CompositesWorld
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111 CompositesWorld.com Floating Vehicular Bridge the more manageable FRP, adding, "Had we gone with concrete, it would have taken longer to complete the design phase." "FRP was a top pick since few materials have the ability to provide such longevity," Fitch sums up. "It doesn't crack easily; it can handle ice pressures; and with the addition of UV-9, an inhib- itor chemical, it should not degrade easily." Te greatest design challenge was working with unknowns, Olund says. Because design codes for FRP bridges do not exist, the team worked with VTrans and UMaine to examine avail- able guidance and research papers and developed its own set of criteria, using those guides as a foundation. "Every single step of the way, we had to reinvent how to approach design," he recalls. T.Y. Lin also determined bridge loading conditions, including ice pressures, live-load confgurations (pedestrian and vehicle) and buoyant-service capacity. During the winter of 2012-2013, ASCC's Sanchez set up sensors at the bridge site to record the pressure developed by the ice on the lake's surface against the existing fota- tion system's vertical surfaces. And as FHWA required for the new bridge, the design team increased the load capacity of the struc- ture, designing for the larger of a single 12-ton truck or 384 lb/linear ft for cars crossing the bridge, Olund explains. "Historically, the bridge had been posted for 3 tons, but it's unclear if this was truly its design capacity." To make construction, transport and installation practical, the FRP portion of the bridge structure was designed in fve separate but identical 15.54m long by 7.01m wide sections called rafts (see drawing, above) each formed from two buoyant FRP pontoon structures, assembled back-to-back. Assembled rafts then would be bolted together, end-to-end, using steel splice-plates designed by T.Y. Lin, to form a monolith, undergirding the entire length of the wooden bridge. "It actually acts as one long beam, with timber on top — this is new for a bridge application," adds Olund. To prevent water overfow that had degraded the lifespan of the previous bridge's timber decking and made it slippery for auto trafc, the team designed rotating on/of ramps with unique bearings and joints to permit the bridge to rise and fall with the lake's natural 1.7m level fuctuations. To enable the most cost-efective solution possible, the design team elected not to dictate specifc materials or substantially high material properties. "Tis allowed the fabricators to have fexibility in meeting project goals with materials and practices they were comfortable with," Olund explains. Illustration / Karl Reque Kenway Composite Pontoons for Floating Bridge › Replacement of foam-flled plastic barrel fotation system by more substantial composite pontoons extends bridge service life to 100 years. › Modular assembly via mechanically fastened "rafts" permits maintenance/repair without removal of entire bridge from water. › Selection of composites over concrete for pontoon material avoided expensive lake bottom dredging step. RAFT CROSS SECTION Pontoon Pontoons adhesively bonded and mechanically fastened here, back-to-back 0.91m Timber railings Timber plank sidewalk Steel plate joins rafts Oak runner board Wood deck paneling (over FRP foat top) FRP RAFT (TWO PONTOONS) WITH WOODEN SUPERSTRUTURE 7m Timber sidewalk risers FRP foat top 15.54m