專業(yè)英語課文翻譯Lesson4PhrasesandExpressionsmoisturecontent含水量，含濕度;watercontentcementpaste水泥漿mortarcapillarytension毛細管張力，微張力gradationofaggregate骨料級配coarsefine(crushedstone,gravel)TheBritishCodePC100英國混凝土規(guī)范PC100;nowadayBS8110coefficientofthermalexpansionofconcrete混凝土熱膨脹系數(shù)7.TheB.SCode英國標準規(guī)范sustainedload永久荷載，長期荷載permanentplasticstrain永久的塑性應變stresscrystallattice晶格,晶格cementgel水泥凝膠體water-cementratio水灰比expansionjoint伸縮縫stabilityofthestructure結(jié)構(gòu)的穩(wěn)定性structuralstability15.fatiguestrengthofconcrete混凝土的疲勞強度VolumeChangesofConcrete Concreteundergoesvolumechangesduringhardening.混凝土在硬結(jié)過程中會經(jīng)歷體積變化。Ifitlosesmoisturebyevaporation,itshrinks,butiftheconcretehardensinwater,itexpands.如果蒸發(fā)失去水分，混凝土會收縮；但如果在水中硬結(jié)，它便膨脹。Thecausesofthevolumechangesinconcretecanbeattributedtochangesinmoisturecontent,chemicalreactionofthecementwithwater,variationintemperature,andappliedloads.混凝土體積變化的原因可歸結(jié)為含水量的變化、水泥與水的水化反應、溫度變化和所施加的荷載。ShrinkageThechangeinthevolumeofdryingconcreteisnotequaltothevolumeofwaterremoved.Theevaporationoffreewatercauseslittleornoshrinkage.混凝土干燥時的體積變化量不等于它所失去的水的體積。自由水的蒸發(fā)基本不產(chǎn)生收縮。Asconcretecontinuestodry,waterevaporatesandthevolumeoftherestrainedcementpastechanges,causingconcretetoshrink,probablyduetothecapillarytensionthatdevelopsinthewaterremaininginconcrete.隨著混凝土的不斷變干，水分蒸發(fā)，受約束水泥漿的體積也變化，導致了混凝土的收縮，這多半是由于殘留在混凝土中的水的毛細張力所致。Emptyingofthecapillariescausesalossofwaterwithoutshrinkage.Butoncetheabsorbedwaterisremoved,shrinkageoccurs.毛細管變空導致無收縮的水分丟失，但一旦失去吸收的水分，收縮便發(fā)生。Manyfactorsinfluencetheshrinkageofconcretecausedbythevariationsinmoistureconditions.許多因素都會影響因水分環(huán)境發(fā)生變化而產(chǎn)生的混凝土收縮。1.Cementandwatercontent.Themorecementorwatercontentintheconcretemix,thegreatertheshrinkage.水灰比：水灰比越大，收縮越大；2.Compositionandfinenessofcement.High-early-strengthandlow-heatcementsshowmoreshrinkagethannormalportlandcement.Thefinerthecement,thegreateristheexpansionundermoistconditions.3.Type,amount,andgradationofaggregate.Thesmallerthesizeofaggregateparticles,thegreateristheshrinkage.Thegreatertheaggregatecontent,thesmalleristheshrinkage.骨料的類型、含量及其級配：骨料的粒徑越小，收縮越大；骨料含量越大，收縮則越小。4.Ambientconditions,moisture,andtemperature.Concretespecimenssubjectedtomoistconditionsundergoanexpansionof200to300×10-6,butiftheyarelefttodryinair,theyshrink.Hightemperaturespeedstheevaporationofwaterand,consequently,increasesshrinkage.外部條件，水分與溫度：潮濕環(huán)境下的混凝土試件的膨脹量為200to300×10-6，但如果讓其在空氣中干燥，它們將收縮。高溫加速了水分的蒸發(fā)，因此也加快了收縮。5.Admixtures.Admixturesthatincreasethewaterrequirementofconcreteincreasetheshrinkagevalue.添加劑：使用水量增加的外加劑也增加了收縮值。6.Sizeandshapeofspecimen.Asshrinkagetakesplaceinareinforcedconcretemember,tensionstressesdevelopintheconcrete,andequalcompressivedevelopinthesteel.Thesestressesareaddedtothosedevelopedbytheloadingaction.Therefore,cracksmaydevelopinconcretewhenahighpercentageofsteelisused.Properdistributionofreinforcement,byproducingbetterdistributionoftensilestressesinconcrete,canreducedifferentialinternalstresses.試件的尺寸和形狀：當收縮在鋼筋混凝土構(gòu)件中發(fā)生時，混凝土中產(chǎn)生拉應力，同樣大小的壓力產(chǎn)生于鋼筋中，這些力與荷載引起的力相迭加。因此，當鋼筋的配筋率高時，可能會使混凝土開裂。鋼筋的合理分布、會使混凝土中的拉應力分布更有利，可減小內(nèi)部應力差。Thevaluesoffinalshrinkageforordinaryconcretevarybetween200and700×10-6.普通混凝土的收縮應變終值在200and700×10-6之間。Fornormal-weightconcrete,avalueof300×10-6maybeused.對常重混凝土，收縮應變終值可取為300×10-6TheBritishCodeCP100givesavalueof500×10-6,whichrepresentsanunrestrainedshrinkageof1.5mmin3mlengthinthin,plainconcretesections.英國CP100規(guī)范不出的收縮應變終值為500×10-6，這表示3m長素混凝土薄截面構(gòu)件的非約束收縮為1.5mm.Ifthememberisrestrained,atensilestressofabout10N/mm2(1400psi)arises.如果此構(gòu)件受到約束，便產(chǎn)生了大約10N/mm2(1400psi)的拉應力。Ifconcreteiskeptmoistforacertainperiodaftersetting,shrinkageisreduced;如果凝結(jié)后將混凝土保持在潮濕環(huán)境中，可減小其收縮。therefore,itisimportanttocuretheconcreteforaperiodofnofewerthan7days.因此，將混凝土至少在潮濕環(huán)境中養(yǎng)護7天非常重要。Exposureofconcretetowindincreasestheshrinkagerateontheupwindside.將混凝土置于風中將增加其在迎風一側(cè)的收縮速率。Shrinkagecausesanincreaseinthedeflectionofstructuralmembers,whichinturnincreaseswithtime.收縮引起結(jié)構(gòu)構(gòu)件的撓度增大，而撓度也隨時間而增長。Symmetricalreinforcementintheconcretesectionmaypreventcurvatureanddeflectionduetoshrinkage.混凝土截面的對稱配筋可防止因收縮而產(chǎn)生的曲率和撓度。Generally,concreteshrinksatahighrateduringtheinitialperiodofhardening,butatlaterstagestheratediminishesgradually.通常，在硬結(jié)初期，混凝土收縮較快，但在后期，收縮速度會慢慢變小。Itcanbesaidthat15%to30%oftheshrinkagevalueoccursin2weeks,40%to80%occursin1month,and70%to85%occursin1year.可以說15%to30%收縮量的發(fā)生在前兩周，40%to80%發(fā)生在前一個月，70%to85%發(fā)生在第一年。ExpansionDuetoRiseinTemperature升溫膨脹Concreteexpandswithincreasingtemperatureandcontractswithdecreasingtemperature.Thecoefficientofthermalexpansionofconcretevariesbetween4and7×10-6perdegreeFahrenheit.混凝土受溫膨脹，變冷時收縮。Anaveragevalueof5.5×10-6perdegreeFahrenheit(12×10-6perdegreeCelsius)canbeusedforordinaryconcrete.一般混凝土的平均收縮值為每華氏度5.5×10-6（或每攝氏度12×10-6）。TheB.S.Codesuggestsavalueof10-5perdegreeCelsius.英國標準規(guī)范的建議值為每攝氏10-5Thisvaluerepresentsachangeoflengthof10mmina30-mmembersubjectedtoachangeintemperatureof33℃.Ifthememberisrestrainedandunreinforced,astressofabout7N/mm2(1000psi)maydevelop.這一值表示如果30m長的構(gòu)件溫度變化33度，其長度變化為10mm.如果該構(gòu)件受到約束但并未配筋，由此產(chǎn)生的應力可能約為7N/mm2(1000psi)Inlongreinforcedconcretestructures,expansionjointsmustbeprovidedatlengthsof100to200ft(30to60m).在較長的鋼筋混凝土結(jié)構(gòu)中，每隔100英尺到200英尺（30m至30m）必須留伸縮縫。Thewidthoftheexpansionjointisabout1in.(25mm).伸縮縫寬度約為1英寸（25mm）Concreteisnotagoodconductorofheat,whereassteelisagoodone.Theabilityofconcretetocarryloadisnotmuchaffectedbytemperature.Greep 徐變Concreteisanelastoplasticmaterial,andbeginningwithsmallstresses,plasticstrainsdevelopinadditiontoelasticones.混凝土是一種彈塑性材料，即使受低應力作用，它就產(chǎn)生伴有彈性應變的塑性應變。Undersustainedload,plasticdeformationcontinuestodevelopoveraperiodthatmaylastforyears.在持續(xù)荷載作用下，在很長時間內(nèi)其塑性變形連續(xù)增長，甚至可達數(shù)年。Suchdeformationincreasesatahighrateduringthefirst4monthsafterapplicationoftheload.Thisslowplasticdeformationunderconstantstressiscalledcreep.此變形在加載后的前4個月最快。這種在持續(xù)應力作用下的緩慢變形稱為徐變。Figure2.5showsaconcretecylinderthatisloaded.圖2.5表示受荷載作用的一混凝土圓柱體。Theinstantaneousdeformationisε1,whichisequaltothestressdividedbythemodulusofelasticity.其瞬時變形為ε1，它等于應力除以彈性模量。Ifthesamestressiskeptforaperiodoftime,anadditionalstrainε2,duetocreepeffect,canberecorded.如果將此應力保持一段時間，便可測出另一由徐變所產(chǎn)生的應變增量ε2，Ifloadisthenreleased,theelasticstrain,ε1,willberecovered,inadditiontosomecreepstrain.如果卸去荷載，彈性應變便得到恢復，同時還產(chǎn)生一些徐變應變。Thefinalpermanentplasticstrain,ε3,willbeleft,asshowninFigure2.5.最終會剩下永久塑性應變ε3，如圖2.5所示。Inthiscaseε3=(1－α)ε2,whereαistheratiooftherecoveredcreepstraintothetotalcreepstrain.在此情況下，ε3=(1－α)ε2,α是恢復徐變應變與總徐變的比值。Theratioαrangesbetween0.1and0.2.其值在0.1至0.2之間。Themagnitudeofcreeprecoveryvarieswiththepreviouscreepanddependsappreciablyupontheperiodofthesustainedload.徐變后效的大小隨先前徐變的大小而變化，且在很大程度上取決于荷載的持續(xù)時間。Creeprecoveryratewillbelessiftheloadingperiodisincreased,probablyduetothehardeningofconcretewhileinadeformedcondition.如果持荷時間增加，徐變恢復率將變小，這大概是由于混凝土在變形硬化所致。Figure2.5Deformationinaloadedconcretecylinder:(a)specimenunloaded,(b)elasticdeformation,(c)elasticpluscreepdeformation,(d)permanentdeformationafterreleaseofload.Theultimatemagnitudeofcreepvariesbetween0.2×10-6and2×10-6perunitstress(1b/in.2)perunitlength.Avalueof1×10-6canbeusedinpractice.Theratioofcreepstraintoelasticstrainmaybeashighas4.Creeptakesplaceinthehardenedcementmatrixaroundthestrongaggregate.Itmaybeattributedtoslippagealongplaneswithinthecrystallattice,internalstressescausedbychangesinthecrystallattice,andgraduallossofwaterfromthecementgelintheconcrete.徐變在強度更高的骨料周圍的硬結(jié)的水泥基中產(chǎn)生，這可能是由于沿晶格中一些平面發(fā)生滑移、晶格變化所產(chǎn)生的內(nèi)部應力、及混凝土凝膠體中水分不斷丟失的結(jié)果。Thedifferentfactorsthataffectthecreepofconcretecanbesummarizedasfollows.影響徐變的各種不同因素可概括如下：1.Thelevelofstress.Creepincreaseswithanincreaseofstressinspecimensmadefromconcreteofthesamestrengthandwiththesamedurationofload.應力大?。?.Durationofloading.Creepincreaseswiththeloadingperiod.About80%ofthecreepoccurswithinthefirst4months;90%occursafterabout2years.持荷時間：3.Strengthandageofconcrete.Creeptendstobesmallerifconcreteisloadedatalateage.Also,creepof2000psi-(14N/mm2-)strengthconcreteisabout1.41×10-6whereasthatof4000psi-(28N/mm2-)strengthconcreteisabout0.8×10-6perunitstressandlengthoftime.混凝土強度和齡期：4.Ambientconditions.Creepisreducedwithanincreaseinthehumidityoftheambientair.周圍環(huán)境條件：5.Rateofloading.Creepincreaseswithanincreaseintherateofloadingwhenfollowedbyprolongedloading.加載速率：6.Percentageanddistributionofsteelreinforcementinareinforcedconcretemember.Creeptendstobesmallerforhigherproportionorbetterdistributionofsteel.鋼筋混凝土結(jié)構(gòu)中鋼筋的配筋率及其分布：7.Sizeoftheconcretemass.Creepdecreaseswithanincreaseinthesizeofthetestedspecimen.混凝土尺寸：8.Type,fineness,andcontentofcement.Theamountofcementgreatlyaffectsthefinalcreepofconcrete,ascementcreepsabout15timesasmuchasconcrete.水泥的類型、細度和含量：9.Water-cementratio.Creepincreaseswithanincreaseinthewater-cementratio.水灰比：10.Typeandgradingofaggregate.Well-gradedaggregatewillproducedenseconcreteandconsequentlyareductionincreep.骨料的類型和級配：11.Typeofcuring.High-temperaturesteamcuringofconcreteaswellastheproperuseofaplasticizerwillreducetheamountofcreep.養(yǎng)護方式：Creepdevelopsnotonlyincompression,butalsointension,bending,andtorsion.徐變不僅產(chǎn)生于受壓，它也產(chǎn)生于受拉，受彎和受扭。Theratiooftherateofcreepintensiontothatincompressionwillbegreaterthan1inthefirst2weeks,butthCreepinconcreteundercompressionhasbeentestedbymanyinvestigators.Troxell,Davis,andRaphaelmeasuredcreepstrainsperiodicallyforupto20yearsandestimatedthatofthetotalcreepafter20years,18%to35%occurredin2weeks,30%to70%occurredin3months,and64%to83%occurredin1year.許多研究者都對混凝土的受壓徐變進行了試驗研究。Fornormalconcreteloadedafter28days,,where=creepstrainperunitstressperunitlength.對28天齡期加載的普通混凝土，，=單位應力下單位時間內(nèi)的徐變。Creepaugmentsthedeflectionofreinforcedconcretebeamsappreciablywithtime.Inthedesignofreinforcedconcretemembers,long-termdeflectionmaybecriticalandhastobeconsideredinproperdesign.隨時間的增加，徐變會大大增加鋼筋混凝土梁的撓度。Extensivedeformationmayinfluencethestabilityofthestructure.過大的變形會影響結(jié)構(gòu)的穩(wěn)定性。Sustainedloadsaffectthestrengthaswellasthedeformationofconcrete.持續(xù)加載會影響混凝土的強度和變形。Areductionofupto30%ofthestrengthofunreinforcedconcretemaybeexpectedwhenconcreteissubjectedtoaconcentricsustainedloadfor1year.當混凝土在持續(xù)集中荷載作用下長達一年時，其鋼筋混凝土的強度可能要降低30%.Thefatiguestrengthofconcreteismuchsmallerthanitsstaticstrength.混凝土的疲勞強度大大低于其靜力強度。Repeatedloadingandunloadingcyclesincompressionleadtoagradualaccumulationofplasticdeformations.受壓的重復加載和卸載會導致混凝土塑性變形的逐漸積累。Ifconcreteincompressionissubjectedtoabout2millioncycles,itsfatiguelimitisabout50%to60%ofthestaticcompressionstrength.Inbeams,thefatiguelimitofconcreteisabout55%ofitsstaticstrength.如果混凝土不斷受壓加載2百萬次，其疲勞強度容許值會降為其靜態(tài)抗壓強度的50%至60%.在梁中，混凝土的疲勞強度約為其靜態(tài)強度的55%.Lesson5LoadsNewwords1.prescribe v.指示，規(guī)定;specify2.criterion n.標準，規(guī)范，準則；standard3.dictate v.規(guī)定，指示，命令；dominate4.survey n.v.調(diào)查，測量；investigation5.variation n.變更，變化，差異；vary,variable,various6.deterministic a.確定性的，nondeterministic,probabilistic7.rational a.合理的，理性的；reasonable,logical8.stochastic a.隨機的；stochasticprocess,butrandomvariable9.pertain v.適合，屬于;pertaining10.cumbersome a.討厭的，麻煩的；troublesome11.landscape n.地形，風景，景觀12.endeavor v.n.盡力，努力；strive，puteffortto13.deviation n.背離；deviate,14.constancy n.恒定性，不變性，constant15.occupant n.居住者16.fluctuation n.波動，起伏；fluctuate17.`tenant n.房客，承租人；18.transient a.瞬時的；instantaneous19.sustain vt.維持，支撐，持續(xù)；sustainable20.premise n.前提；vt.假定；PhrasesandExpressions1.nominalvalue 標準值，名義值2.inpractice 實際上，在實踐上3.atbest 充其量，最好也只不過4.buildingcode 5.seismicaction 地震作用，earthquakeload6.reference(return)period 重現(xiàn)期7.semiempiricalequation 半經(jīng)驗公式 empirical8.windtunneltesting 風洞試驗9.meanvalue 均值；平均10.code-prescribeddata 規(guī)范規(guī)定數(shù)值11.transientliveload 瞬時活荷載12.sustainedliveload 持續(xù)活荷載TextLoads(1)IntroductionNormally,adesignspecificationdoesnotprescribethemagnitudesoftheloadsthataretobeusedasthebasicinputtothestructuralanalysis,withtheexceptionofspecialcasessuchascranedesignspecifications.除特殊情況設計規(guī)程外（如起重機設計規(guī)程），一般設計規(guī)程并不規(guī)定荷載的大小，盡管它是作為結(jié)構(gòu)分析所輸入的基本變量。Itistheroleofthespecificationtodetailthemethodsandcriteriatobeusedinarrivingatsatisfactorymemberandconnectionsizesforthestructuralmaterialinquestion,giventhemagnitudesoftheloadsandtheireffects.設計規(guī)程的作用就是對于給定的荷載值及其效應，詳細說明用設計材料能得到滿意構(gòu)件及其連接尺寸的方法和準則。Thespecificationthereforereflectstherequirementsthatmustbesatisfiedbythestructureinorderthatitwillhavearesponsethatallowsittoachievetheperformancethatisneeded.因此，規(guī)程反映了結(jié)構(gòu)必須滿足的各種要求，從而使其具有這樣一種結(jié)構(gòu)反應，它能使其達到所要求的性能。Loads,ontheotherhand,aregovernedbythetypeofoccupancyofthebuilding,whichinturnisdictatedbytheapplicablelocal,regional,andnationallawsthataremorecommonlyknownatbuildingcodes.另一方面，荷載取決于建筑物的使用類型，這反過來取決于相應的地區(qū)，地方和國家法規(guī)，即常說的建筑規(guī)范。 Thebuildingcodeloadshavetraditionallybeengivenasnominalvalues,determinedonthebasisofmaterialproperties(e.g.,deadload)orloadsurveys(e.g.,liveloadandsnowload).建筑規(guī)范的荷載傳統(tǒng)上都是作為標準值給出，它們是根據(jù)材料特性確定（如恒載）或荷載調(diào)查所確定（如活荷載及雪荷載）Tobereasonablycertainthattheloadsarenotexceededinagivenstructure,thecodevalueshavetendedtobehigherthantheloadsonarandomstructureatanarbitrarypointintime.為了適當?shù)卮_保作用在任一結(jié)構(gòu)上的荷載不超過規(guī)范值，后者往往都要比任一時刻作用任一結(jié)構(gòu)上的荷載值大些。Thismay,iffact,beoneofthereasonswhyexcessivegravityloadsarerarelytheobviouscauseofstructuralfailures.事實上，這可能就是過大的重力荷載大都不會導致結(jié)構(gòu)破壞的原因所在。Bethatatitmay,thefactofthematteristhatallofthevarioustypesofstructuralloadsexhibitrandomvariationsthatarefunctionsoftime,andthemannerofvariationalsodependsonthetypeofload.盡管可能如此，實際上結(jié)構(gòu)上的各種荷載都具有隨時間而變化的隨機變化特性，且這種變化也取決于荷載類型。Ratherthandealingwithnominalloadsthatappeartobedeterministicanature,arealisticdesignprocedureshouldtakeloadvariabilityintoaccountalongwiththatofthestrength,inorderthatadequatestructuralsafetycanbeachievedthroughrationalmeans.不是去處理看上去具有定值特征的標準荷載，現(xiàn)實的設計方法應同時考慮荷載和強度的變異性，以便以合理的手段得到足夠的結(jié)構(gòu)安全度。 Sincetherandomvariationoftheloadsisafunctionoftimeaswellasanumberofotherfactors,themodeling,strictlyspeaking,shouldtakethisintoaccountbyusingstochasticanalysestoreflectthetimeandspaceinterdependence.由于荷載的隨機變化是時間以及許多因素的函數(shù)，嚴格地講，通過采用隨機分析方法以反映時間與空間的相互影響，應使建模對此加以考慮。Manystudieshavedealtwiththishighlycomplexphenomenon,especiallyasitpertainstoliveloadinbuildings.許多研究工作都涉及了這一高度復雜的現(xiàn)象，特別是當其屬于活荷載時。Inpractice,however,theuseoftime-dependentloadsiscumbersomeatbest,althoughtherelationshipmustbeaccountedforincertaincases(i.e.,seismicaction).然而，實踐中采用時間相關荷載至少半是麻煩的，盡管在一些情況下必須考慮其相關性（即在有地震作用時）。Formostdesignsituationsthecodewillspecifythemagnitudeoftheloadsasiftheywerestatic.對于大多數(shù)設計，規(guī)范將規(guī)定荷載的大小，就象它們是靜載似的。Theirtimeandspacevariationarecoveredthroughtheuseofthemaximumloadoccurringoveracertainreference(return)period,anditsstatistics.通過采用出現(xiàn)在某一參照期（重現(xiàn)周期）內(nèi)的最大荷載及其統(tǒng)計特性，將它們的時間和空間的變異加以考慮。Forexample,Americanliveloadcriteriaarebasedonareferenceperiodof50years,whileCanadiancriteriausea30-yearinterval.例如，美國活荷載準則基于50年重現(xiàn)周期，而加拿大準則是30年。 Thegeographicallocationofthestructureplaysanimportantroleforcertainloads.結(jié)構(gòu)的地理位置對某些荷載起很重要的作用。Itisparticularlyapplicabletosnow,windandseismicaction,thefirstbeingofspecialimportanceinnorth-centralandnorth-easternareasoftheUnitedStates,thesecondinhighwindcoastalandmountainareas,andthelastinareashavingearthquakefaultlines.特別是對于雪、風和地震作用更是如此。第一種荷載對美國中北部和東北部地區(qū)非常重要，第二種對具有大風的沿海地區(qū)和山嶺地區(qū)特別重要，第三種對具有地震斷裂帶的地區(qū)則特別重要。 Deignforwindeffectsiscomplicatedbyanumberofphenomena.由于數(shù)個現(xiàn)象使風作用效果的設計復雜化。Likesnowloadsandearthquakeaction,windloadsaregivenmoreattentionincertainpartsofthecountry.類似于雪荷載和地震作用，在本國的一些地區(qū)，對風荷載更加重視。Atthesametimewindloadsareneitherstaticnoruniformlyvarying,andareheavilyinfluencedbythegeometryofthestructureaswellsthesurroundingstructuresandlandscape.同時，風載不但非靜態(tài)，而且也非均勻變化，同時還受結(jié)構(gòu)幾何形式和周圍結(jié)構(gòu)物及地形的影響。Toacertaindegreethisalsoappliestothemagnitudeofthesnowload.在一定程度上，這也適用于雪荷載。Buildingcodestreattheseeffectsasstaticphenomenaandrelatethemtotheactualconditionsthroughsemiempiricalequations.建筑規(guī)范將這些作用作為靜力荷載并用半經(jīng)驗公式將其與實際情況相聯(lián)系。Thisgivesthedesignerabetterhandleonadifficultproblem,butcanleadtodifficultieswhentherealstructuredepartssignificantlyfromthebasesofthecode.這使設計者能更好地處理復雜問題，但當實際結(jié)構(gòu)與設計規(guī)范出入太大時，便導致了一些困難。Forthatreasonwindloads,andsometimesearthquakeandsnowloads,aredeterminedonthebasisofmodeltest.為此，風荷載、有時地震荷載和雪荷載都要用模型試驗來確定。Inparticular,windtunneltestinghasbecomeausefulandpracticaltoolintheseendeavors.特別是風洞試驗，它已經(jīng)成為這些努力中一個有用且實用工具。 Theloadsonthestructurearenormallyassumedtobeindependentofthetypeofstructureandstructuralmaterial,withtheexceptionofdeadloads.除恒載外，通常都假定結(jié)構(gòu)上的荷載與結(jié)構(gòu)類型及其材料無關。Theresponseofabuilding,however,willbedifferentfordifferentmaterials,dependingonthetypeofload.然而，一建筑物的反應將隨其建材的不同而不同，這取決于荷載的類型。Forexample,thebehaviorofamoment-resistantsteelframewillbequiteunlikethatofabracedframe,whensubjectedtolateralloads,especiallythoseduetoanearthquake.例如，在側(cè)向荷載作用下，特別是當其由地震所引起時，抗彎鋼框架的工作性能將全然不同于有支撐框架的性能。Ontheotherhand,theresponseofthesetwoframestogravityloadswillnotbeallthatdifferent.另一方面，這兩種框架對重力荷載反應的差別卻并沒有那么大。 Thesizeofastructure(height,floorarea)hasasignificantimpactonthemagnitudesofmostloads.一個結(jié)構(gòu)物的大小（如其高度，樓層面積）對大多數(shù)荷載的量值影響很大。Allloadsareinfluencedbytheincreasingheightofamultistorybuilding,forexample.例如，所有的荷載都受多層建筑高度增加的影響。Similarly,thegreaterthefloorareathatistobesupportedbyasinglemember,thesmallerwillbetheprobabilitythatthecodeliveloadwillappearwithitsfullintensityovertheentirearea.與這類似，單個構(gòu)件所支承的樓層面積越大，整個樓層上滿載規(guī)范規(guī)定的活荷載的可能性將越小。Insuchcasesaliveloadreductionmethodisusedtoarriveatmorerealisticdesigndata.在此情況下，將采用活荷載折減法以得到更真實的設計值。Lesson8StructuralSteelBehavior1.behavior n.行為，性能，特點，特性；perty n.性能，特點，特性3.fracture n.v.斷裂，破裂4.manganese n.錳5.quote vt.引用，引證，提供，cite6.scatter n.分散disperse,distribute,part,separate,splitup,spread7.accentuate n.強調(diào);emphasize8.damage n.損傷，破壞9.raiser n.抬起者，興起者，提出者10.initiate vt.起始，開始begin11.arrest vt.阻止，抑制apprehend,capture,catch,check,stop12.flaw n.缺陷，裂紋blemish,crack,damage,defect,fault,weakness13.defect14.transitional a.過渡的，不穩(wěn)定的，transit轉(zhuǎn)變，過渡transitionPhrasesandExpressionmechanicalproperty力學性能，機械性能Young’smodulusofelasticityaccountfor是（造成……的）原因，說明（原因）heattreatmentcoldworkingstandardtensiontest標準拉伸試驗normaldistributioncurvefluctuatingload交變荷載，脈動荷載high-cyclefatigue高周疲勞lowcyclefatiguestressrange應力幅（范圍）fatiguelife 疲勞壽命Text:StructuralSteelBehaviorMechanicalPropertiesunderStaticLoadTheimportantmechanicalpropertiesofmoststructuralsteelsunderstaticloadareindicatedintheidealizedtensilestress-straindiagramshowninFig.8.1.在靜力荷載作用下大多數(shù)結(jié)構(gòu)鋼的一些力學性能如圖8.1所示，Initiallythesteelhasalinearstress-straincurvewhoseslopeistheYoung’smodulusofelasticity.最初鋼材的線性應力-應變曲線為線性，其斜率為楊氏彈性模量。ThevaluesofEvaryintherange2000000~210000Mpa,andtheapproximatevalueof2000000Mpaisoftenassumed.彈性模量E的變化范圍是2~2.1兆帕，通常假定其值約為2兆帕。Thesteelremainselasticwhileinthislinearrange,recoversperfectlyonunloading.鋼材在此線性階段保持彈性，卸載后可完全恢復。ThelimitofthelinearelasticbehaviorisoftencloselyapproximatedbytheyieldstressFy,andthecorrespondingyieldstrainy=Fy/E.線彈性性能的極限常近似為屈服應力Fy，相應的屈服應變?yōu)镕y/E。Beyondthislimitthesteelflowsplasticallywithoutandincreaseinstressuntilthestrain-hardeningstrainstisreached.超過此極限后，鋼材出現(xiàn)塑流，直至應變增止應變硬化應變st前，其應力都不再增加。Thisplasticrangeisusuallyconsiderable,andaccountsfortheductilityofthesteel.些塑性范圍通常都比較大，它是鋼材具有延性的原因。ThestressincreasesabovetheyieldstressFywhenthestrain-hardeningstrainisexceeded,andthiscontinuesuntiltheultimatetensilestressFuisreached.當超過應變硬化應變后，應力便超過屈服應為Fy，趕到應力達到極限應力Fu。Afterthis,largelocalreductioninthecross-sectionoccur,andtheloadcapacitydecreaseuntiltensilefracturetakesplace.至此，截面出現(xiàn)局部減小，承載力下降，最終出現(xiàn)拉斷破壞。 TheyieldstressFyisperhapsthemostimportantstrengthcharacteristicofastructuralsteel.屈服應力Fy也許是一結(jié)構(gòu)鋼最重要的強度特性。Thisvariessignificantlywiththechemicalconstituentsofthesteel,themostimportantofwhicharecarbonandmanganese,bothofwhichincreasetheyieldstress.它隨鋼材的化學成分的不同具有明顯變化，最為主要的化學成分包括碳和錳，它們的增加都能提高鋼材的屈服應力。Theyieldstressvarieswiththeheattreatmentusedandwiththeamountofworkingwhichoccursduringtherollingprocess.屈服應力隨所采用的熱處理和軋制過程中的加工量而變化。Thusthinnerplateswhicharemoreworkedhavehigheryieldstressesthanthickerplatesofthesameconstituency.因此，加工較多的較薄的鋼板比具有同樣化學成分的較厚鋼板的屈服應力高些。Theyieldstressisalsoincreasedbycoldworking.冷加工也提高鋼材的屈服應力。Therateofstrainingaffectstheyieldstress,andhighratesofstrainincreasetheupperorfirstyieldstress,andwellastheloweryieldstressFy.應力速率影響屈服應力，高的應變速率使上（或第一）屈服應力以及下屈服應力Fy都有提高。Thestrainratesusedinteststodeterminetheyieldstressofaparticularsteeltypearesignificantlyhigherthanthenearlystaticrateoftenencounteredinactualstructures.確定某一鋼材屈服應力的試驗中所采用的加載速率大大高于實際結(jié)構(gòu)常遇到的大致靜態(tài)加載的速率。 Fordesignpurpose,a‘minimum’yieldstressisidentifiedforeachdifferentsteelclassification.為了設計目的，為每一不同級別的鋼材指定一個所謂的‘最小’屈服應力。InAustraliaandEngland,theseclassificationsaremadeonthebasisofthechemicalcompositionandtheheattreatment,andsotheyieldstressesineachclassificationdecreaseasthegreatestthicknessoftherolledsectionorplateincreases.在澳大利亞和英國，這些分級是基于鋼材的化學成分和熱處理，從而在每一分級中，屈服應力隨軋制型鋼或鋼板的最大厚度而降低。Ontheotherhand,Americanpracticeistovarythecompositionandheattreatmentwitheachclassification,andthequotedyieldstressdonotvarywiththickness.而在美國，每一級別的化學成分和熱處理不同，引用的屈服應力并不隨厚度而變化。The‘minimum’yieldstressofaparticularsteelisdeterminedfromtheresultsofanumberofstandardtensiontests.某一鋼材所謂‘最小’屈服應力由許多標準試驗結(jié)果所確定。Thereisasignificantscatterintheseresultsbecauseofsmallvariationsinthelocalcomposition,heattreatment,amountofworking,thicknessandrateoftesting,andthisscattercloselyfollowsanormaldistributioncurve.由于局部化學成分，熱處理，加工量，厚度以及加載速率的小的差異，標準試驗的結(jié)果具有比較大的離散性。Becauseofthis,the‘minimum’yieldstressFyquotedforaparticularsteelandusedindesignisusuallyacharacteristicvaluewhichhasaparticularchance(often95%)ofbeingexceededinanystandardtensiontest.鑒于此，為某一特定鋼材所指定的、用于設計的所謂‘最小’屈服應力Fy通常是一個標準值，它具有在任一標準試驗中被超過的一特定幾率（常為95%）。Consequently,itislikelythatanisolatedtestresultwillbesignificantlyhigherthanthequotedyieldstress.因此，某一試驗結(jié)果可能會大大高于所指定的屈服應力。Thisdifferencewill,ofcourse,beaccentuatedifthetestismadeforanybutthethickestportionofthecross-section.當然，如果對除了截面最厚部分以外的任何部分進行試驗，這一差異將會更大。 TheyieldstressFydeterminedforuniaxialtensionisusuallyacceptedasbeingvalidforuniaxialcompression.通常認為根據(jù)單軸拉伸所確定的屈服應力也適用于單軸受壓。FatigueFailureunderRepeatedLoadsStructuralsteelmayfractureatlowaveragetensilestressesafterlargenumberofcyclesoffluctuatingload.在許多輪脈動荷載作用下，結(jié)構(gòu)鋼可能在平均拉應力較低時就斷裂。Thishigh-cyclefatiguefailureisinitiatedbylocaldamagecausedbytherepeatedloads,whichleadstotheformationofasmalllocalcrack.這種高周疲勞破壞始于反復加載產(chǎn)生的局部破壞，因為它導致了局部細小裂縫的出現(xiàn)。Theextentofthefatiguecrackisgraduallyincreasedbythesubsequentloadrepetitions,untilfinallytheeffectivecross-sectionissoreducedthatcatastrophicfailuremayoccur.而后的反復加載導致疲勞裂縫逐漸延伸，直到最終有效截面減小太多以致出現(xiàn)災難性破壞為止。High-cyclefatigueisonlyadesignconsiderationwhenalargenumberofloadingcyclesislikelytooccurduringthedesignlifeofthestructure.只有當在結(jié)構(gòu)設計壽命內(nèi)可能出現(xiàn)大量加載次數(shù)時才在設計中考慮高周疲勞。Thisisoftenthecaseforbridge,cranes,andstructureswhichsupportmachinery,whilewindandwaveloadingmayalsoleadtofatigueproblem.這種情況常見于橋梁、起重機以及支撐機械的結(jié)構(gòu)，而風浪荷載也可能導致疲勞問題。Factors,whichsignificantlyinfluencetheresistancetofatiguefailureincludethenumberofloadcycles,therangeofstressduringaloadcycle,andthemagnitudesoflocalstressconcentrations.嚴重影響抗疲勞破壞的因素包括加載次數(shù)、每一加載的應力幅以及局部應力集中的大小。Designagainstfatigueinvolvesaconsiderationofjointarrangementaswellasofpermissiblestress.防止疲勞的設計要考慮結(jié)點的布置以及容許應力，Jointsshouldgenerallybearrangedsoastominimizestressconcentrationsandtoproduceassmootha‘stressflow’throughthejointasispracticable.通常結(jié)點應布置得使應力集中最小，且應盡可能產(chǎn)生通過結(jié)點的平滑的應力流。Welddetailsshouldalsobedeterminedwiththisinmind,andunnecessary‘stress-raisers’shouldbeavoided.在確定焊接詳圖時也應謹記此點，應避免不必要的‘應力提高因素’。Itwillalsobeadvan`tageoustorestrict,wherepracticable,thelocationsofjointstolowstressregionssuchasatpointsofcontraflexureorneartheneutralaxis.在可能的地方，將結(jié)點位置限定在低應力區(qū)也是有益的，如反彎點或中性軸附近。BrittleFractureunderImpactLoad Structuralsteeldoesnotalwaysexhibitaductilebehavior,andundersomecircumstancessuddenandcatastrophicfracturemayoccur,eventhoughthenominaltensilestressesarelow.結(jié)構(gòu)鋼并不總是表現(xiàn)為延性，在一些情況下，突然的災難性的破壞也可能發(fā)生，即使其名義拉應力較低時。Brittlefractureisinitiatedbytheexistenceorformationofasmallcrackinaregionofhighlocalstress.脆性破壞始于高局部應力區(qū)細小裂縫的存在或形成，Onceinitiated,thecrackmaypropagateinaductile