磨损面，人行道以及临边小路，同样也应对管道系统等悬挂荷载进行留余。1.3LoadsLiveloadsconsistchieflyofoccupancyloadsinbuildingsandtrafficloadsonbridges.Theymaybeeitherfullyorpartiallyinplaceornotpresentatall,andmayalsochangeinlocation.Theirmagnitudeanddistributionatanygiventimeareuncer­tain,andeventheirmaximumintensitiesthroughoutthelifetimeofthestructurearenotknownwithprecision.Theminimumliveloadsforwhichthefloorsandroofofabuildingshouldbedesignedareusuallyspecifiedinthebuildingcodethatgovernsatthesiteofconstruction.活载occupancy使用distribution分布1.3LoadsLiveloadsconsistchieflyofoccupancyloadsinbuildingsandtrafficloadsonbridges.Theymaybeeitherfullyorpartiallyinplaceornotpresentatall,andmayalsochangeinlocation.Theirmagnitudeanddistributionatanygiventimeareuncer­tain,andeventheirmaximumintensitiesthroughoutthelifetimeofthestructurearenotknownwithprecision.Theminimumliveloadsforwhichthefloorsandroofofabuildingshouldbedesignedareusuallyspecifiedinthebuildingcodethatgovernsatthesiteofconstruction.活载主要由建筑使用荷载以及桥梁交通荷载组成。它们可能整体或局部分布，也可能完全没有，也可能改变作用位置。对于给定的时间，它们的大小以及分布是不确定的，甚至结构的整个寿命周期中活载的最大值也是无法准确知道的。建筑楼板以及屋顶的最小设计活载可在相关场地建筑设计规范中查得。1.3LoadsEnvironmentalloadsconsistmainlyofsnowloads,windpressureandsuction,earthquakeloads(i.e.,inertiaforcescausedbyearthquakemotions),soilpressuresonsubsurfaceportionsofstructures,loadsfrompossiblepondingofrainwateronflatsur­faces,andforcescausedbytemperaturedifferentials.Likeliveloads,environmentalloadsatanygiventimeareuncertaininbothmagnitudeanddistribution.Thebook,MinimumDesignLoadsforBuildingsandOtherStructures,containsmuchinformationonenvironmentalloads,whichisoftenmodifiedlocallydepending,forinstance,onlocalclimaticorseismicconditions.环境荷载suction吸力subsurface表面下的ponding积水differential差别1.3LoadsEnvironmentalloadsconsistmainlyofsnowloads,windpressureandsuction,earthquakeloads(i.e.,inertiaforcescausedbyearthquakemotions),soilpressuresonsubsurfaceportionsofstructures,loadsfrompossiblepondingofrainwateronflatsur­faces,andforcescausedbytemperaturedifferentials.Likeliveloads,environmentalloadsatanygiventimeareuncertaininbothmagnitudeanddistribution.Thebook,MinimumDesignLoadsforBuildingsandOtherStructures,containsmuchinformationonenvironmentalloads,whichisoftenmodifiedlocallydepending,forinstance,onlocalclimaticorseismicconditions.环境荷载主要由雪荷载，风压力与风吸力，地震荷载（即地震运动引起的惯性力），土对结构地面以下部分的压力，平板的积水荷载，温度差异引起的荷载组成。与活载类似，环境荷载在给定的时间上也无法准确得知其大小与分布。建筑及其他结构最小设计荷载这本书包含了许多环境荷载的信息，这些信息通常根据当地信息，如当地气温以及地震情况等进行了修正。1.4Serviceability,Strength,andStructuralSafetyToserveitspurpose,astructuremustbesafeagainstcollapseandserviceableinuse.Serviceabilityrequiresthatdeflectionsbeadequatelysmall;thatcracks,ifany,bekepttotolerablelimits;thatvibrationsbeminimized;etc.Safetyrequiresthatthestrengthofthestructurebeadequateforallloadsthatmayforeseeablyactonit.Ifthestrengthofastructure,builtasdesigned,couldbepredictedaccurately,andiftheloadsandtheirinternaleffects(moments,shears,axialforces)wereknownaccurately,safetycouldbeensuredbyprovidingacarryingcapacityjustbarelyinexcessoftheknownloads.However,thereareanumberofsourcesofuncertaintyintheanalysis,design,andconstructionofreinforcedconcretestructures.结构的可用性，强度以及结构安全性collapse倒塌serviceability可用性foreseeably可预知地1.4Serviceability,Strength,andStructuralSafetyToserveitspurpose,astructuremustbesafeagainstcollapseandserviceableinuse.Serviceabilityrequiresthatdeflectionsbeadequatelysmall;thatcracks,ifany,bekepttotolerablelimits;thatvibrationsbeminimized;etc.Safetyrequiresthatthestrengthofthestructurebeadequateforallloadsthatmayforeseeablyactonit.Ifthestrengthofastructure,builtasdesigned,couldbepredictedaccurately,andiftheloadsandtheirinternaleffects(moments,shears,axialforces)wereknownaccurately,safetycouldbeensuredbyprovidingacarryingcapacityjustbarelyinexcessoftheknownloads.However,thereareanumberofsourcesofuncertaintyintheanalysis,design,andconstructionofreinforcedconcretestructures.为了达到这个目的，结构必须足够安全以抵抗倒塌，并在使用时可供使用。可用性要求变形足够小；任何裂缝都要控制在可接受的范围内；振动要尽量减小等等。安全性要求结构的强度要足以抵抗可预见的所有荷载。如果荷载以及他们的内力（弯矩，剪力，轴力）已经准确知道的话，可以仅提供比已知荷载稍大的承载力来保证安全性。然而，在分析，设计以及建造混凝土结构时均有一系列不确定性问题。1.4.1VariabilityofLoadsSincethemaximumloadthatwilloccurduringthelifeofastructureisuncertain,itcanbeconsideredarandomvariable.Inspiteofthisuncertainty,theengineermustprovideanadequatestructure.Aprobabilitymodelforthemaximumloadcanbedevisedbymeansofaprobabilitydensityfunctionforloads,asrepresentedbythefre­quencycurveofFig.1.14a.Theexactformofthisdistributioncurve,foranyparticulartypeofloadingsuchasofficeloads，canbedeterminedonlyonthebasisofstatisticaldataobtainedfromlarge-scaleloadsurveys.Anumberofsuchsurveyshavebeencom­pleted.Fortypesofloadsforwhichsuchdataarescarce,fairlyreliableinformationcanbeobtainedfromexperience,observation,andjudgment.荷载概率模型randomvariable随机变量distributioncurve分布曲线scarce缺乏的1.4.1VariabilityofLoadsSincethemaximumloadthatwilloccurduringthelifeofastructureisuncertain,itcanbeconsideredarandomvariable.Inspiteofthisuncertainty,theengineermustprovideanadequatestructure.Aprobabilitymodelforthemaximumloadcanbedevisedbymeansofaprobabilitydensityfunctionforloads,asrepresentedbythefre­quencycurveofFig.1.14a.Theexactformofthisdistributioncurve,foranyparticulartypeofloadingsuchasofficeloads，canbedeterminedonlyonthebasisofstatisticaldataobtainedfromlarge-scaleloadsurveys.Anumberofsuchsurveyshavebeencom­pleted.Fortypesofloadsforwhichsuchdataarescarce,fairlyreliableinformationcanbeobtainedfromexperience,observation,andjudgment.因为结构生命周期里可能出现的最大荷载不确定，因此它可以被考虑为一个随机变量。尽管存在这一不确定性，工程师应该提供足够的结构强度。最大荷载的概率模型可以通过荷载的概率密度方程的形式来获得，如图1.14a所示的频率曲线。对于任意一种荷载，如办公室荷载，其分布曲线的确切形式应以大规模荷载调查得到的统计数据为基础进行确定。大量的调查已经完成。对于数据比较稀缺的荷载类型，可以根据经验，观察以及判断来得到较为可信的信息。1.4.2StrengthThestrengthofastructuredependsonthestrengthofthematerialsfromwhichitismade.Forthispurpose,minimummaterialstrengthsarespecifiedinstandardizedways.Actualmaterialstrengthscannotbeknownpreciselyandthereforealsoconsti­tuterandomvariables.Structuralstrengthdepends,furthermore,onthecarewithwhichastructureisbuilt,whichinturnreflectsthequalityofsupervi­sionandinspection.Membersizesmaydifferfromspecifieddimensions,reinforce­mentmaybeoutofposition,poorlyplacedconcretemay