拱桥的设计和桥梁裂缝产生的原因分析土木工程毕业设计外文翻译.docx
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拱桥的设计和桥梁裂缝产生的原因分析土木工程毕业设计外文翻译.docx
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拱桥的设计和桥梁裂缝产生的原因分析土木工程毕业设计外文翻译
Designofarchbridgesandthebridgecrack
producedthereasontosimplyanalyse
Thischapterconsidersthefullrangeofarchbridgetypesandarangeofmaterialspresentingseveralcasestudiesanddescribingthedesigndecisionsthatweremade.Ageneraltreatmentoftheanalysisofarchesispresented,includingthederivationofthebasicequationsthatcanbeusedtoundertakehandcalculationswhichmaybeusedtovalidatecomputeranalysisoutput.Detailedarchbridgedesignisoutsidethescopeofthischaptersoonlygeneralissuesarediscussed.Mostofthechapterisdevotedtomasonryarchbridges.Masonryarchbridgeconstructionisdiscussedinitshistoricalcontextandtheimportanceforengineerstotakeaholisticapproachtobridgeassessmentanddesignisemphasized.Thereisasignificantsectiononbridgeassessmentwhichincludesguidanceintheapplicationofcurrentandemergingassessmentmethods.Thisisunderpinnedwithbackgroundinformationregardingthematerialpropertiesofmasonry.Thechapterconcludeswithatreatmentofrepairandmaintenancestrategiesincludingacomprehensivetablewhichconsiderscommonremedialandstrengtheningmeasures.
Theoriginsoftheuseofarchesasastructuralforminbuildingscanbetracedbacktoantiquity(VanBeek,1987).IntryingtoarriveatasuitabledefinitionforanarchwemaylooknofurtherthanHooke’sanagramof1675whichstated‘Utpendetcontinuumflexile,sicstabatcontinuumrigiduminversum’–‘ashangstheflexibleline,sobutinvertedwillstandtherigidarch’.Thissuggeststhatanygivenloadingtoaflexiblecableiffrozenandinvertedwillprovideapurelycompressivestructureinequilibriumwiththeappliedload.Clearly,anyslightvariationintheloadingwillresultinamomentbeinginducedinthearch.Itisarrivingatappropriateproportionsofarchthicknesstoaccommodatetherangeofeccentricitiesofthethrustlinethatisthechallengetothebridgeengineer.EvenintheMiddleAgesitwasappreciatedthatmasonryarchesbehavedessentiallyasgravitystructures,forwhichgeometryandproportiondictatedaestheticappealandstability.Compressivestrengthcouldberelieduponwhilsttensilestrengthcouldnot.Baseduponexperience,manyempiricalrelationshipsbetweenthespanandarchthicknessweredevelopedandappliedsuccessfullytoproducemanyelegantstructuresthroughoutEurope.
Theexpansionoftherailwayandcanalsystemsledtoanexplosionofbridgebuilding.Brickworkarchesbecameincreasinglypopular.WiththeconstructionoftheCoalbrookdaleBridge(1780)aneweraofarchbridgeconstructionbegan.Bytheendofthenineteenthcenturycastiron,wroughtironandfinallysteelbecameincreasinglypopular;onlytobechallengedbyferrocement(reinforcedconcrete)attheturnofthecentury.
Duringthenineteenthcenturyanalyticaltechniquedevelopedapace.Inparticular,Castigliano(1879)developedstrainenergytheoremswhichcouldbeappliedtoarchesprovidedtheyremainedelastic.Thisconditioncouldbesatisfiedprovidedthelineofthrustlaywithinthemiddlethird,thusensuringthatnotensilestresseswereinduced.Therequirementtoavoidtensilestressesonlyappliedtomasonryandcastiron;itdidnotapplytosteelorreinforcedconcrete(ortimberforthatmatter)asthesematerialswerecapableofresistingtensilestresses.
Twentiethcenturyarchbridgeshavebecomeincreasinglysophisticatedstructurescombiningmodernmaterialstocreateexcitingfunctionalurbansculptures.
Typesofarchbridge
TherelevanttermsthatareusedtodescribethevariouspartsofanarchbridgeareshowninFigure1.Archesmaybegroupedaccordingtothefollowingparameters:
1.thematerialsofconstruction
2.thestructuralarticulation
3.theshapeofthearch
Historically,archbridgesareassociatedwithstonemasonry.Thisgavewaytobrickworkinthenineteenthcentury.Becausethesewereproportionedtominimisethepossibilityoftensilestress,theytendtobefairlymassivestructures.Bycomparisontheuseofreinforcedconcreteandmodernstructuralsteelgivestheopportunityforslender,elegantarches.
Nowadays,timberisrestrictedtosmallbridgesoccasionallyinatrussformbutmoreusuallyaslaminatedcurvedarches.Althoughtimberhasahighstrengthtodensityratioparalleltothegrain,itisanisotropicandstrengthpropertiesperpendiculartothegrainarerelativelyweak.Thisrequirescarefuldetailingofconnectionstoensureeconomicuseofthematerial.
Withregardtostructuralarticulationthearchcanbefixedorhinged.Inthelattercaseeitherone,twoorthreehingescanbeincorporatedintothearchrib.Whilstthefixedarchhasthreeredundancies,theintroductionofeachhingereducedtheindeterminacybyoneuntil,withthreehinges,thearchisstaticallydeterminateandhence,theoretically,freeoftheproblemsofsecondarystresses.Figure2showsarangeofpossiblearrangements.Thearticulationofthearchisnotonlydependentuponthenumberofhingesbutisalsofundamentallyinfluencedbythepositionofthedeckandthenatureoftheloadtransferfromthedecktothearch.
Thetraditionalfilledspandrel,wherethevehicularloadingistransferredthroughthebackfillmaterialontotheextradosofthearch,representsatfirstglancethesimpleststructuralcondition.Aswillbeseenlaterthisisnotthecaseandhasledtomuchresearchforthespecificcaseofthemasonryarchbridgeinanattempttoimproveourunderstandingofsuchstructures.
Thespandrelmaybeopenwithcolumnsand/orhingesusedtotransferthedeckloadstothearch.Inanattempttominimisethehorizontalthrustontheabutments,thedeckmaybeusedto‘tie’thearch.Tiedarchesareparticularlyappropriatewhendeckconstructiondepthsarelimitedandlargeclearspansareneeded(particularlyifgroundconditionsarealsodifficultandwouldrequireextensivepilingtoresistthehorizontalthrusts).
ReturningtoHooke’sanagram,theperfectshapeforanarchwouldbeaninvertedcatenary–thiswouldonlybethecaseforcarryingitsownself-weight.Vehicleloadingandvaryingsuperincumbentdeadloadbothinducebendingmoments.Consequentlythearchhastohavesufficientthicknesstoaccommodatethe‘wandering’thrustline.
Foreaseofsettingoutandconstructionsimplershapesareadoptednowadayssegmentalorparabolicshapesareused.Althoughinsituationswheremaximumwidthsofheadroomhavetobeprovided(sayoverarailway,roadorcanal)anellipticalshapemayberequiredoritsnearest‘easy’equivalentthethree-centredarch.
Itisworthcommentingatthisstageregardingtheidealizationofarchstructures.Traditionallyarchesareperceivedasbeingtwo-dimensionalstructures;this,ofcourseisnottrue–buttheextenttowhichitisnottrueshouldbeofconcerntothedesigner/assessor.Eveninthecaseofathree-hingedarchwhichostensiblyisstaticallydeterminatethe‘pins’arecapableoftransmittingsheareventhoughtheytheoreticallycannottransmitmoments.Inthecaseofnon-uniformtransversedistributionofloadingthehingeswilltransmitavaryingshearwhichwillproducetorsioninthearch.Moreover,inthecaseofskewarchesornon-verticalribsthestructurehasamuchhigherredundancyandhencewillrequiregreaterattentiontodetailinregardtothereleaseswhichareengineeredintothestructure.
Fromanaestheticpointofview,archeshaveauniversalappeal.Inspiteofthis,caremustbetakenastheimpactofevenmodestsizedbridgesissignificant.Filledarchesareinvariablymasonry(orwideningofmasonry)bridges.Cleannessofline,honestyofconceptionandtheattentiontodetailarevitalingredientstoasuccessfulbridge.Certainly,simplestringcoursesandcopingsarepreferabletoelaboratedetailswhichwouldbeexpensiveandinappropriateformostmodernbridges.Wherestoneisuseditisimportanttobesensitivetothenatureofthematerial.Modernquarryingtechniquesshouldbeemployed(lasercutting,diamondsawing,flametexturingandsandblasting)reservingtraditionaldressingtoconservationschemes.Ifbrickworkisuseddifferenttexturedorcolouredbricksandmortarcanbespecified.Herestringcoursescanbeparticularlyusefultomaskchangesinbeddingangle.
Historicallyabutmentscomprisedeitherrock,orelseweremassivemasonrysupportsrelyingontheirweighttoresistthethrustofthearch.Intermsofstructuralhonestythisisnecessaryasitisinstinctivetoexpectsuchsupport.
Reinforcedconcreteandsteelarchesarealtogethermuchlighterstructures.‘Thestructureconsistsbasicallyofthearch,thedeckandusuallysomesupportsfromthearchtothedeck–inthatorderofimportance.Theseelementsshouldbeexpressedinbothformanddetail,andwithdueregardfortheirhierarchy’(HighwaysAgency,1996).
Itisimportantthatthedeck,ifitrestsonthecrownofthearch,shouldnotmaskitinanyway.Anysupportwhetherspandrelcolumnsorhinges(inthecaseofthetiedarch)shouldnotbeallowedtodominate.Preferablytheyshouldberecessedrelativetotheparapetandstringcourse.
Concretearchescanbeeitherafullwidthcurvedslaboraseriesofribs.Steelisalmostalwaysaseriesofribs.Whereribsareusedthoughtshouldbegiven(iftheyaregoingtobeseenfromunderneath)tothechiaroscuroofthesoffit.
Theratioofspantoriseshouldgenerallybeintherange2:
1to10:
1.Theflatterthearchthegreaterthehorizontalthrust;thismayaffectthestructuralformselected,i.e.whetherornotatieshouldbeintroduced,orthestiffnessofthedeckrelativetothearch.
Inrecentyears,thetrafficcapitalconstructionofourcountryg
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