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Lesson7Bridge,课程学习指南：1.wordslearningPedestrian(行人)ped(pod，词根)-足pedal(足的，脚的、脚踏)Subsoil(底土)=sub（前缀：下、次）+soilOverlap(重叠，搭接)=over(前缀：=super)+lap(接)Intersect(交叉)=inter(前缀：相互之间的)+sect(段、部分=part),Bridgeisastructurethatspansobstacles,suchasriversandvalleys,toprovidearoadwayfortraffic.Byfarthemajorityofbridgesaredesignedtocarryautomobileorrailroadtraffic,butsomeareintendedforpedestriansonly.pedestriannounapersonwalkingalongaroadorinabuilt-uparea步行者，行人adjectivelackinginspirationorexcitement;dull缺乏想像力的，平淡无奇的，乏味的,为提供交通道路，桥梁是跨越如河流、山谷这样障碍的一种结构形式，到目前为止，大部分桥梁都是公路桥或铁路桥，仅有少部分是人行桥。,Anumberofaqueductbridges,mostlyerectedinEuropeinthe19thcentury,carrycanalsandtheirbargetraffic;andatleastonebridge,atNewYorkCitysKennedyAirport,servestocarrytaxyingaircraftoverahighway.aqueductbridge高架水渠verb(taxis,taxied,taxiingortaxying)(intransitive)(ofanaircraft)moveslowlyalongthegroundbeforetakeofforafterlanding:theplanetaxisuptoawaitingcar.takeataxiasameansoftransport:Iwouldtaxihomeandsleeptilleight.大量的高架渠桥于19世纪在欧洲建成，以保证运河和船舶交通的运行。最小的一座桥在纽约市的肯尼迪机场，它主要是把滑行飞机拖到跑道上服务的。,Thefirstbridgesbuiltbymanprobablyresembledthosestillbeingconstructedbyprimitivepeoplesinisolatedregions.primitiveadj.原始的,早期的简单的;粗糙的原始人,原始事物人类建成的首批桥类似于原始人在孤立地带建成的桥。Thetoolsandbuildingskillsofearlyman,likethoseofprimitivepeoplestoday,weresoelementarythathewasundoubtedlyforcedtouseeasilytransportablematerialsthatcouldbeputinplacewithaminimumofformingandshaping.早期人类的工具和建筑技术如同原始人类一样都是最初级的，只有应用经过简单加工成型的材料。,Inforestregions,wherestouttimbersorlogscouldbeobtained,bridgesverylikelyweremadeofonemoreparallellogs,possiblycoveredwithcrossbranchesormattingforbetterfooting.stout结实的；厚实的在森林里，随处可得结实的木材，那时侯的桥极可能是由并排的几根圆木建成，可能在其上覆一些木枝或草垫以方便行走。,IntropicalregionsofIndia,Africa,andSouthAmerica,fibrousvineswereusedtobuildsuspensionbridges.Thevinesweretiedtotreesorrocksoneachsideofthestreamorvalleytobecrossed.Oneormorevineswereusedtotreadon.Othervines,strungseveralfeethigher,wereusedforhandholds.streamvalley河谷处于热带地区的印度、非洲、和南美洲，纤维藤用来建成悬索桥，这些藤系在小河或山谷两边的树上或岩石上，一根或更多的藤用于在上面行走，其它的在几英尺高的地方，用作扶手用。Althoughvineridgesareusuallyunstable,somebuiltbytheIncaswerestrongandstableenoughtobeusedbytheinvadingSpanishsoldiersandhorses.invade侵入;侵略(别国);侵犯(主权等)拥入;挤满Thetownwasinvadedbyacrowdoftourists.虽然藤索桥通常不稳定，但印加人建成的藤索桥足够的坚固和稳定性，被用于西班牙士兵和他们马匹的通行。,Inrockyregions,stonewasusedforbridges.Pilesofstonewereplacedatshortintervalsacrosstheriver,providingthebridgepiers,andthenapathfrontbanktobankwasmadebylayingflatstoneslabsacrossadjacentpiers.Afewstonebridgesofthistype,calledclapperbridges,canstillbeseeninDartmoor,England;however,theydateonlyfromtheMiddleAgesorevenlater.在岩石地区，石头被用来建桥，横跨河流以很小的间距布置石碓作为桥墩，然后用平坦的石头跨过相邻的桥墩就建成连接两岸的通道，大部分的石桥就是这种类型，叫做鼓掌桥。现在在Dartmoor、英格兰仍然可见，不过它们都建于中世纪甚至更晚。,ThefirstinnovationbeyondtheprimitivebridgeformsisbelievedtohaveoccurredinancientChinaandthenspreadtoIndia.Tobridgestreamwiderthanasingletreelength,theChineseandIndiansusedtwopilesofthreetrunks,buildingtowardthestreamcenterfromeachbank.pile桩，一堆，一叠原始桥梁的第一步变革被认为出现在中国古代，随后传入印度。河床一般比树要宽，中国人和印度人从河的两岸到河中央用两堆树干来建造。,Ineacharmofthestructure,thelogswerepiledontopofoneanotherwithaslightupwardslant,witheachlayerprojectedseveralfeetbeyondtheoneimmediatelybelowit.Forstability,eachpileoftimberswasanchoredbyamassivepileofstoneoneachbank.Nearmidstream,thegapbetweentheendsofthearmswasclosedbytheadditionofasimplebeambetweenthetwoends.每个分支中，圆木一个接一个的布置并微微向上倾斜，使其每一层都比它下面的高几英尺。为了增加稳定性，每个木桩在两岸都用一堆大而重的石头锚固；接近河中央，在河中间的两个分支的两端则用简支梁连接。,Inthiskindofstructure,acrudecantileverbridge,theachievablespanlengthismadegreaterbytheadditionofacentralsectionbetweenthefreeendsofthearms.crude天然的,未加工的，简陋的,粗糙的,未加修饰，粗鲁的,粗俗的,粗野的在这种结构中，天然支架桥在两个分支的两端加上额外构件后可达到很宽的跨度。,Asearlyas4000B.C.inMesopotamiaand3000B.C.inEgypt,overlappinghorizontallayersofstoneorsun-bakedbrickwereusedinsteadofoverlappingtimbers.早在公元前4000年的esopotamia和在公元前3000年的埃及，用石头或日光烤干的砖代替木头被用于横向覆盖层。thisconstruction,whichlookslikeamasonryarchthatissteppedratherthansmoothontheundersideiscalledacorbeledarch这种结构看起来像的砖拱，一层层铺起，被叫做突拱。,T0changethecorbeledarchintoatruearch,itwasonlynecessarytoreorienttheinnerstonessothattheyformedasmoothcurve.要使突拱变为真正的拱，需要调整内部石头的构造以形成光滑的曲面。Thetruearch，whichwasusedasearlyas3500B.C.,ismuchstrongerthanthecorbeledarch.这种拱比突拱更坚固，且早在公元前500年就被使用。,Thetruemasonryarchwasefficient,economical,anddurable.Itcouldspansmallriversbymultiplearchesrestingonpiers;moreover,itwasgenerallygraciousinappearance.这种石拱经济和经久耐用，它可以由多个跨过桥墩的拱而跨过小的河流，而且外观非常漂亮。Thesequalitieswerehardtomatchbyanypreviousformofconstruction.ThetruemasonryarchwaswidelyusedinbridgeconstructionbyboththeancientChineseandtheRomans;itremainedinwideuseuntilthe19thcentury.而它的质量比先前的任何结构都要好。在中国和罗马的古代，这种整体石拱被广泛地用于桥梁结构。它一直被广泛地使用直到19世纪。,Basicallytherearejustfourtypesofstructuresthatcanbeusedtobridgeastreamorotherobstacle:rigidbeams,cantilevers,arches,andsuspensionsystems.通常用作河流或障碍物上的桥，有4类基本结构：刚性梁、悬臂、拱、和悬索体系。,RigidBeam.Thesimplestway一andprobablythefirstused一istolayarigidbeamacrossthestreamsothatitstwoendsrestonoppositebanks.刚性梁.最简单也可能是最早使用的刚性梁布置在河流上，两端固定在河的两岸。Therigidbeaminthistypeofbridgemaybeashapedwoodenbeam,agirderofsteelorconcrete,orevenacomplextruss.桥的刚性梁可以是某种形状的木梁、钢筋混凝土梁或复杂的绗架。ThespanofarigidbeamtypeofbridgecanbeincreasedbybuildingIntermediatepiersandbridgingthegapsbetweenthepierswithseveralbeams.采用在中间建桥墩或用几根横梁连接间隙的方法，可以增加桥刚性梁的跨度。,Thematerialsusedforrigidbeamsmustbeabletowithstandbothcompressionandtension.刚性梁的材料必须能够承担压力和拉力，Thedoublerequirementstemsfromthefactthatwhenaloadisplacedonarigidbeam,thebeambends-inspiteofitsname.这种双重要求源于梁在荷载作用会产生弯曲。Asaresultofbendingtheupperpartofthebeamiscompressedandthelowerhalfisstretched.弯曲的结果，使梁上部受压，下部受拉，Ifitscompressivestrengthistoolow,itwillbuckle;ifitstensilestrengthistoolow,itwillbreak.如果它的受压承载力太弱，它将会屈曲，如果受拉承载力太弱，他将会拉断。,Cantilever.Oftenitisnotfeasibletoconstructlongspanbridgesbymeansofintermediatepiers.Indeepswift-runningriversorinsoftsubsoil,forexample,itmaybedifficulttoconstructthepiersortomakethemdeepenoughtoreachafirmfoundationrock.悬臂。利用中间桥墩方式，建造大跨度桥通常是不可行的。举个例子，在深而流速急的河流或软泥中，很难建造墩使它有足够深度达到基岩层。Insuchcasesthespanofarigid-beamstructurecanbeextendedsimplybyusingtwobeams-oneextendingfromeachbankwithoneendofeachbeamfirmlyanchoredtoitsfoundation,ratherthansimplyrestingonitasinordinaryrigid-beamconstruction.在这种情况下，刚性梁结构用两根梁就可以延伸从每岸伸出一根梁而在两根梁的端部基础进行锚固，这一点与普通刚性梁不同。,Eachoftheanchoredbeamsinthiskindofstructureiscalledacantilever.Perhapsthesimplestfamiliarexampleofasinglecantileveristhefamiliardivingboard.每一根锚固的梁称之为悬臂，或许这种最简单而熟悉的悬臂例子便是跳水板。Inanordinarycantileverbridge,thegapbetweentheendsofthecantileversisclosed,providingacontinuousdeckfortheroadway,butifthebridgewerecutintwoatthepointofclosureeachcantileverwouldsupportitself.在普通的悬臂式桥梁中，悬臂梁端部之间的间隙是闭合的，为道路提供了连续的桥面。但是假如把这种桥梁在其闭合点断开，那么每一根悬臂梁都不需要另外设置支撑而可保持稳定。Usuallythegapbetweenthecantileversisclosedbymeansofarigidbeam,thusextendingthespanofthecantilevers.通常悬臂梁中间间隙使用刚性梁来闭合，这样，可延伸悬臂梁的跨度。,Suspension.Suspensionbridgescanspanevengreaterdistanceswithoutintermediatepiersthancancantileverbridges.悬索。在没有中间桥墩的情况下，悬索桥比悬臂桥跨越更大的距离。Thesupportingmembersofasuspensionbridgearecontinuousflexiblecables,witheachcableanchoredatbothofitsends.悬索桥的支撑体系是两端锚固的连续可弯曲的缆绳。,Thesimplestexampleofthesuspensionbridgeisthecircusaerialiststightrope,andprimitivesuspensionbridgeswereoftennomorethanseveralsuchtightropestiedtogethertoprovidebothhandandfootholds.悬索桥最简单的例子是杂技场高空走钢丝杂技演员用的钢丝。原始的悬索桥常常是一把很小的几根这样的钢丝系在一起来提供扶手和立足点的。Inmodernsuspensionbridgesalevelroadwayisprovidedbystringingthecablesoverheadonhightowersandsuspendingtheseparateroadwaybelow.在现代悬索桥内，水平道路是由悬吊在高的塔架上缆绳提供，缆绳在车行道的下面。,Arch.Thearchisinasensetheoppositeofasuspensioncable.Wherethesuspensioncablehangsfreelyfromitssupportingtowers，thearchcurvesrigidlyupwardfromitsabutments.拱则是相反于悬索的作用，在那些悬索的缆绳自由地悬挂在塔架上，拱却是从它的两端拱座处紧紧地向上弯曲。Becauseofthisdifferenceinshape,thesuspensioncabletendstopullitsanchoragestogether,whilethearchtendstopushitsabutmentsapart.由于在形状上的不同，悬索使锚固趋向拉在一起，而拱的拱座则趋向于分开。,Forthisreasonsuspensioncablesmustbeabletowithstandstretching,whilearchesmustbemadeofmaterialsthatcanwithstandcompression.由于这些原因，悬索桥的缆绳必须承受一定的延伸作用，而拱材料则尽可能地抵抗压缩作用。Becausetensilestrengthisnotnecessarilyrequiredforarchconstruction,archbridgescanbemadeofbricksorstoneblocksthatareheldtogetherbythecompressiveforcecharacteristicofthearch因为拱结构不一定要求材料具有抗拉强度，所以拱桥可以用砖或石头建造，砖或石头通过拱传递压力的特性结合在一起。Suchmaterialsareuselessfortheotherbasicbridgestructures.这种材料在其它的基本桥梁结构中却毫无用处。,Inanarchbridge,theloadsaretransmittedverticallydownfromtheroadwayuntiltheyintersectthearchform.在拱桥中，荷载由公路上垂直传递下来，直到汇集在拱处。Atthearchboundarytheloadsareredirectedalongthepathofthearch,whichisinastateofsimplecompression.在拱桥中，荷载由公路上垂直传递下来，直到汇集在拱处。在拱的边缘处，荷载便会改变力的传递路径，变为一种简单的受压状态。Thecompressivethrustforcesarethentransmittedtothegroundthroughtheabutmentsorpiers.Thearch,withitssimpleandelegantstructures,hasbecomeaclassicbridgeconfiguration.有压缩力的推力通过节点或墩传到地面。拱这种简单而优美的结构成为桥梁中的一种基本结构。,
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