M.S. Palanichamy - Basic Civil Engineering (2011, Tata McGraw-Hill Education) 256-273.docx

M.S. Palanichamy - Basic Civil Engineering (2011, Tata McGraw-Hill Education) 256-273.docx

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Bridges 7.9

Chapter7

BRIDGES

7.1INTRODUCTION

Abridgeisastructureprovidingpassageoveranobstaclesuchasavalley,road,railway,canal,river,withoutclosingthewaybeneath.Therequiredpassagemaybeforroad,railway,canal,pipeline,cycletrackorpedestrians.

Thebranchofcivilengineeringwhichdealswiththedesign,planningconstructionandmaintenanceofbridgesisknownasbridgeengineering.

7.2NECESSITYOFBRIDGES

Bridgesarevitalforthedevelopmentofacountrysincetheseenabletransportingmaterialsfromoneareatotheotherwhichmaybeseparatedbystreamsandrivers,therebymaintaininguniformflowofessentialgoodsfordevelopment.Intimesofwar,materialsareswiftlytransportedforthedefenceneedsofthecountrybyrailroadbridges.Bridgeslinkthewholecountrywithroadandrailwaycommunicationmaintainingauniformflowofpeople,goodsandotheressentialcommodities.Thenecessityofbridgesmaythereforebesummarisedasfollows:

1.Bridgesenablethefreeflowoftrafficduringmonsoonsandotherperiodsofinclementweather.

2.Bridgesprovideadditionalcommunicationfacilities.

3.Thedevelopmentofthebackwarddistrictswhichmayberichagriculturallycriticallydependsontheexistenceofbridges.

4.Bridgesprovidemoresocio-economicbenefitstothepeople.

5.Bridgesalsoenablemovementoftroopsandmilitaryvehiclesduringhostilities.

7.3SITEINVESTIGATION

Beforeabridgeisconstructed,asuitablesiteisselectedbasedoncertainfactorswhichhavebearingontheeconomyandstabilityofthebridge.Reconnaissanceisthereforemadeandthefollowingdataarerecordedtodeterminethefeasibilityofbridgeconstruction:

Consideredwhileselectingthesiteforabridgearethefollowing:

1.Thebridgeshouldcrosstheriveratrightanglestothedirectionofflowofstreamorriverwatersoastominimisethelengthofthebridge.

2.Thebanksoneithersidesoftherivershouldhavefirmsoilandbestraightandwell-defined.Thiswillincreasethestabilityofthebridgeandreducethepossibilityoftheerosionofthebanks.Also,thesoilneednotbestabilisedorgivenanyothertreatmentwhichwillincreasethecost.

3.Theselectedsiteshouldbeataplacewheretheriverisnarrowandtheflowisstreamlinedwithoutseriouswhirlsandcrosscurrents.Smallwidthreducesthelengthofthebridgewhichmeanslesscostofconstructionandmaintenance.Iftheflowofwaterisuniformandparallel,itisareliableguardagainstscour.

4.Precautionsshouldbetakentoseethattheselectedsiteshouldbefarawayfromwheretheriverislikelytochangethecourse.Ifthebridgehasbeenconstructedandtheriverhaschangeditscourse,thebridgewillberendereduseless.

5.Hardinerodablestrataorrockshouldbeavailableclosetotheriver-bedlevel.

6.Thereshouldnotbeanysharpcurvesinroadapproaches.

7.4PRELIMINARYDATATOBECOLLECTED

Theengineerin-chargeoftheinvestigationforabridgeshouldcollectthefollowinginformationbeforetheconstructionofabridgeisundertaken:

1.VolumeandNatureofTrafficNotonlythepresentvolumeandnatureoftrafficbutalsothefuturevolumewhichisexpectedinthenexttenyearsshouldbecollected.Thesizeandthetypeofthebridgerequireddependonthisdata.

2.VelocityoftheStreamandHighFloodLevel（HFL）AttainedThedischargeofwaterpassingthroughthebridgedependsonthevelocityofwater.Itwillhelpindesigningthepropersizeofthewaterwayandpierthickness.Thevelocityofthestreamduringhighfloodandalsoduringnormalflowcanbedeterminedwiththehelpofthecurrentmeterorvelocityrods.TheHighFloodLevel（HFL）willenableabridgeengineertodeterminetheheightoffreeboardwhichistheheightoftheroadwayabovetheHFL.Itisnecessarytopreventthewashingawayofthebridgeduringheavyfloods.

3.CatchmentAreaItisanareaofthatportionofwatershedfromwhichwaterflowsandfeedstheriver.Thisisnecessarytocalculatethedischargeofthestream.

4.StrengthandNatureofSoilandExtentandTypeofVegetationThedepthoffoundationforthepiersandabutmentsdependsonthestrengthofsoil.Thisisdeterminedbycarryingoutboringsatseveralplacesandtestingthesoilsamples.Extentandtypeofvegetationalongwithclimaticconditionsarealsonoted.

5.FrequencyofFloodOccurrenceandRainfallDetailsThedeterminationoffloodfrequencyisanimportantfactorasmostofthebridgesaredesignedforafloodfrequencyof50years.AmountofrainfallandtheHFLarealsonotedforfuturedesign.

6.ScourDepthDeterminationErodingofthebedofriverduetoheavydischargeandthevelocityofwaterknownasscourhasagreatbearingonthedesignofthedepthoffoundationofpiersandabutments.Hence,theextenttowhichthebedoftherivermayscourbelowthe