chapter  4
46 Pages

The use of models in nineteenth-century British suspension bridge design

Theearliestreferencetoamodeltestinconnectionwith suspensionbridgedesignrelatestotheproposedbridgeoverthe MerseyatRuncornundertakenbyThomasTelfordinI8I4.A petitiontoParliamentwasdrawnupinI8I3tobringinaBillto establishtheRiverWeaverFisheryCompany,'topreservethefry inthesaidRiverWeaver,andincertainpartsoftheRiverMersey ...andalsotoerectaBridgeovertheRiverMerseyatorneara placecalledRuncornGap'. 1TelfordwasapproachedduringI814 andinSeptemberanewspapersaid'Amagnificentbridgeis intendedtobeerectedovertheMerseyatRuncorn,whichunite LancasterandChester...Aplanworthythisgreatobject,hasbeen producedbyMr.Telford'. 2SomeyearslaterTelfordsaid'the crossingoftheRiverMerseyatRuncornhasalwaysbeenthemost importantconsiderationwhenevershorteningthecommunication betweenLondonandLiverpoolhascomeunderdiscussion.Inthe yearI8I4Ihadmyattentionparticularlydrawntothesubject,and wasemployedtoinvestigateit'.3FewdetailsofhisworkinI8I4 surviveandtheprincipalsourceisTelford'sReporttothe RuncornBridgeCommitteeofI8I7.However,threeofhispocket notebookssurviveinthelibraryoftheInstitutionofCivil Engineers,oneofwhichistitled,inhisownhand,'Runcorn BridgeI8J4'. 4ThiscontainsriversurveynotesatRuncornand detailsofvariousironchainsseeninhistravels,recordingthesize oflinksandtheloadscarried.Inaddition,twopagesmarked 'model'givedetailsofwirejointsandincrementalloadingupto 3,033lbs.Thisdoesnottellusmuchofthemodelexceptthatthe testsweremadeon8July.Anotherpartofthenotebookmentions 'ExperimentsatBrunton's'madefrom5April,andthesehe describedinhisReporttotheCommitteethreeyearslater:

onconsideringthemattermaturely,itoccuredtome,thaton thesuspendingprinciple,chainsarenotthemostperfect meanstobeemployed,butthatthemetalshouldbekept,as faraspracticable,instraightlines,andalsohavefewjoinings. Inordertoproceedwithduecautionuponthisprinciple,I

172ModelsinBritishSuspensionBridgeDesign

DMETHODS

DenisSmith173

Severalquestionsarisefromthis;firstlythatofthegeometricscale ofthemodel.Telford'sdesignwasforabridgehavingacentral spanofI,ooofeettogetherwithtwosidespansofsoofeeteachmakingitabridgeofunprecedentedspan.Whenhesaid'amodel ofsofeetinlength'wecannotbecertainifthisincludedallthree spans;althoughW.A.Pro viswasmorespecificsaying'amodelof thecentralopeningwasconstructed' 11andthisiscertainlymore likelyandlogical.Ifso,thegeometricscalefactorwasso/1000= 1/20or20:Isofarasthespanwasconcerned,butunfortunatelywe havenofurtherdetailsofthemodel.Secondly,asTelfordloaded thismodelitindicatesthatitwasnotmerelyofthevisualtype,and indeedweknowthatanother,smaller,modelwasmade presumablyforthispurpose. 12Lackingdetailsofthemodelchains itisdifficulttounderstaindwhatTelfordmeantbydescribingthe modelasbeing'onetwelvehundredthpartofthestrengthofthe intendedbridge'.Inusingthewordstrengthwemayreasonably assumethatheisconcernedwiththebreakingstressofamember subjectedtopuretension,inwhichcaseasquarelawrelatesthe tensilestresstothegeometricscalefactor.IfweassumeTelford's spanscalefactortobeI/20andthatthisfactorwasalsousedto scaledownthedimensionoftheproposedchainthenthemodel wouldhavebeenI/400(i.e.I/202)thestrengthofthefull-sized structure.Thechainsofthismodelwere,however,'composedof wireonly'and'withoutproperjoints'andunlessthegeometric scalefactorforthesewasI/\/I200=I/34·6itisdifficulttoadd anythingtoTelford'sstatementrelatingthestrengthofhismodel tothatofthefull-sizedstructure.Themodelwasloadedupto c.3,000lbs,'withoutanysymptomofitspartsbeingderanged'; inotherwordsitwasnottestedtodestruction,andindeedProvis addedthatit'wouldhavecarriedconsiderablymorewithoutbeing injured'.13AfterthetestsTelfordsaid:'ThisfarIproceededin I8I4.Iwasperfectlyconvincedofthepracticabilityofthescheme, andamnowsatisfiedIwentfurtherthannecessaryinthequantity ofmaterial,andalsoastosundryprecautionsinformingtheplan, butIwaswillingtoerronthesafeside.'14Telford'sdesignforthe chainsexist;theyweretoconsistof'I6ironcables,eachformedof 36squarehalf-inchironbars;andofthesegmentsofcylinders properforformingthemintooneimmensecylindricalironcable, whichwastobenearlyhalfamilelong(includingthefixingson shore),andabout4-!inchesdiameter'.15