《土木工程专业英语 第2版》 课件 Unit1 Introduction to Reinforced Concrete；Unit2 Prestressed Concrete

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,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,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,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,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,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,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,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1.2StructuralFormsFloorsupportsystemsforbuildingsincludethemonolithicslab-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.1.2StructuralFormsWherelongclearspansarerequiredforroofs,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.1.2StructuralFormsCylindricalconcretetanksarewidelyusedforstorageofwaterorinwastepurifi­cationplants.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.)1.3LoadsDeadloadsarethosethatareconstantinmagnitudeandfixedinlocationthroughoutthelifetimeofthestructure.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1.3LoadsLiveloadsconsistchieflyofoccupancyloadsinbuildingsandtrafficloadsonbridges.Theymaybeeitherfullyorpartiallyinplaceornotpresentatall,andmayalsochangeinlocation.Theirmagnitudeanddistributionatanygiventimeareuncer­tain,andeventheirmaximumintensitiesthroughoutthelifetimeofthestructurearenotknownwithprecision.Theminimumliveloadsforwhichthefloorsandroofofabuildingshouldbedesignedareusuallyspecifiedinthebuildingcodethatgovernsatthesiteofconstruction.活载occupancy使用distribution分布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1.4Serviceability,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1.4.1VariabilityofLoadsSincethemaximumloadthatwilloccurduringthelifeofastructureisuncertain,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1.4.2StrengthThestrengthofastructuredependsonthestrengthofthematerialsfromwhichitismade.Forthispurpose,minimummaterialstrengthsarespecifiedinstandardizedways.Actualmaterialstrengthscannotbeknownpreciselyandthereforealsoconsti­tuterandomvariables.Structuralstrengthdepends,furthermore,onthecarewithwhichastructureisbuilt,whichinturnreflectsthequalityofsupervi­sionandinspection.Membersizesmaydifferfromspecifieddimensions,reinforce­mentmaybeoutofposition,poorlyplacedconcretemayshowvoids,etc.结构强度precisely准确地supervi­sion监管inspection检查void孔洞1.5DesignBasisThesinglemostimportantcharacteristicofanystructuralmemberisitsactualstrength,whichmustbelargeenoughtoresist,withsomemargintospare,allforeseeableloadsthatmayactonitduringthelifeofthestructure,withoutfailureorotherdistress.Itislogical,therefore,toproportionmembers,i.e.,toselectconcretedimensionsandrein­forcement,sothatmemberstrengthsareadequatetoresistforcesresultingfromcertainhypotheticaloverloadstages,significantlyaboveloadsexpectedactuallytooccurinservice.Thisdesignconceptisknownasstrengthdesign.强度设计margin边缘distress危害；危险logical合理的hypothetical假设的Unit2IntroductiontoPrestressedConcreteEnglishforCivilEngineering——Unit2IntroductiontoPrestressedConcrete

2.1Introduction2.2EffectsofPrestressing2.2.1ConcreteStressControlbyPrestressing2.2.2EquivalentLoads2.2.3PrestressedConcreteasaVariationofReinforcedConcrete2.3SourcesofPrestressForce2.4PrestressingSteels2.5ConcreteforPrestressedConstructionLogic

1.Introductionwhywedevelopprestressedconcreteeconomicstructures,usehigherstrengthmaterials,reinforcedconcreteproblem:crackinganddeflectiondevelopmentofprestressedconcrete2.Threealternativewaystolookattheprestressingofconcrete(分析方法)(1)concretestresscontrolexample:arectangularbeamcross-section(2)equivalentLoadsexample:asimplespan(简支跨)beam(3)specialvariationofreinforcedconcrete3.SourcesofPrestressForcemethods:

pretensioning,post-tensioning4.Steelsstressloseproblem(应力损失)

example：超张拉5.whyuseHigh-strengthConcrete4reasons2.1Introduction

Modernstructuralengineeringtendstoprogresstowardmoreeconomicstructuresthroughgraduallyimprovedmethodsofdesignandtheuseofhigherstrengthmaterials.Thisresultsinareductionofcross-sectionaldimensionsandconsequentweightsavings.Suchdevelopmentsareparticularlyimportantinthefieldofreinforcedconcrete,wherethedeadloadrepresentsasubstantialpartofthetotaldesignload.Also,inmultistorybuildings,anysavingindepthofmembers,multipliedbythenumberofstories,canrepresentasubstantialsavingintotalheight,loadonfoundations,lengthofheatingandelectricalducts,plumbingrisers,andwallandpartitionsurfaces.structuralengineering结构工程；cross-sectionaldimensions截面尺寸；reinforcedconcrete钢筋混凝土；deadload恒载；foundations基础；ducts管线；plumbingrisers管道吊组绳2.1Introduction

Theselimitingfeaturesofordinaryreinforcedconcretehavebeenlargelyovercomebythedevelopmentofprestressedconcrete.Aprestressedconcretemembercanbedefinedasoneinwhichtherehavebeenintroducedinternalstressesofsuchmagnitudeanddistributionthatthestressesresultingfromthegivenexternalloadingarecounteractedtoadesireddegree.Concreteisbasicallyacompressivematerial,withitsstrengthintensionalowandunreliablevalue.Prestressingappliesaprecompressiontothememberthatreducesoreliminatesundesirabletensilestressesthatwouldotherwisebepresent.Crackingunderserviceloadscanbeminimizedorevenavoidedentirely.Deflectionsmaybelimitedtoanacceptablevalue;infact,memberscanbedesignedtohavezerodeflectionunderthecombinedeffectsofserviceloadandprestressforce.预应力混凝土构件介绍bedefinedas定义为；magnitudeanddistribution大小和分布；

tensilestresses拉应力；Cracking开裂；deflection变形2.2EffectsofPrestressing2.2.1ConcreteStressControlbyPrestressingManyimportantfeaturesofprestressedconcretecanbedemonstratedbysimpleexamples.Considerfirsttheplain,unreinforcedconcretebeamshowninFig.2-1a.Itcarriesasingleconcentratedloadatthecenterofitsspan.Theself-weightofthememberwillbeneglectedhere.AstheloadWisgraduallyapplied,longitudinalflexuralstressesareinduced.Iftheconcreteisstressedonlywithinitselasticrange,theflexuralstressdistributionatmidspanwillbelinear,asshown.Atarelativelylowload,thetensilestressintheconcreteatthebottomofthebeamwillreachthetensilestrengthoftheconcretefr,andacrackwillform.Becausenorestraintisprovidedagainstupwardextensionofthecrack,thebeamwillcollapsewithoutfurtherincreaseofload.concentratedload集中荷载；thecenterofitsspan跨中；self-weight自重;longitudinalflexuralstresses纵向弯曲应力；elasticrange弹性范围；

tensilestrength

抗拉强度2.2EffectsofPrestressing2.2.1ConcreteStressControlbyPrestressing（1）Butitwouldbemorelogicaltoapplytheprestressingforcenearthebottomofthebeam,tocompensatemoreeffectivelyfortheload-inducedtension.Apossibledesignspecification,forexample,mightbetointroducethemaximumcompressionatthebottomofthebeamwithoutcausingtensionatthetop,whenonlytheprestressingforceacts.Itiseasilyshownthat,forabeamwitharectangularcrosssection,thepointofapplicationoftheprestressingforceshouldbeatthelowerthirdpointofthesectiondepthtoachievethis.预应力施加过程prestressingforce预应力；

designspecification设计方案；rectangularcrosssection矩形截面；sectiondepth截面高度2.2EffectsofPrestressing2.2.1ConcreteStressControlbyPrestressing（2）TheforceP,withthesamevalueasbefore,butappliedwitheccentricitye=h/6relativetotheconcretecentroid,willproducealongitudinalcompressivestressdistributionvaryinglinearlyfromzeroatthetopsurfacetoamaximumof2fc=P/Ac+Pe·c2/Icatthebottom,wherefcistheconcretestressattheconcretecentroid,c2isthedistancefromtheconcretecentroidtothebottomofthebeam,andIcisthemomentofinertiaofthecrosssection.ThisisshowninFig.2-1c.Thestressatthebottomwillbeexactlytwicethevalueproducedbeforebyaxialprestressing.预应力施加过程Eccentricity偏心；concretecentroid混凝土中部；longitudinalcompressivestressdistribution纵向压应力分布；themomentofinertiaofthecrosssection横截面的惯性矩2.2EffectsofPrestressing2.2.1ConcreteStressControlbyPrestressingForeachcharacteristicloaddistribution,thereisabesttendonprofilethatproducesaprestressmomentdiagramthatcorrespondstothatoftheappliedload.Iftheprestresscountermomentismadeexactlyequalandoppositetotheload-inducedmoment,theresultisabeamthatissubjectonlytouniformaxialcompressivestressintheconcreteallalongthespan.Suchabeamwouldbefreeofflexuralcracking,andtheoreticallyitwouldnotbedeflectedupordownwhenthatparticularloadisinplace,comparedtoitspositionasoriginallycast.Sucharesultwouldbeobtainedforaloadof1/2×2Q=Q,asshowninFig.2-le，forexample.基于荷载分布的最佳钢筋束布置方案besttendonprofile最佳钢筋束布置方案；prestressmomentdiagram预应力弯矩图；Countermoment反弯矩，恢复力矩；allalongthespan沿着跨长；flexuralcracking弯曲开裂2.2EffectsofPrestressing2.2.1ConcreteStressControlbyPrestressing2.2EffectsofPrestressing2.2.1ConcreteStressControlbyPrestressing2.2EffectsofPrestressing2.2.2EquivalentLoadsInFig.2.2a,forexample,atendonthatappliesforcePatthecentroidoftheconcretesectionattheendsofabeamandthathasauniformslopeatangleθbetweentheendsandmidspanintroducesatransverseforce2Psinθatthepointofchangeofslopeatmidspan.Attheanchorages,theverticalcomponentoftheprestressingforceisPsinθandthehorizontalcomponentisPcosθ.ThehorizontalcomponentisverynearlyequaltoPfortheusualflatslopeangles.ThemomentdiagramforthebeamofFig.2-2aisseentohavethesameformasthatforanycenter-loadedsimplespan.预应力的力等效过程uniformslope

等斜率，等坡度；midspan跨中；transverseforce横向力，剪力；anchorages锚固端；momentdiagram弯矩图2.2EffectsofPrestressing2.2.2EquivalentLoadsIfastraighttendonisusedwithconstanteccentricity,asshowninFig.2-2c,therearenoverticalforcesontheconcrete,butthebeamissubjecttoamomentPeateachend,aswellastheaxialforceP，andadiagramofconstantmomentresults.TheendmomentmustalsobeaccountedforinthebeamofFig.2-2d,inwhichaparabolictendonisusedthatdoesnotpassthroughtheconcretecentroidattheendsofthespan.Inthiscase,auniformlydistributedupwardloadplusendanchorageforcesareproduced,asshowninFig.2-2b,butinaddition,theendmomentsM=Pecosθmustbeaccountedfor.预应力的力等效过程straighttendon直线型预应力筋；verticalforces竖向力；axialforce轴向力；constantmoment固定弯矩；endmoment端弯矩；parabolictendon抛物线型的预应力钢筋；uniformlydistributed均布2.2EffectsofPrestressing2.2.2EquivalentLoads2.2EffectsofPrestressing2.2.3PrestressedConcreteasaVariationofReinforcedConcreteInthedescriptionsoftheeffectsofprestressingintheparagraphsabove,itwasimpliedthattheprestressforceremainedconstantastheverticalloadwasintroduced,thattheconcreterespondedelastically,andthatnoconcretecrackingoccurred.Theseconditionsmayprevailuptoabouttheserviceloadlevel,butiftheloadsshouldbeincreasedmuchbeyondthat,flexuraltensilestresseswilleventuallyexceedthemodulusofruptureandcrackswillform.Loadscanusuallybeincreasedmuchbeyondthecrackingloadinwell-designedprestressedbeams.作为钢筋混凝土的变形体flexuraltensilestresses弯曲拉应力；exceedthemodulusofruptureandcrackswillform超过断裂模量而产生裂缝；well-designed设计良好的2.2EffectsofPrestressing2.2.3PrestressedConcreteasaVariationofReinforcedConcreteEachofthethreeviewpointsdescribed—concretestresscontrol,equivalentloads,andreinforcedconcreteusingprestrainedsteel—isusefulintheanalysisanddesignofprestressedconcretebeams,andnoneofthethreeissufficientinitself.Neitheranelasticstressanalysisnoranequivalentloadanalysisprovidesinformationaboutstrengthorsafetymargin.However,thestressanalysisishelpfulinpredictingtheextentofcracking,andtheequivalentloadanalysisisoftenthebestwaytocalculatedeflections.Strengthanalysisisessentialtoevaluatesafetyagainstcollapse,butittellsnothingaboutcrackingordeflectionsofthebeamunderserviceconditions.总结三类观点equivalentloads等效荷载；strengthorsafetymargin强度和安全界限；extentofcracking开裂的程度；Strengthanalysis强度分析；evaluatesafetyagainstcollapse评估抵抗倒塌的安全度2.3SourcesofPrestressForcePrestresscanbeappliedtoaconcretememberinmanyways.Perhapsthemostobviousmethodofprecompressingistheuseofjacksreactingagainstabutments,asshowninFig.2-4a.Suchaschemehasbeenemployedforlargeprojects.Manyvariationsarepossible,includingreplacingthejackswithcompressionstrutsafterthedesiredstressintheconcreteisobtainedorusinginexpensivejacks,thatremaininplaceinthestructure,insomecaseswithacementgroutusedasthehydraulicfluid.Theprincipaldifficultyassociatedwithsuchasystemisthatevenaslightmovementoftheabutmentswilldrasticallyreducetheprestressforce.预应力的施加Precompressing先张法；Jacks千斤顶；abutments台座；cementgrout水泥浆；hydraulicfluid液压流体2.3SourcesofPrestressForceInmostcases,thesameresultismoreconvenientlyobtainedbytyingthejackbasestogetherwithwiresorcables,asshowninFig.2-4b.Thesewiresorcablesmaybeexternal,locatedoneachsideofthebeam;moreusuallytheyarepassedthroughahollowconduitembeddedintheconcretebeam.Usually,oneendoftheprestressingtendonisanchored,andalltheforceisappliedattheotherend.Afterattainmentofthedesiredprestressforce,thetendoniswedgedagainsttheconcreteandthejackingequipmentisremovedforreuse.Notethatinthistypeofprestressing,theentiresystemisself-containedandisindependentofrelativedisplacementofthesupports.施加预应力wiresorcables钢线或锚索；a

hollowconduitembeddedintheconcretebeam嵌入混凝土梁的空心管道；attainmentofthedesiredprestressforce达到所需预应力；wedged

嵌入；self-contained独立的；relativedisplacement相对位移2.3SourcesofPrestressForceMostofthepatentedsystemsforapplyingprestressincurrentusearevariationsofthoseshowninFig.2-4bandc.Suchsystemscangenerallybeclassifiedaspretensioningorpost-tensioningsystems.Inthecaseofpretensioni