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8BridgeDesignandConstruction桥的设计与建造,1.Newwordsanalysisaccessory=access(increased)oryDerrick=fromDerrick,thesurnameofaLondonhangmanDebris=orignearly18thcent.:fromFrenchdbris,fromobsoletedbriserbreakdown,Planning.Thefirststepleadingtotheconstructionofamodernmajorbridgeisacomprehensivestudytodeterminewhetherabridgeisneeded.规划设计现代重要的桥梁建造的第一步是广泛地研究以确定桥梁的必要性。Ifitistobeahighwaybridge,intheUnitedStatesforexample，aplanningstudyisinitiatedbyastatebridgeauthority，possiblyincooperationwithlocalgovernmentorthefederalgovernment.比如：如果是美国的高速公路桥，是由州桥管理局研究规划并确定，同时当地的政府或联邦政府一起参与。,Studiesaremadetoestimatetheamountofbridgetraffic,thereliefofjammedtrafficinnearbyhighwaynetwork,theeffectsontheregionaleconomy,andthecostofthebridge.桥梁的交通流量、对附近高速公路网交通堵塞的调剂，对当地经济的影响和桥的造价等因素进行评估研究。Themeansforfinancingtheproject,suchaspublictaxesorsaleofrevenuebondsrepaidbytollcharges,areconsidered.Bondn.结合(物),粘结(剂),联结,公债,债券,合同revenuebond收益债券这就决定了工程的投资方式，如公众收费，依靠过通行费来支付的发行债券的规模都要被考虑进来。Ifthestudiesleadtoadecisiontogoaheadwiththeproject,thelandneededforthebridgeanditsapproachesisacquiredattheselectedsite.如果研究认为其可行信，那么桥选址和占地问题将着手处理。,Atthispoint,fieldengineeringworkisstarted.Accuratelandsurveysaremade.landsurvey土地测量在确定场地时，现场测绘工作开始进行，做好精确的实地测量。Tides,floodconditions，currents,andothercharacteristicsofthewaterwayarecarefullystudied.潮汐，洪水因素，水流和排水的其它的特征都要仔细研究，Boringsamplesofsoilandrockaretakenatpossiblefoundationlocations,bothonlandandunderthewater.在陆地和水下的泥土和岩石的钻孔取样在基础可能的位置都要尽可能地进行。,Selectionofbridgedesign.桥梁设计类型的选择Thechieffactorsindecidingwhetherabridgewillbebuiltasagirder,cantilever,truss,arch,suspension,orsomeothertypeare：(1)location；forexample,crossariver;(2)purposes;forexample,abridgeforcarryingmotorvehicles;(3)spanlength;(4)strengthofavailablematerials;(5)cost;(6)beautyandharmonywiththelocation.决定把桥建成梁、悬臂、桁架、拱、悬索或其他类型结构的主要因素是：（1）地点，如跨越河流；（2）目的，如建桥为了方便交通；（3）跨度；（4）可用材料强度；（5）花费；（6）美观和和谐性。,Eachtypeofbridgeismosteffectiveandeconomicalonlywithinacertainrangeofspanlengths,asshowninthefollowingtable:.每种结构类型的桥，只有在一定跨度范围内，才能是最经济和最有效。如下表所示：,Asindicatedinthetable,thereisaconsiderableoverlapintherangeofapplicabilityofthevarioustypesInsomecases，alternativepreliminarydesignsarepreparedforseveraltypesofbridgeinordertohaveabetterbasisformakingthefinal上表表明了不同结构类型的适用性有很多交叉的地方。在一些实例中，在不同的初步设计中，用来比较不同类型的桥是为了在最后有最好的选择。,Selectionofmaterials材料的选择Thebridgedesignercanselectfromanumberofmodernhigh-strengthmaterials，includingconcrete，steel，andawidevarietyofcorrosion-resistantalloysteels桥梁设计者能选用大量的现代高强材料，包括混泥土、钢筋和多种耐腐蚀的合金钢。,FortheVerrazano-NarrowsBridge，forexample，thedesignerusedatleastsevendifferentkindsofalloysteel，oneofwhichhasayieldstrengthof5000poundspersquareinch(psi)(3，515kgsqcm)anddoesnotneedtobepaintedbecauseanoxidecoatingformsonitssurfaceandinhibitscorrosioninhibit抑制,约束,化医抑制拿Varian-Narrows大桥来说，设计者使用了七种不同的合金钢，其中之一的合金的屈服强度为50000英镑每平方英寸（3115kg/c）,而且不需要油漆保护，因为有一种氧化膜覆盖在它的表面而防止腐蚀。Thedesigneralsocanselectsteelwiresforsuspensioncablesthathavetensilestrengthsupto250，000psi(17，577kgsqcm)设计者还选用钢丝绳作为悬索，它的抗拉强度超过250000英镑每平方英寸（17577kg/c）,Concretewithcompressivestrengthsashighas8,000psi(5625kgsqCm)cannowbeproducedforuseinbridges,anditcanbegivenhighdurabilityagainstchippingandweatheringbytheadditionofspecialchemicalagentsandcontrolofthehardeningprocess.chipping碎屑；小碎片weathering侵蚀,风化现在，抗压强度高达8000英镑每平方英尺（562.5kg/c）的混泥土被生产用于桥梁工程，而且它在增加特殊化学物质后具有很高的抗脆裂性能和抗风化性能，Concretethathasbeenpresressedandreinforcedwithsteelwireshasatensilestrengthof250,000psi(17,577kg/sqcm).用钢绞线加强的预应力混凝土，其强度达到250000英镑每平方英寸（17577kg/c）,Otherusefulmaterialsforbridgesincludealuminumalloysandwood.桥梁使用的其它材料还有铝合金和木材。Modernstructuralaluminumalloyhaveyieldstrengthsexceeding40,000psi(2,812kg/sqcm).现在的铝合金的屈服强度超过了40000每平方英寸（2818kg/c）。Laminatedstripsofwoodgluedtogethercanbemadeintobeamswithstrengthstwicethatofnaturaltimber;glue-laminatedsouthernpine,forexample,canbearworkingstressesapproaching30,000psi(210.9kg/sqcm).laminated由薄片叠成的把片状的木条粘在一起做成的梁是自然木材强度的二倍。例如用南部松树而胶结的梁能承受的工作应力达到了3000英镑每英寸（210.9kg/c）。,Analysisofforces力的分析Abridgemustresistacomplexcombinationoftension,compression,bending,shear,andtorsionforces.一座桥要抵抗一系列的合力，如拉力，压力，剪力和扭力。Inaddition,thestructuremustprovideasafetyfactorasinsuranceagainstfailure.另外，结构还需要一定的安全系数作为防止破坏的安全储备。Thecalculationoftheprecisenatureoftheindividualstressesandstrainsinthestructure,calledanalysis,isperhapsthemosttechnicallycomplexaspectofbridgebuilding.对结构的各种应力和应变的准确特性进行计算，这就叫应力分析，这或许是桥梁结构中最复杂的技术。Thegoalofanalysisistodeterminealloftheforcesthatmayactoneachstructuralmember.应力分析的目的是为了确定作用在结构上的各种力。,Theforcesthatactonbridgestructuralmembersareproducedbytwokindsofloads-staticanddynamic.作用在桥梁结构的力都可以分为二类荷载：动荷载和静荷载。Thestaticload-thedeadweightofthebridgestructureitself-isusuallythegreatestload.Thedynamic,orlive,loadhascomponents,includingvehiclescarriedbythebridge,windforces,andaccumulationsoficeandsnow.静荷载即桥的自重它往往也是最大的荷载。动荷载或静荷载有很多，包括桥面上的机动车，风荷载，和积冰积雪荷载。,Althoughofthetotalweightofthevehiclesmovingoverabridgeatanytimeisgenerallyasmallfractionofthestaticanddynamicload,itpresentsspecialproblemstothebridgedesignerbecauseofthevibrationandimpactstressescreatedbymovingvehicles.虽然随时在桥面上移动的车辆的总重量相对于静荷载和动荷载来说是一个很少的部分，而对设计者来说，因为机动车辆产生的振动和冲击压力而会出现特殊问题。Forexample,thesevereimpactscausedbyirregularitiesofvehiclemotionorbumpsintheroadwaymaymomentarilydoubletheeffectoftheliveloadonthebridge.例如：在路面上机动车的不规则的运动或碰撞对桥面产生瞬时双倍活荷载的作用力。,Windexertsforceonabridgebothdirectlybystrikingthebridgestructureandindirectlybystrikingvehiclesthatarecrossingthebridge.风在桥上的作用的力包括：直接作用桥结构的力和间接的通过作用在通行车辆的力。Ifthewindinducesaeroelasticvibration,asinthecaseoftheTacomaNarrowsBridge,itseffectmaybegreatlyamplified.如果出现空气弹性振动，在这种情况下的TacomaNarrows大桥的风作用被大大地增大，Becauseofthisdanger,thebridgedesignermakesprovisionsforthestrongestwindsthatmayoccuratthebridgelocation.provisionn.供应,提供,供给，准备,防备；食物和饮料规定,条款,条件由于这种危险的存在，桥的设计者必须知道桥址处可能发生的最大的风。Otherforcesthatmayactonthebridge,suchasstressescreatedbyearthquaketremors,mustalsobeprovidedfor.还有其它的力作用在桥上，如：地震产生的压力也必须注意。,Specialattentionmustoftenbegiventothedesignofthebridgepiers,sinceheavyloadsmaybeimposedonthembycurrents,waves,andfloatingiceanddebris.Occasionallyapiermayevenbehitbyapassinghip.对桥墩的设计要给予特殊的关注，因为桥墩承担水流，浮冰和漂浮物而产生的重荷载，桥墩通常还有被船撞击的可能。,Electroniccomputersareplayinganeverincreasingroleinassistingbridgedesignersintheanalysisofforces.电脑在力分析上协助桥梁设计者，起着很重要的作用。Theuseofprecisemodeltesting,particularlyforstudyingthedynamicbehaviorofbridges,alsohelpsdesigners.用一个精确的模型试验，尤其对桥的动力的活动状态的研究也可以帮助设计者。Ascaled-downmodelofthebridgeisconstructed,andvariousgaugestomeasurestrains,accelerations,anddeformationsareplacedonthemodel.一个缩尺比例的桥模结构中，布置很多位移计对桥模各处的应力，加速度和变形进行测量。,Themodelbridgeisthensubjectedtovariousscaled-downloadordynamicconditiontofindoutwhatwillhappen.桥模在同样比例的荷载和动力条件作用下，来分析桥的受力行为。WindtunneltestsmayalsobemadetoensurethatnothingliketheTacomaNarrowsBridgefailurecanoccur.Withmoderntechnologicalaids,thereismuchlesschanceofbridgefailurethaninthepast.风洞试验也可以确保不再发生TacomaNarrows大桥的失败。在现代技术的帮助下，桥梁事故出现的机会将大大少于以前。,Constructionthefoundations.基础施工Constructionstartswiththefoundation，whichmaycostalmostasmuchasthesuperstructure.施工都是从基础开始的，基础的花费几乎大大超过上层建筑。Foundationsbuiltinwaterusuallypresentthegreatestdifficulties.Oneoftheoldermethods,whichisstillusedinshadowwaters,istoerectcofferdamssimilartotheringofcloselyspacedpilesthattheRomansused.水下基础通常会遇到很大的困难，有个古老的方法常被用于浅水中，即在小范围内布置垂直围堰而建桥墩。罗马人常用这种方法。,Forconstructingfoundationsindeepwater,caissonshavelongbeenused.在深水中建基础一般用沉箱法。thecaisson,whichisahugeboxclosedonallsidesexceptthebottom,isloweredontotheriverbed.沉箱是一个底部开口其余封闭的大盒子而沉入河床上，Workersinsidethecaisson,whichisfilledwithcompressedairtokeepoutthewater,digdeeperanddeeper,andthecaissonsinksasthediggingproceeds,Whenasuitabledepthisreached，thecaissonisfilledwithconcreteandbecomespartofthefoundationitself.工人们在为挡水而充满压缩空气的沉箱里，越挖越深，沉箱也跟着下沉。当达到合适的深度后在箱内填入混泥土而成为基础的一部分。,Anotherdeep-watermethod,lesshazardousandlesscostlythanthecaissonmethod,usessteelorconcretepiles.在深水中建基础的另一种方法比沉箱法更安全和更低的成本，用于钢或混泥土桥墩。Withmodernpiledrivers,longheavypilescanbedrivenevenIndeepwater.用现代的打桩机，长重桩可以打入深水中。Thepilescanbecutoffandcappedeitherabovethewaterlevelorbelowit.桩可以在水面或水下截断或做成桩帽。Iftheyarecappedbelowthewaterlevel,aprefabricatedhollowpiercaseisfloatedouttothesite,sunkonthepiles,andthenfilledwithconcretetoformthepier.如在水下把它们做成桩帽，可把一个预制空心桥墩浮运到场地内并沉入到桩上，然后灌入混泥土形成桥墩。,Erectingthesuperstructure.建造上部结构Afterallpiersandabutmentsareinplace,theerectionofthesuperstructurebegins当所有的桩和支柱建好后，则上部结构开始建造。Themethodofconstructionuseddependslargelyonthetypeofbridgebeingbuilt.Therearesixconstructionmethods:falsework,flotation,cantilevering，sliding,directlifting,andSuspension.结构的施工方法有很多种类，共有六类施工方法：脚手架、浮运、悬臂、滑移、直升和悬挂法。,Infalsworkconstruction,mainlyusedinbuildingconcretearchbridges,metalorwoodsupportsarebuilttemporarilytosupporttheerection.在用脚手架施工时，主要用来建混泥土拱桥，金属或木支撑都是临时搭设为支撑上部结构。Agreatdealofingenuityisoftenrequiredjusttoerectthefalsework,especiallyforstructuresoverswiftriversordeepcanyons.Temporarypilesandtrestlesarecommonlyusedinwideshallowrivers.Trestle脚手架,高架桥,三角凳脚手架都是根据需要而灵活搭建的，尤其结构在激流河流或深谷上时，临时桥墩和架子一般在宽而浅的河上使用。,Inthefloatationmethod,mainlyusedinbuildinglongbridges,largebridgesectionareprefabricatedonshoreandfloatedoutonbargestothebridgesite.浮运法主要用来施工很长的桥梁，主桥部分是在河岸预制的，然后用驳船浮移到桥梁位置。Thesectionsarethenhoistedintoplace,eitherbyfloatingderricksorbywinchesplacedonpreviouslyconstructedsectionsofthebridge.浮吊起重机或卷扬机安装在已建成桥的部位，然后把预知的主桥部分精确吊到大桥的施工部位。,Thecantileveringtechniqueisusednotonlyforcantileverbridgesbutalsoforsteelarchbridges.悬臂技术不仅用于悬臂桥中，也用于刚拱桥上，Constructionstartsatanabutmentandextendstowardthecenterpiecebypiece.Movingderricksandcranesonthecompletedportionofthestructurehandletheheavymaterial.施工时先建成一个桥台，然后一步步延伸到中央，在已建好的结构部分的起重机和吊车，来回移动完成施工材料的运输。,Slidingconstructionisusedonlyrarely.Inthismethod,aprefabricatedunit,suchasatruss,iserectedonshoreandslidoutoveratemporaryorpermanentsupportuntilitcomestorestonanotherSupport.滑移法在施工中很少用到。这种方法，如一个预制构件，如桁架结构，竖立在河岸上，然后在临时或永久性的支撑滑动，直到它进入安装的另一个支撑上。Inthedirectliftingmethod,mainlyusedforlight，short-spanhighwaybridge,aprefabricatedbridgeunitisliftedbyahoistandswungdirectlyontothebridgesupports.直升法主要用于轻质小跨度的公路桥。每一个预制桥单元被垂直悬起并旋转到桥梁支撑点上。,Intheconstructionofsuspensionbridges,thecablesarestrungbetweenthebridgetowersandusedasasupportforthebridgedeck.在由悬索桥的施工中，一串缆绳连接到两侧的桥塔上，被用作桥面支撑点。Thedeckerectionstartsattheendsofthebridgeandprogressestowardthecenter.Atravelingderrick,movingonthecompletedpartofthedeck,isusedtohandleheavymaterial.桥面施工从桥的两端开始，向中间进行，在已完成的桥面上移动，用来运送重材。Temporarysuspensioncablesareoccasionallyusedintheconstructionofothertypeofbridgestoconveymaterialacrossthespan.悬挂钢缆，有时在其他类型的桥梁中被用来运输材料。,Inallmethodsofconstruction,itisnecessarytodeterminethestressesanddeformationateverystageofconstruction.Stressesinapartlycompletedbridge-constructedbythecantilevermethod-canexceedthestressesinacompletedbridgebecauseofthetotallydifferentconditionsofsupportandloading.所有的施工方法中，都需要验算施工阶段的应力和变形，在用悬臂梁法施工的桥梁中，因为完全不同的支撑和荷载条件，未竣工桥梁内的应力可能会超过已竣工桥梁内的应力。Whentheroadwayissurfaced,andsigns,lighting,guardrails,andotheraccessoriesareinstalled,thebridgeisreadyforuse.当公路的铺装，标志，灯光，护栏和附属设施完成后，桥梁就准备投入使用了。,
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