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Arch bridges are one of the oldest types of bridges and have great natural strength. Instead of pushing straight down, the weight of an arch bridge is ... ArchBridge ArchBridge BixbyCreekBridge,Monterey,CA Archbridgesareoneoftheoldesttypesofbridgesandhavegreatnatural strength.Insteadofpushingstraightdown,theweightofanarchbridgeis carriedoutwardalongthecurveofthearchtothesupportsateachend.These supports,calledtheabutments,carrytheloadandkeeptheendsofthebridge fromspreadingout. TryIt! Howdotheabutmentssupportanarchbridge? Cutastripofcardboardthat'saboutoneinchby11inches.Gentlybendthe stripsothatithasacurve.Positionthecardboardonatablesothatit resemblesanarch.Pressdownonthecenterofthearch.Whathappenstothe endsofthecardboard? Next,placeastackofbooksateachendofthearch.Pressagain.Nowwhat happens?Noticehowthestacksofbooksactasabutments,keepingtheendsof thearchfromspreadingapart. Whensupportingitsownweightandtheweightofcrossingtraffic,everypart ofthearchisundercompression.Forthisreason,archbridgesmustbemadeof materialsthatarestrongundercompression. ThePontduGardaqueduct TheRomansusedstones.Oneofthemostfamousexamplesoftheirhandiworkis thePontduGardaqueductnearNîmes,France.Builtbeforethebirthof Christ,thebridgeisheldtogetherbymortaronlyinitstoptier;thestones intherestofthestructurestaytogetherbythesheerforceoftheirown weight. TheNewRiverbridge Todaymaterialslikesteelandpre-stressedconcretehavemadeitpossibleto buildlongerandmoreelegantarches,includingaspectacular1700footspanin NewRiverGorge,WestVirginia.(Moretypically,modernarchbridgesspan between200-800feet.) Centeringunderconstruction Constructinganarchbridgecanbetricky,sincethestructureiscompletely unstableuntilthetwospansmeetinthemiddle.Onetechniqueistobuild elaboratescaffolding,or"centering,"belowthespanstosupportthemuntil theymeet.Anewermethodsupportsthespansusingcablesanchoredtothe Archconstructionusingcablesupports groundoneithersideofthebridge.Insituationswherethereisanactive waterorroadwaybelow,thismethodallowscontractorstobuildwithout disruptingtraffic. TheNatchezTracebridge OneofthemostrevolutionaryarchbridgesinrecentyearsistheNatchezTrace BridgeinFranklin,Tennessee,whichwasopenedtotrafficin1994.It'sthe firstAmericanarchbridgetobeconstructedfromsegmentsofpre-cast concrete,ahighlyeconomicalmaterial.Twogracefularchessupportthe roadwayabove.Usuallyarchbridgesemployverticalsupportscalled "spandrels"todistributetheweightoftheroadwaytothearchbelow,butthe NatchezTraceBridgewasdesignedwithoutspandrelstocreateamoreopenand aestheticallypleasingappearance.Asaresult,mostoftheliveload isrestingonthecrownsofthetwoarches,whichhavebeenslightlyflattened tobettercarryit.Alreadythewinnerofmanyawards,thebridgeisexpected toinfluencebridgedesignforyearstocome. (backtointro) (nextbridgedescription:beambridge) Photos:(1)Caltrans;(2)©TimHauf/VisualsUnlimited;(3)DavidBowen/WVDOH;(4)Corbis-Bettmann;(5,6)Figg Engineering,Inc. BuildABridge| Resources| Guide| Transcript| BridgeHome Editor'sPicks| PreviousSites| JoinUs/E-mail| TV/WebSchedule AboutNOVA| Teachers| SiteMap| Shop| Jobs| Search| Toprint PBSOnline| NOVAOnline| WGBH ©|UpdatedOctober2000