《土木工程專業(yè)英語 第2版》 課件 Unit1 Introduction to Reinforced Concrete；Unit2 Prestressed Concrete

Unit1IntroductiontoReinforcedConcreteDesignEnglishforCivilEngineering——Unit1IntroductiontoReinforcedConcreteDesign1.1Concrete,ReinforcedConcrete,andPrestressedConcrete（混凝土，鋼筋混凝土，以及預(yù)應(yīng)力混凝土）1.2StructuralForms（結(jié)構(gòu)形式）1.3Loads（荷載）1.4Serviceability,Strength,andStructuralSafety（可用性，強度以及結(jié)構(gòu)安全性）1.4.1VariabilityofLoads（荷載變異性）1.4.2Strength（強度）1.4.3StructuralSafety（結(jié)構(gòu)安全性）1.5DesignBasis（設(shè)計基礎(chǔ)）1.1Concrete,ReinforcedConcrete,andPrestressedConcreteConcreteisastonelikematerialobtainedbypermittingacarefullyproportionedmixtureofcement,sandandgravelorotheraggregate,andwatertohardeninformsoftheshapeanddimensionsofthedesiredstructure.Thebulkofthematerialconsistsoffineandcoarseaggregate.Cementandwaterinteractchemicallytobindtheaggre-gateparticlesintoasolidmass.Additionalwater,overandabovethatneededforthischemicalreaction,isnecessarytogivethemixturetheworkabilitythatenablesittofilltheformsandsurroundtheembeddedreinforcingsteelpriortohardening.混凝土的組成stonelike像石頭一樣的

cement水泥

gravel砂礫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)permitanevenwidervari-etyofpropertiestobeobtained.特種混凝土property

特性

constituent構(gòu)成的

admixture外加劑plasticizer塑化劑air-entrainingagent加氣劑silicafume硅粉flyash粉煤灰

steam-curing蒸汽養(yǎng)護1.1Concrete,ReinforcedConcrete,andPrestressedConcreteThesepropertiesdependtoaverysubstantialdegreeontheproportionsofthemix,onthethoroughnesswithwhichthevariousconstituentsareintermixed,andontheconditionsofhumidityandtemperatureinwhichthemixismaintainedfromthemomentitisplacedintheformsuntilitisfullyhardened.Theprocessofcontrollingconditionsafterplacementisknownascuring.Toprotectagainsttheunintentionalproductionofsubstandardconcrete,ahighdegreeofskillfulcontrolandsupervisionisnecessarythroughouttheprocess,fromtheproportioningbyweightoftheindivid-ualcomponents,throughmixingandplacing,untilthecompletionofcuring.混凝土的制作與養(yǎng)護substantial實質(zhì)的

thoroughness充分性

intermix混合humidity濕度

curing養(yǎng)護1.1Concrete,ReinforcedConcrete,andPrestressedConcreteThefactorsthatmakeconcreteauniversalbuildingmaterialaresopronouncedthatithasbeenused,inmoreprimitivekindsandwaysthanatpresent,forthousandsofyears,startingwithlimemortarsfrom12,000to6000BCEinCrete,Cyprus,Greece,andtheMiddleEast.Thefacilitywithwhich,whileplastic,itcanbedepositedandmadetofillformsormoldsofalmostanypracticalshapeisoneofthesefactors.Itshighfireandweatherresistanceisanevidentadvantage.混凝土的歷史與優(yōu)點pronounced顯著的

primitive原始的

limemortars水泥砂漿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).混凝土組成材料的特性additive添加劑

constructionsite施工現(xiàn)場

compressivestrength抗壓強度tensilestrength抗拉強度

1.1Concrete,ReinforcedConcrete,andPrestressedConcreteTooffsetthislimitation,itwasfoundpossible,inthesecondhalfofthenineteenthcentury,tousesteelwithitshightensilestrengthtoreinforceconcrete,chieflyinthoseplaceswhereitslowtensilestrengthwouldlimitthecarryingcapacityofthemember.Thereinforcement,usuallyroundsteelrodswithappropriatesurfacedeformationstoprovideinterlocking,isplacedintheformsinadvanceoftheconcrete.將鋼材引入到混凝土中（即鋼筋混凝土）capacity承載力rod棒，桿interlocking咬合作用1.1Concrete,ReinforcedConcrete,andPrestressedConcreteWhencompletelysurroundedbythehardenedconcretemass,itformsanintegralpartofthemember.Theresultingcombinationoftwomaterials,knownasreinforcedconcrete，combinesmanyoftheadvantagesofeach:therelativelylowcost,goodweatherandfireresistance,goodcompressivestrength,andexcellentformabilityofconcreteandthehightensilestrengthandmuchgreaterductilityandtoughnessofsteel.Itisthiscombinationthatallowsthealmostunlimitedrangeofusesandpossi-bilitiesofreinforcedconcreteintheconstructionofbuildings,bridges,dams,tanks,reservoirs,andahostofotherstructures.鋼筋混凝土優(yōu)點hardened變硬的formability可成形性toughness韌性reservoirs水庫1.1Concrete,ReinforcedConcrete,andPrestressedConcreteAspecialwayhasbeenfound,however,tousesteelsandconcretesofveryhighstrengthincombination.Thistypeofconstructionisknownasprestressedconcrete.Thesteel,intheformofwires,strands,orbars,isembeddedintheconcreteunderhightensionthatisheldinequilibriumbycompressivestressesintheconcreteafterhard-ening.Becauseofthisprecompression,theconcreteinaflexuralmemberwillcrackonthetensionsideatamuchlargerloadthanwhennotsoprecompressed.Prestressinggreatlyreducesboththedeflectionsandthetensilecracksatordinaryloadsinsuchstructures,andtherebyenablesthesehigh-strengthmaterialstobeusedeffectively.預(yù)應(yīng)力混凝土prestressed預(yù)應(yīng)力的wire網(wǎng)strand串bar棒equilibrium平衡precompression預(yù)壓力1.2StructuralFormsFloorsupportsystemsforbuildingsincludethemonolithicslab-and-beamfloorshowninFig.1-1,theone-wayjoistsystemofFig.1-2,andtheflatplatefloor,withoutbeamsorgirders,showninFig.1-3.TheflatslabfloorofFig.1-4,frequentlyusedformoreheavilyloadedbuildingssuchaswarehouses,issimilartotheflatplatefloor,butmakesuseofincreasedslabthicknessinthevicinityofthecolumns,aswellasflaredcolumntops,toreducestressesandincreasestrengthinthesupportregion.Thechoiceamongtheseandothersystemsforfloorsandroofsdependsuponfunc-tionalrequirements,loads,spans,andpermissiblememberdepths,aswellasoncostandestheticfactors.樓層支撐體系monolithic整體的warehouse倉庫vicinity臨近flared向外展開的esthetic審美的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.混凝土殼roof屋頂shell殼cylindrical圓柱形的curvature曲率；彎曲1.2StructuralFormsFig.1-5Foldedplateroofof125ftspan,inadditiontocarryingordinaryroofloads,carriesthesecondflooraswellfromasystemofcablehangers;thegroundflooriskeptfreeofcolumns.Fig.1-6Cylindricalshellroofprovidingcolumn-freeinteriorspace.1.2StructuralFormsCylindricalconcretetanksarewidelyusedforstorageofwaterorinwastepurifi-cationplants.ThedesignshowninFig.1.11isproofthatasanitaryengineeringfacilitycanbeestheticallypleasingaswellasfunctional.Cylindricaltanksareoftenpre-stressedcircumferentiallytomaintaincompressionintheconcreteandeliminatethecrackingthatwouldotherwiseresultfrominternalpressure.圓柱形混凝土水箱storage儲存purifi-cation凈化sanitary衛(wèi)生的esthetically審美地circumferentially圓周地internal內(nèi)部的1.2StructuralFormsFig.1-11CircularconcretetanksusedasapartofthewastewaterpurificationfacilityatHowden,England.(NorthumbrianWaterAuthoritywithLuderandJones,Architects.)1.3LoadsDeadloadsarethosethatareconstantinmagnitudeandfixedinlocationthroughoutthelifetimeofthestructure.Usuallythemajorpartofthedeadloadistheweightofthestructureitself.Thiscanbecalculatedwithgoodaccuracyfromthedesigncon-figuration,dimensionsofthestructure,anddensityofthematerial.Forbuildings,floorfill,finishfloors,andplasteredceilingsareusuallyincludedasdeadloads,andanallowanceismadeforsuspendedloadssuchaspipingandlightingfixtures.Forbridges,deadloadsmayincludewearingsurfaces,sidewalks,andcurbs,andanallowanceismadeforpipingandothersuspendedloads.恒載deadloads恒載magnitude幅值plasteredceiling灰泥吊頂piping管道系統(tǒng)suspendedload懸掛荷載sidewalk人行道curb路邊1.3LoadsLiveloadsconsistchieflyofoccupancyloadsinbuildingsandtrafficloadsonbridges.Theymaybeeitherfullyorpartiallyinplaceornotpresentatall,andmayalsochangeinlocation.Theirmagnitudeanddistributionatanygiventimeareuncer-tain,andeventheirmaximumintensitiesthroughoutthelifetimeofthestructurearenotknownwithprecision.Theminimumliveloadsforwhichthefloorsandroofofabuildingshouldbedesignedareusuallyspecifiedinthebuildingcodethatgovernsatthesiteofconstruction.活載occupancy使用distribution分布1.3LoadsEnvironmentalloadsconsistmainlyofsnowloads,windpressureandsuction,earthquakeloads(i.e.,inertiaforcescausedbyearthquakemotions),soilpressuresonsubsurfaceportionsofstructures,loadsfrompossiblepondingofrainwateronflatsur-faces,andforcescausedbytemperaturedifferentials.Likeliveloads,environmentalloadsatanygiventimeareuncertaininbothmagnitudeanddistribution.Thebook,MinimumDesignLoadsforBuildingsandOtherStructures,containsmuchinformationonenvironmentalloads,whichisoftenmodifiedlocallydepending,forinstance,onlocalclimaticorseismicconditions.環(huán)境荷載suction吸力subsurface表面下的ponding積水differential差別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.結(jié)構(gòu)的可用性，強度以及結(jié)構(gòu)安全性collapse倒塌serviceability可用性foreseeably可預(yù)知地1.4.1VariabilityofLoadsSincethemaximumloadthatwilloccurduringthelifeofastructureisuncertain,itcanbeconsideredarandomvariable.Inspiteofthisuncertainty,theengineermustprovideanadequatestructure.Aprobabilitymodelforthemaximumloadcanbedevisedbymeansofaprobabilitydensityfunctionforloads,asrepresentedbythefre-quencycurveofFig.1.14a.Theexactformofthisdistributioncurve,foranyparticulartypeofloadingsuchasofficeloads，canbedeterminedonlyonthebasisofstatisticaldataobtainedfromlarge-scaleloadsurveys.Anumberofsuchsurveyshavebeencom-pleted.Fortypesofloadsforwhichsuchdataarescarce,fairlyreliableinformationcanbeobtainedfromexperience,observation,andjudgment.荷載概率模型randomvariable隨機變量distributioncurve分布曲線scarce缺乏的1.4.2StrengthThestrengthofastructuredependsonthestrengthofthematerialsfromwhichitismade.Forthispurpose,minimummaterialstrengthsarespecifiedinstandardizedways.Actualmaterialstrengthscannotbeknownpreciselyandthereforealsoconsti-tuterandomvariables.Structuralstrengthdepends,furthermore,onthecarewithwhichastructureisbuilt,whichinturnreflectsthequalityofsupervi-sionandinspection.Membersizesmaydifferfromspecifieddimensions,reinforce-mentmaybeoutofposition,poorlyplacedconcretemayshowvoids,etc.結(jié)構(gòu)強度precisely準確地supervi-sion監(jiān)管inspection檢查void孔洞1.5DesignBasisThesinglemostimportantcharacteristicofanystructuralmemberisitsactualstrength,whichmustbelargeenoughtoresist,withsomemargintospare,allforeseeableloadsthatmayactonitduringthelifeofthestructure,withoutfailureorotherdistress.Itislogical,therefore,toproportionmembers,i.e.,toselectconcretedimensionsandrein-forcement,sothatmemberstrengthsareadequatetoresistforcesresultingfromcertainhypotheticaloverloadstages,significantlyaboveloadsexpectedactuallytooccurinservice.Thisdesignconceptisknownasstrengthdesign.強度設(shè)計margin邊緣distress危害；危險logical合理的hypothetical假設(shè)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(簡支跨)beam(3)specialvariationofreinforcedconcrete3.SourcesofPrestressForcemethods:

pretensioning,post-tensioning4.Steelsstressloseproblem(應(yīng)力損失)

example：超張拉5.whyuseHigh-strengthConcrete4reasons2.1Introduction

Modernstructuralengineeringtendstoprogresstowardmoreeconomicstructuresthroughgraduallyimprovedmethodsofdesignandtheuseofhigherstrengthmaterials.Thisresultsinareductionofcross-sectionaldimensionsandconsequentweightsavings.Suchdevelopmentsareparticularlyimportantinthefieldofreinforcedconcrete,wherethedeadloadrepresentsasubstantialpartofthetotaldesignload.Also,inmultistorybuildings,anysavingindepthofmembers,multipliedbythenumberofstories,canrepresentasubstantialsavingintotalheight,loadonfoundations,lengthofheatingandelectricalducts,plumbingrisers,andwallandpartitionsurfaces.structuralengineering結(jié)構(gòu)工程；cross-sectionaldimensions截面尺寸；reinforcedconcrete鋼筋混凝土；deadload恒載；foundations基礎(chǔ)；ducts管線；plumbingrisers管道吊組繩2.1Introduction

Theselimitingfeaturesofordinaryreinforcedconcretehavebeenlargelyovercomebythedevelopmentofprestressedconcrete.Aprestressedconcretemembercanbedefinedasoneinwhichtherehavebeenintroducedinternalstressesofsuchmagnitudeanddistributionthatthestressesresultingfromthegivenexternalloadingarecounteractedtoadesireddegree.Concreteisbasicallyacompressivematerial,withitsstrengthintensionalowandunreliablevalue.Prestressingappliesaprecompressiontothememberthatreducesoreliminatesundesirabletensilestressesthatwouldotherwisebepresent.Crackingunderserviceloadscanbeminimizedorevenavoidedentirely.Deflectionsmaybelimitedtoanacceptablevalue;infact,memberscanbedesignedtohavezerodeflectionunderthecombinedeffectsofserviceloadandprestressforce.預(yù)應(yīng)力混凝土構(gòu)件介紹bedefinedas定義為；magnitudeanddistribution大小和分布；

tensilestresses拉應(yīng)力；Cracking開裂；deflection變形2.2EffectsofPrestressing2.2.1ConcreteStressControlbyPrestressingManyimportantfeaturesofprestressedconcretecanbedemonstratedbysimpleexamples.Considerfirsttheplain,unreinforcedconcretebeamshowninFig.2-1a.Itcarriesasingleconcentratedloadatthecenterofitsspan.Theself-weightofthememberwillbeneglectedhere.AstheloadWisgraduallyapplied,longitudinalflexuralstressesareinduced.Iftheconcreteisstressedonlywithinitselasticrange,theflexuralstressdistributionatmidspanwillbelinear,asshown.Atarelativelylowload,thetensilestressintheconcreteatthebottomofthebeamwillreachthetensilestrengthoftheconcretefr,andacrackwillform.Becausenorestraintisprovidedagainstupwardextensionofthecrack,thebeamwillcollapsewithoutfurtherincreaseofload.concentratedload集中荷載；thecenterofitsspan跨中；self-weight自重;longitudinalflexuralstresses縱向彎曲應(yīng)力；elasticrange彈性范圍；

tensilestrength

抗拉強度2.2EffectsofPrestressing2.2.1ConcreteStressControlbyPrestressing（1）Butitwouldbemorelogicaltoapplytheprestressingforcenearthebottomofthebeam,tocompensatemoreeffectivelyfortheload-inducedtension.Apossibledesignspecification,forexample,mightbetointroducethemaximumcompressionatthebottomofthebeamwithoutcausingtensionatthetop,whenonlytheprestressingforceacts.Itiseasilyshownthat,forabeamwitharectangularcrosssection,thepointofapplicationoftheprestressingforceshouldbeatthelowerthirdpointofthesectiondepthtoachievethis.預(yù)應(yīng)力施加過程prestressingforce預(yù)應(yīng)力；

designspecification設(shè)計方案；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.預(yù)應(yīng)力施加過程Eccentricity偏心；concretecentroid混凝土中部；longitudinalcompressivestressdistribution縱向壓應(yīng)力分布；themomentofinertiaofthecrosssection橫截面的慣性矩2.2EffectsofPrestressing2.2.1ConcreteStressControlbyPrestressingForeachcharacteristicloaddistribution,thereisabesttendonprofilethatproducesaprestressmomentdiagramthatcorrespondstothatoftheappliedload.Iftheprestresscountermomentismadeexactlyequalandoppositetotheload-inducedmoment,theresultisabeamthatissubjectonlytouniformaxialcompressivestressintheconcreteallalongthespan.Suchabeamwouldbefreeofflexuralcracking,andtheoreticallyitwouldnotbedeflectedupordownwhenthatparticularloadisinplace,comparedtoitspositionasoriginallycast.Sucharesultwouldbeobtainedforaloadof1/2×2Q=Q,asshowninFig.2-le，forexample.基于荷載分布的最佳鋼筋束布置方案besttendonprofile最佳鋼筋束布置方案；prestressmomentdiagram預(yù)應(yīng)力彎矩圖；Countermoment反彎矩，恢復(fù)力矩；allalongthespan沿著跨長；flexuralcracking彎曲開裂2.2EffectsofPrestressing2.2.1ConcreteStressControlbyPrestressing2.2EffectsofPrestressing2.2.1ConcreteStressControlbyPrestressing2.2EffectsofPrestressing2.2.2EquivalentLoadsInFig.2.2a,forexample,atendonthatappliesforcePatthecentroidoftheconcretesectionattheendsofabeamandthathasauniformslopeatangleθbetweentheendsandmidspanintroducesatransverseforce2Psinθatthepointofchangeofslopeatmidspan.Attheanchorages,theverticalcomponentoftheprestressingforceisPsinθandthehorizontalcomponentisPcosθ.ThehorizontalcomponentisverynearlyequaltoPfortheusualflatslopeangles.ThemomentdiagramforthebeamofFig.2-2aisseentohavethesameformasthatforanycenter-loadedsimplespan.預(yù)應(yīng)力的力等效過程uniformslope

等斜率，等坡度；midspan跨中；transverseforce橫向力，剪力；anchorages錨固端；momentdiagram彎矩圖2.2EffectsofPrestressing2.2.2EquivalentLoadsIfastraighttendonisusedwithconstanteccentricity,asshowninFig.2-2c,therearenoverticalforcesontheconcrete,butthebeamissubjecttoamomentPeateachend,aswellastheaxialforceP，andadiagramofconstantmomentresults.TheendmomentmustalsobeaccountedforinthebeamofFig.2-2d,inwhichaparabolictendonisusedthatdoesnotpassthroughtheconcretecentroidattheendsofthespan.Inthiscase,auniformlydistributedupwardloadplusendanchorageforcesareproduced,asshowninFig.2-2b,butinaddition,theendmomentsM=Pecosθmustbeaccountedfor.預(yù)應(yīng)力的力等效過程straighttendon直線型預(yù)應(yīng)力筋；verticalforces豎向力；axialforce軸向力；constantmoment固定彎矩；endmoment端彎矩；parabolictendon拋物線型的預(yù)應(yīng)力鋼筋；uniformlydistributed均布2.2EffectsofPrestressing2.2.2EquivalentLoads2.2EffectsofPrestressing2.2.3PrestressedConcreteasaVariationofReinforcedConcreteInthedescriptionsoftheeffectsofprestressingintheparagraphsabove,itwasimpliedthattheprestressforceremainedconstantastheverticalloadwasintroduced,thattheconcreterespondedelastically,andthatnoconcretecrackingoccurred.Theseconditionsmayprevailuptoabouttheserviceloadlevel,butiftheloadsshouldbeincreasedmuchbeyondthat,flexuraltensilestresseswilleventuallyexceedthemodulusofruptureandcrackswillform.Loadscanusuallybeincreasedmuchbeyondthecrackingloadinwell-designedprestressedbeams.作為鋼筋混凝土的變形體flexuraltensilestresses彎曲拉應(yīng)力；exceedthemodulusofruptureandcrackswillform超過斷裂模量而產(chǎn)生裂縫；well-designed設(shè)計良好的2.2EffectsofPrestressing2.2.3PrestressedConcreteasaVariationofReinforcedConcreteEachofthethreeviewpointsdescribed—concretestresscontrol,equivalentloads,andreinforcedconcreteusingprestrainedsteel—isusefulintheanalysisanddesignofprestressedconcretebeams,andnoneofthethreeissufficientinitself.Neitheranelasticstressanalysisnoranequivalentloadanalysisprovidesinformationaboutstrengthorsafetymargin.However,thestressanalysisishelpfulinpredictingtheextentofcracking,andtheequivalentloadanalysisisoftenthebestwaytocalculatedeflections.Strengthanalysisisessentialtoevaluatesafetyagainstcollapse,butittellsnothingaboutcrackingordeflectionsofthebeamunderserviceconditions.總結(jié)三類觀點equivalentloads等效荷載；strengthorsafetymargin強度和安全界限；extentofcracking開裂的程度；Strengthanalysis強度分析；evaluatesafetyagainstcollapse評估抵抗倒塌的安全度2.3SourcesofPrestressForcePrestresscanbeappliedtoaconcretememberinmanyways.Perhapsthemostobviousmethodofprecompressingistheuseofjacksreactingagainstabutments,asshowninFig.2-4a.Suchaschemehasbeenemployedforlargeprojects.Manyvariationsarepossible,includingreplacingthejackswithcompressionstrutsafterthedesiredstressintheconcreteisobtainedorusinginexpensivejacks,thatremaininplaceinthestructure,insomecaseswithacementgroutusedasthehydraulicfluid.Theprincipaldifficultyassociatedwithsuchasystemisthatevenaslightmovementoftheabutmentswilldrasticallyreducetheprestressforce.預(yù)應(yīng)力的施加Precompressing先張法；Jacks千斤頂；abutments臺座；cementgrout水泥漿；hydraulicfluid液壓流體2.3SourcesofPrestressForceInmostcases,thesameresultismoreconvenientlyobtainedbytyingthejackbasestogetherwithwiresorcables,asshowninFig.2-4b.Thesewiresorcablesmaybeexternal,locatedoneachsideofthebeam;moreusuallytheyarepassedthroughahollowconduitembeddedintheconcretebeam.Usually,oneendoftheprestressingtendonisanchored,andalltheforceisappliedattheotherend.Afterattainmentofthedesiredprestressforce,thetendoniswedgedagainsttheconcreteandthejackingequipmentisremovedforreuse.Notethatinthistypeofprestressing,theentiresystemisself-containedandisindependentofrelativedisplacementofthesupports.施加預(yù)應(yīng)力wiresorcables鋼線或錨索；a

hollowconduitembeddedintheconcretebeam嵌入混凝土梁的空心管道；attainmentofthedesiredprestressforce達到所需預(yù)應(yīng)力；wedged

嵌入；self-contained獨立的；relativedisplacement相對位移2.3SourcesofPrestressForceMostofthepatentedsystemsforapplyingprestressincurrentusearevariationsofthoseshowninFig.2-4bandc.Suchsystemscangenerallybeclassifiedaspretensioningorpost-tensioningsystems.Inthecaseofpretensioni