ASPIRE Summer 08 - Aspire - The Concrete Bridge Magazine

The final set of 10 anchors was towed to the Hood Canal and set accurately on the channel floorusing GPS, tilt meters, and gyroscopes. The anchor cables will be attached between the anchorsand pontoons in 2009.A concrete floating bridge providesa cost-effective solution.Pontoons are assembled together withspliced post-tensioning tendons beforebeing towed to the Hood Canal Bridge toreplace the old pontoons.conducting a series of qualification testsand constructing a mock-up pontoonwall, the contractor successfullydemonstrated that this “new” mixcould be placed without segregation.Testing and acceptance of this concretewas accomplished using the flow testthat is common with self-consolidatingconcrete (SCC).Another innovation implemented was toprecast portions of two pontoons thatmake up the moveable draw pontoons.These pontoons have heavy mechanicalcomponents cast into the walls 21 ftoverhead. The precasting operationimproved overall safety in supportingthese massive guides and facilitated thetight alignment tolerances needed forthe mechanical draw span operations.The precast elements consisted ofportions of the exterior walls with allnecessary reinforcement and posttensioningto tie into the top andbottom slabs. Once the precast pieceswere set into place, reinforcement andpost-tensioning was tied into the baseslab and the wall closure regions. Thebase slab, wall closures, and top slabwere then constructed with cast-inplaceconcrete. By precasting portionsof these pontoons, construction timewas reduced. Precasting also allowedmuch of the work to be shifted off-siteand away from the heavily congestedgraving dock facility.Elevated RoadwayConstructionTo withstand the regular pounding ofsaltwater waves that crash over thebridge during the storm season fromOctober through April, the elevatedroadway built atop the pontoonsis constructed primarily of reinforcedand precast, prestressed concrete.With project activities since early 2006focusing on constructing pontoonsand anchors and assembling the drawspan section, the elevated roadwayremains a main element of work to beaccomplished.The elevated roadway on a floatingstructure compels the designer to selectan optimal span length to minimizethe dead loads and to evenly distributecolumn loads to the pontoon structure,which behaves like a beam on elasticfoundation from water buoyancy. Forthe Hood Canal Bridge, this equates toshallow I-girders with 60-ft span lengths.Built on two-column piers, the two-spancontinuous units have a hinge diaphragmat the center pier. The floating structure isisolated from seismic events with specialconnections to the fixed structures, so thefloating bridge is governed by dynamicloads from wind, waves, and currentsinstead of seismic loads.The prestressed concrete I-girders aretypical WSDOT 32-in.- and 42-in.-deepsections, but all reinforcement and0.5-in.-diameter prestressing strands areepoxy coated and the bottom flangeconcrete clear cover is 1-1/4 in. The7-1/2-in.-thick roadway deck consistsof 4 in. of reinforced concrete cast on3-1/2 in.-thick, stay-in-place precast,prestressed concrete deck panels.Using stay-in-place panels significantlydecreased the time required to constructthe deck and increased safety whenworking over water.ASPIRE, Summer 2008 | 19