《土木工程专业英语 第2版》 翻译版 课件 Unit 2 Introduction to Prestressed Concrete

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

Modernstructuralengineeringtendstoprogresstowardmoreeconomicstructures（throughgraduallyimprovedmethodsofdesignandtheuseofhigherstrengthmaterials.）Thisresultsinareductionofcross-sectionaldimensionsandconsequentweightsavings.Suchdevelopmentsareparticularlyimportantinthefieldofreinforcedconcrete,wherethedeadloadrepresentsasubstantialpartofthetotaldesignload.Also,inmultistorybuildings,anysavingindepthofmembers,multipliedbythenumberofstories,canrepresentasubstantialsavingintotalheight,loadonfoundations,lengthofheatingandelectricalducts,plumbingrisers,andwallandpartitionsurfaces.现代结构工程倾向于通过逐步改进的设计方法和利用高强材料来设计更经济的结构。这减小了构件的截面尺寸和相应的重量。这在钢筋混凝土领域中尤其重要，因为其中恒载占了设计荷载的大部分。同样，在多层建筑中，构件高度的减小值，乘以总层数，可以代表总高度，基础上的荷载，供热供电管线，管道吊组绳，墙及隔墙表面的减小值。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2.1Introduction

Theselimitingfeaturesofordinaryreinforcedconcretehavebeenlargelyovercomebythedevelopmentofprestressedconcrete.Aprestressedconcretemembercanbedefinedasone（inwhichtherehavebeenintroducedinternalstressesofsuchmagnitudeanddistribution）（thatthestressesresultingfromthegivenexternalloadingarecounteractedtoadesireddegree）.Concreteisbasicallyacompressivematerial,（withitsstrengthintensionalowandunreliablevalue）.Prestressingappliesaprecompressiontothemember（thatreducesoreliminatesundesirabletensilestresses（thatwouldotherwisebepresent））.Crackingunderserviceloadscanbeminimizedorevenavoidedentirely.Deflectionsmaybelimitedtoanacceptablevalue;infact,memberscanbedesignedtohavezerodeflectionunderthecombinedeffectsofserviceloadandprestressforce.预应力混凝土的发展已经很大程度上克服了上述限制。预应力混凝土构件可通过施加一定大小和分布的内部应力以一定程度上抵消外加荷载产生的应力。混凝土基本上是受压材料，受拉强度很小且不可靠。预应力施加给构件一个预压力以减小或消除拉应力。使用荷载下的开裂可以减小甚至避免。变形可以限制在一个可接受范围，事实上，构件能够设计成在使用荷载和预应力共同作用下，变形为零。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2.2EffectsofPrestressing2.2.1ConcreteStressControlbyPrestressingManyimportantfeaturesofprestressedconcretecanbedemonstratedbysimpleexamples.Considerfirsttheplain,unreinforcedconcretebeam（showninFig.2-1a）.Itcarriesasingleconcentratedloadatthecenterofitsspan.(Theself-weightofthememberwillbeneglectedhere.)AstheloadWisgraduallyapplied,longitudinalflexuralstressesareinduced.Iftheconcreteisstressedonlywithinitselasticrange,theflexuralstressdistributionatmidspanwillbelinear,asshown.Atarelativelylowload,thetensilestressintheconcreteatthebottomofthebeamwillreachthetensilestrengthoftheconcretefr,andacrackwillform.Becausenorestraintisprovidedagainstupwardextensionofthecrack,thebeamwillcollapsewithoutfurtherincreaseofload.许多预应力混凝土的重要特性能够通过简单的例子来说明。先来考虑一根素混凝土没有钢筋的梁，如图21.1a。在跨中有一个集中力，忽略梁的自重。逐步施加荷载W，产生纵向弯曲应力。如果混凝土的应力一直在弹性范围，那么跨中弯曲应力将是如图所示的线性分布。在荷载较小时，梁底混凝土的拉应力达到混凝土的抗拉强度，fr，从而产生裂缝。由于没有约束来限制裂缝的开展，梁在荷载不再增加的情况下也会失效。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（1）Butitwouldbemorelogicaltoapplytheprestressingforcenearthebottomofthebeam,（tocompensatemoreeffectivelyfortheload-inducedtension）.Apossibledesignspecification,（forexample）,mightbetointroducethemaximumcompressionatthebottomofthebeam（withoutcausingtensionatthetop）,whenonlytheprestressingforceacts.Itiseasilyshownthat,（forabeamwitharectangularcrosssection）,thepointofapplicationoftheprestressingforceshouldbeatthelowerthirdpointofthesectiondepthtoachievethis.但是，在梁的底部施加预应力将更加合理，可以更有效地补偿外荷载产生的拉力。比如，一个可能的设计方案是，当只有预应力作用时，在梁底产生最大压力而在顶部不产生拉力。可以看到，矩形截面梁，预应力作用在截面高度的下三分点处可以达到上述要求。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2.2EffectsofPrestressing2.2.1ConcreteStressControlbyPrestressing（2）TheforceP,（

withthesamevalueasbefore,butappliedwitheccentricitye=h/6relativetotheconcretecentroid,）willproducealongitudinalcompressivestressdistribution

（varyinglinearlyfromzeroatthetopsurfacetoamaximumof2fc=P/Ac+Pe·c2/Icatthebottom,）

（wherefcistheconcretestressattheconcretecentroid,c2isthedistancefromtheconcretecentroidtothebottomofthebeam,andIcisthemomentofinertiaofthecrosssection.）ThisisshowninFig.2-1c.Thestressatthebottomwillbeexactlytwicethevalueproducedbeforebyaxialprestressing.当之前同样大小的P作用在离截面中心e=h/6的偏心时，纵向压应力在梁顶为零，梁底为最大值2fc=P/Ac+Pec2/Ic，线性变化，其中fc是混凝土中部的应力，c2是从中部到梁底的距离，Ic是横截面的惯性矩，如图21.1c所示。梁底应力将是轴向预应力施加前的值的2倍。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2.2EffectsofPrestressing2.2.1ConcreteStressControlbyPrestressingForeachcharacteristicloaddistribution,thereisabesttendonprofile

(thatproducesaprestressmomentdiagram(thatcorrespondstothatoftheappliedload)).Iftheprestresscountermomentismadeexactlyequalandoppositetotheload-inducedmoment,theresultisabeamthatissubjectonlytouniformaxialcompressivestressintheconcreteallalongthespan.Suchabeamwouldbefreeofflexuralcracking,andtheoreticallyitwouldnotbedeflectedupordownwhenthatparticularloadisinplace,comparedtoitspositionasoriginallycast.Sucharesultwouldbeobtainedforaloadof1/2×2Q=Q,asshowninFig.2-le，forexample.对于各具特点的荷载分布，都会有一个最佳钢筋束布置方案，以产生与施加荷载相应的预应力弯矩图。如果预应力产生的反弯矩正好与外荷载产生的弯矩数量相等，方向相反，则梁中仅产生沿着跨长分布的均布轴向压应力。这种梁不会产生开裂，理论上来说，只要该外荷载不变，与浇注时的位置相比，它也不会产生变形。这种情况在外荷载为Q的时候产生，图21.1e给出了一个例子。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2.2EffectsofPrestressing2.2.2EquivalentLoadsInFig.2-2a,forexample,atendon（thatappliesforcePatthecentroidoftheconcretesectionattheendsofabeamandthathasauniformslopeatangleθbetweentheendsandmidspan）introducesatransverseforce2Psinθatthepointofchangeofslopeatmidspan.Attheanchorages,theverticalcomponentoftheprestressingforceisPsinθandthehorizontalcomponentisPcosθ.ThehorizontalcomponentisverynearlyequaltoPfortheusualflatslopeangles.ThemomentdiagramforthebeamofFig.2-2aisseentohavethesameformasthat（foranycenter-loadedsimplespan.）以图21.2a为例，P沿着倾角θ作用在混凝土截面质心，从梁端至跨中的θ为等斜率，在跨中倾角转变的地方产生2Psin(θ)的横向力。在锚固端，预应力的竖向力分量为Psin(θ)，水平分量为Pcos(θ)。对于通常的较平的倾角，水平力分量与P的大小接近。图21.2a所示的梁弯矩图与跨中作用荷载的简支梁弯矩图是一致的。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2.2EffectsofPrestressing2.2.2EquivalentLoadsIfastraighttendonisusedwithconstanteccentricity,asshowninFig.2-2c,therearenoverticalforcesontheconcrete,butthebeamissubjecttoamomentPeateachend,aswellastheaxialforceP，andadiagramofconstantmomentresults.TheendmomentmustalsobeaccountedforinthebeamofFig.2-2d,(

inwhichaparabolictendonisused(thatdoesnotpassthroughtheconcretecentroidattheendsofthespan)).Inthiscase,auniformlydistributedupwardloadplusendanchorageforcesareproduced,asshowninFig.2-2b,butinaddition,theendmomentsM=Pecosθmustbeaccountedfor.如图2-2c，采用固定偏心率的直线型预应力筋，此时不会在混凝土中产生竖向力，但是会在梁端受到弯矩Pe和轴向力P，以及数值为常数的弯矩图。图2-2d所示的梁，抛物线型的预应力钢筋没有通过梁端混凝土的中心，其端部弯矩必须考虑。这种情况下，产生了如图2-2b的朝上的均布力，端部锚固力，此外，端部弯矩M=Pecosθ也必须考虑。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2.2EffectsofPrestressing2.2.3PrestressedConcreteasaVariationofReinforcedConcrete（Inthedescriptionsoftheeffectsofprestressingintheparagraphsabove,）

itwasimpliedthattheprestressforceremainedconstantastheverticalloadwasintroduced,thattheconcreterespondedelastically,andthatnoconcretecrackingoccurred.Theseconditionsmayprevailuptoabouttheserviceloadlevel,butiftheloadsshouldbeincreasedmuchbeyondthat,flexuraltensilestresseswilleventuallyexceedthemodulusofruptureandcrackswillform.Loadscanusuallybeincreasedmuchbeyondthecrackingloadinwell-designedprestressedbeams.在上述各段对于预应力作用的描述中，隐含着预应力在竖向荷载作用下保持不变，混凝土保持弹性，且未开裂。这些条件在使用荷载水平下是成立的，但是如果荷载大大超过了使用荷载，则弯曲拉应力最终将超过断裂模量而产生裂缝。在设计良好的预应力梁中，荷载可在开裂荷载之后增加很多。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2.2EffectsofPrestressing2.2.3PrestressedConcreteasaVariationofReinforcedConcreteEachofthethreeviewpointsdescribed（—concretestresscontrol,equivalentloads,andreinforcedconcreteusingprestrainedsteel—）isusefulintheanalysisanddesignofprestressedconcretebeams,andnoneofthethreeissufficientinitself.Neitheranelasticstressanalysisnoranequivalentloadanalysisprovidesinformationaboutstrengthorsafetymargin.However,thestressanalysisishelpfulinpredictingtheextentofcracking,andtheequivalentloadanalysisisoftenthebestwaytocalculatedeflections.Strengthanalysisisessentialtoevaluatesafetyagainstcollapse,butittellsnothingaboutcrackingordeflectionsofthebeamunderserviceconditions.三种观点（混凝土应力控制，等效荷载和采用预拉钢筋的钢筋混凝土）的任意一种在分析和设计预应力混凝土梁时都是有用的，但是没有一种是足够的。弹性应力分析和等效荷载分析都不能提供有关强度和安全边际的信息。然后，应力分析有助于预测开裂的程度，等效荷载分析往往是变形计算的最佳方式。强度分析对评估抵抗倒塌的安全度有重要作用，但是不能给出使用条件下的裂缝或者变形情况。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2.3SourcesofPrestressForcePrestresscanbeappliedtoaconcretememberinmanyways.Perhapsthemostobviousmethodofprecompressingistheuseofjacks(reactingagainstabutments),asshowninFig.2-4a.Suchaschemehasbeenemployedforlargeprojects.Manyvariationsarepossible,（includingreplacingthejackswithcompressionstrutsafterthedesiredstressintheconcreteisobtainedorusinginexpensivejacks,thatremaininplaceinthestructure,insomecaseswithacementgroutusedasthehydraulicfluid.）Theprincipaldifficultyassociatedwithsuchasystemisthatevenaslightmovementoftheabutmentswilldrasticallyreducetheprestressforce.预应力可以以很多方式施加到混凝土构件。也许最常用的先张法是使用抵抗基台的千斤顶，如图2-4a所示。这个方案已经用于大型项目。可能也有许多变形，包括在获得混凝土中所需的应力之后用受压柱替代千斤顶，或使用廉价千斤顶置于结构之中，有时用水泥浆作为液压流体，与这种系统相关的主要困难在于，即使是基台的轻微移动也将大大降低预应力。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2.3SourcesofPrestressForceInmostcases,thesameresultismoreconvenientlyobtainedby（tyingthejackbasestogetherwithwiresorcables）,asshowninFig.2-4b.Thesewiresorcablesmaybeexternal,（locatedoneachsideofthebeam）;moreusuallytheyarepassedthroughahollowconduitembeddedintheconcretebeam.Usually,oneendoftheprestressingtendonisanchored,andalltheforceisappliedattheotherend.Afterattainmentofthedesiredprestressforce,thetendoniswedgedagainsttheconcreteandthejackingequipmentisremovedforreuse.Notethatinthistypeofprestressing,theentiresystemisself-containedandisindependentofrelativedisplacementofthesupports.在大多数情况下，如图2-4b所示，通过将插座底座与钢线或钢缆捆绑在一起，可以更方便地获得相同的结果。这些钢线或钢缆可以在外部，位于梁两端;它们更通常地是贯通嵌入在混凝土梁中的中空导管。通常，预应力筋的一端锚定，所有的力都施加在另一端。在达到所需的预应力之后，将腱楔入混凝土，并且套筒被移除以重新使用。注意，在这种类型的预应力中，整个系统是独立的并且独立于支撑件的相对位移。2.3SourcesofPrestressForceMostofthepatentedsystemsforapplyingprestressincurrentusearevariationsofthoseshowninFig.2-4bandc.Suchsystemscangenerallybeclassifiedaspretensioningorpost-tensioningsystems.Inthecaseofpretensioning,thetendonsarestressedbeforetheconcreteisplaced,asinFig.2-4c.Thissystemiswellsuitedformassproduction,sincecastingbedscanbemadeseveralhundredfeetlong,theentirelengthcastatonce,andindividualbeamscuttothedesiredlengthinasinglecasting.Figure2-5showsworkersusingahydraulicjacktotensionstrandsattheanchorageofalongpretensioningbed.Althougheachtendonisindividuallystressedinthiscase,largecapacityjacksareoftenusedtotensionallstrandssimultaneously.先张法post-tensioning后张法；massproduction工厂生产；castingbeds浇筑平台；ahydraulicjack液压千斤顶2.3SourcesofPrestressForceMostofthepatentedsystemsforapplyingprestressincurrentusearevariationsofthoseshowninFig.2-4bandc.Suchsystemscangenerallybeclassifiedaspretensioningorpost-tensioningsystems.Inthecaseofpretensioning,thetendonsarestressedbeforetheconcreteisplaced,asinFig.2-4c.Thissystemiswellsuitedformassproduction,sincecastingbedscanbemadeseveralhundredfeetlong,（theentirelengthcastatonce,andindividualbeamscuttothedesiredlengthinasinglecasting）.Figure2-5showsworkersusingahydraulicjacktotensionstrandsattheanchorageofalongpretensioningbed.Althougheachtendonisindividuallystressedinthiscase,largecapacityjacksareoftenusedtotensionallstrandssimultaneously.在当前使用中施加预应力的大多数专利系统是图2-4b和c所示的变形形式。这种系统通常可以分类为先张法和后张法。先张法的情况下，在混凝土被放置之前对钢筋施加预应力，如图2-4c所示。该系统非常适合批量生产，因为铸造床可以制成几百英尺长，整个长度一次浇筑，并且单个梁在单个铸件中切割成所需长度。图2-5显示了在一个先张法的长浇筑床的锚固处使用液压千斤顶张拉的工人。尽管在这种情况下每个锚固单独受力，但通常使用大容量千斤顶来同时拉紧所有钢索。2.3SourcesofPrestressForce2.3SourcesofPrestressForceAlargenumberofparticularsystems,steelelements,jacks,andanchoragefittingshavebeendevelopedinthiscountryandabroad,manyofwhichdifferfromeachotheronlyinminordetails.Asfarasthedesignerofprestressedconcretestructuresisconcerned,itisunnecessaryandperhapsevenundesirabletospecifyindetailthetechniquethatistobefollowedandtheequipmenttobeused.Itisfrequentlybesttospecifyonlythemagnitudeandlineofactionoftheprestressforce.Thecontractoristhenfree,inbiddingthework,toreceivequotationsfromseveraldifferentprestressingsubcontractors,withresultantcostsavings.Itisevident,however,thatthedesignermusthavesomeknowledgeofthedetailsofthevarioussystemscontemplatedforuse,sothatinselectingcross-sectionaldimensions,anyoneofseveralsystemscanbeaccommodated.预应力设计原则和要求themagnitudeandlineofactionoftheprestressforce预应力的大小和作用线；contractor承包商；subcontractors分包商；accommodated适合的2.3SourcesofPrestressForceAlargenumberofparticularsystems,steelelements,jacks,andanchoragefittingshavebeendevelopedinthiscountryandabroad,(manyofwhichdifferfromeachotheronlyinminordetails).Asfarasthedesignerofprestressedconcretestructuresisconcerned,itisunnecessaryandperhapsevenundesirabletospecifyindetailthetechnique（thatistobefollowedandtheequipmenttobeused）.Itisfrequentlybesttospecifyonlythemagnitudeandlineofactionoftheprestressforce.Thecontractoristhenfree,（inbiddingthework）,toreceivequotationsfromseveraldifferentprestressingsubcontractors,withresultantcostsavings.Itisevident,however,thatthedesignermusthavesomeknowledgeofthedetailsofthevarioussystemscontemplatedforuse,sothatinselectingcross-sectionaldimensions,anyoneofseveralsystemscanbeaccommodated.在国内外已经开发了大量特殊系统，钢构件，千斤顶和锚固配件，其中很多不同之处仅在细节上。就预应力混凝土结构的设计者来说，不必要甚至不希望详细说明要遵循的技术和要使用的设备。通常最好仅指定预应力的大小和作用线。然后，承包商在投标工作时选择接收来自几个不同预应力分包商的报价，从而节省成本。然而，显而易见的是，设计人员必须了解所使用的各种系统的细节，使得在选择横截面尺寸时，可采纳多个系统中的任何一个。2.4PrestressingSteels

2.4PrestressingSteels

2.4PrestressingSteels

2.4PrestressingSteels

2.4PrestressingSteels（