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Structures2008:CrossingBorders©2008ASCENewInnovativeFeaturesofMaine’sFirstCableStayBridgeAuthor:ChristopherJ.Burgess,P.E.,S.E.,FIGG,Denver,CO,cburgess@figgbridge.comThesteelsuspensionWaldo-HancockBridgecarriedU.S.Route1travelersoverthePenobscotRiver,nearthetownofBucksport,Mainefor74years.Thisvitallyimportanttransportationlinkprovidedsafepassageforcarloadsoftouriststhroughtheyears,inadditiontosignificantcommercialtrafficrelatedtoregionalpaper-making,granite-quarrying,boatbuilding,manufacturedhousingproductionandahostoffreightitems,suchasdeliveryofhomeheatingoil.In2002,ascheduledrenovationbeganandinthesummerof2003,themainsuspensioncableswereunwrappedtoprepareforavisualinspectionoftheindividualwiresinthe37cablesineachmaincable.Deteriorationofthemaincableonthesouthsideofthebridgewasmuchmoreadvancedthanexpected,leadingthebridgeowner,MaineDepartmentofTransportation,tosimultaneouslyundertakeastrengtheningprojectontheWaldo-HancockBridgeandthedesignofthePenobscotNarrowsBridge&Observatoryonaparallelalignmenttotheexistingbridge.Inordertospeedtheprocess,Maineundertookthereplacementinaninnovativeowner-facilitateddesignbuildprocessandgroundwasbrokeninDecemberof2003forthenewcable-stayedbridge,afirstforMaine.Akeyelementinthedesignofthenewcast-in-placeconcretesegmentalbridgeistheconsiderableattentionandeffortinvestedtoprovideadurableandeasytomaintainstructurewithaplannedservicelifeexceeding100years.Thebridgeisthefirsttoutilizeauniquecombinationoffeaturesdesignedspecificallytoprotectthebridgeandonlythesecondtobenefitfromapioneeringcradlesystem,developedandpatentedbythebridge’sdesigner.FIGURE2ADJACENTCRADLESPOSITIONEDINTHEUPPERPYLONALONGWITHFORMWORK,ASTHEJUMPFORMSAREBEINGPREPAREDTOMOVEUPANDCASTTHENEXTSECTION.THECRADLESAREFABRICATED,DELIVEREDTOTHESITE,POSITIONEDANDSURVEYEDINPOSITIONINTHEFRAMEWORK,THENLIFTEDINTOPLACEANDTHEENTIREASSEMBLYSURVEYEDFORCORRECTPOSITIONINGPRIORTOCASTING.FIGURE1THEWALDO-HANCOCKBRIDGEANDNEWPENOBSCOTNARROWSBRIDGE&OBSERVATORYSIDE-BY-SIDEOVERTHEPENOBSCOTRIVER,LINKINGWALDOANDHANCOCKCOUNTIESINMAINE.THENEWBRIDGEOPENEDTOTRAFFICONDECEMBER30,2006,JUST42MONTHSAFTERTHEEMERGENCYREPLACMENTNEEDWASIDENTIFIED.Aninnovativenitrogengasprotectionandmonitoringsystemprovidesanenclosedenvironmentofpressurizedinertgasaroundeachcablestay.Themaincomponentsofthesystemarethegas,HDPEsheathing,reservoirtanks,anchorage-sealingcapsandmonitoringhardware.Nitrogengaswasselectedduetoitsnon-corrosivepropertiesandeaseofavailability.Anenvironmentofpressurizedpurenitrogenessentiallyeliminatesoxygen,chloridesandmoisture,allpotentiallycorrosiveelements.Aprimarygoalwastocost-effectivelycreateagas-tightsealedsystemthatcompletelyenclosesthestays.Afterinstallationofthestays,theywerepurgedofmoistureandnitrogengasinjectedtopurgetheexistingairfromtheclosedenvironment.Thenitrogenwasthenpressurizedtotwopoundspersquareinch.Asealingcapcoversthestrandtailsateachanchorageandfullyencapsulatesallanchoragehardware.Aclearendplateallowsdirectvisualinspectionoftheanchorarea.Eachstayisprovidedwitha300cubicfootnitrogengasreservoirtorechargethegasintheeventofadropinpressure.GaugesrecordallfluctuationsinpressureandserveasatoolfortheDepartmenttoeasilymonitorthestatusofthesystemandtakecorrectiveaction,ifnecessary.Theannularspacebetweenthecablestaystrandandthecradlesleevesallowsgastoflowfreelythroughthestaysysteminanisolatedenvironment.Tothebestofourknowledge,thestaycablegasprotectionsystemutilizedinthenewPenobscotNarrowsBridgeisaworldfirst.Thisuniquesystemprovidestwoadditionallayersofprotectionbycompletelysurroundingthestayswithaninertgas,andthroughautomaticandcontinualmonitoringofthegaspressurelevel.MonitoringallowstheMaineDepartmentofTransportationreal-timeknowledgeofanypotentialcompromiseinthesystem.Thereareatotaloffournestedlevelsofprotection,providingredundancy:•Epoxycoatingonthestaystrands•OuterlayerofHDPEsheathingaroundthestays•Systemfilledwithnitrogengastopurgepotentialcorrosives•SealedsystemwithmonitoringequipmentThisproactiveapproachallowsmaintenancecrewstoidentifyandaddresspotentialproblemsatanearlystage,withminimalmonitoringeffort.AnadditionalmonitoringtoolforthePenobscotNarrowsBridgeisaseriesofforcemonitoringsystemsoneachstay.DywidagSystemsInternationalprovidedtheforcemonitoringsystems,whichcanaccuratelydeterminetheforcewithin1%usingaportablefieldlaptopunit.Thesystemisveryrugged,requiresnomaintenance,hasnomovingpartsandwillprovidemonitoringthroughouttheprojectedservicelifeofthebridge,allowingDepartmentemployeeseaseinregularlymonitoringtheforcesinthecableaspartoftheirinspectionprocedures,withouttheneedforlift-offequipmentortheutilizationofspecialexpensivetechniquessuchasvibrationmeasurements.Withthismonitoringsystem,theforceinthecableisobtainedinminuteswithoutanyinterferencewiththetrafficonthebridge.Theleadsfromthesensorsareinagastightboxlocatedinsidetheboxgirderjustunderthecableanchorage,maintainingtheintegrityofthesealedsystemofthegasmonitoringsystem.FIGURE3CONSTRUCTIONTOOKPLACEINTHREEDIRECTIONSONBOTHPYLONS–TWOCANTILEVERSEXTENDINGFROMTHEPYLONASTHEPYLONSWER