CompositesWorld
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OCTOBER 2015 110 CompositesWorld FOCUS ON DESIGN By Johanna Knapschaefer / Contributing Writer Composite pontoons undergird update of 1820s-vintage floating bridge Modular composite flotation system triples potential lifespan of New England town's signature single-lane auto/pedestrian timber bridge. » In the central Vermont town of Brookfeld (pop. 1,292), the world's frst foating fber-reinforced polymer (FRP) bridge has replaced a 1978 version that carried state Route 65 trafc over Sunset Lake. Designed by T.Y. Lin International (San Francisco, CA, US), it is the eighth version of a foating log bridge built in 1820 after a resident fell through winter ice and drowned while crossing the lake nearly two centuries ago. It has since been the focal point of the town's life. Construction of the new 97m long, 6.7m wide, US$2.4-million single-lane Brookfeld Bridge began in April 2014 and was complete before its opening, Memorial Day weekend 2015. Replacement became necessary when 380 of the 50-gal barrels that kept it afoat began to leak. "Te plastic styrofoam-flled barrels were becoming saturated and deteriorating," says Jennifer Fitch, project manager at the Vermont Agency of Transportation (VTrans). As the barrels lost fotation and the timber got saturated, the bridge gradually sank and was fnally closed in 2008. Its use had been seasonal, recalls John Benson, chair of the Brookfeld Selectboard and a registered civil engineer, so "there were no plans for quick replacement, since only 110 cars cross the bridge daily in summer." But two years later, increasing awareness of the bridge as a community landmark and a contributing element to Brookfeld's listing on the National Historic Register spurred formation of a foating bridge committee. Its members — including represen- tatives of the town, VTrans and the US Federal Highway Admin. (FHWA) — identifed the need for a state-of-the-art fotation system with a 100-year life, says Fitch. Design evolution VTrans structures program manager Wayne Symonds frst toyed with using FRP for the bridge while at the Maine Composites Showcase at the University of Maine (UMaine, Orono). He visited some of Maine's frst bridge-in-a-backpack structures (designed at UMaine, see "Learn More") and the manufacturing facilities at Kenway Corp. (Augusta, ME), and Brunswick, ME-based Harbor Technologies. Te latter was building foating docks for cruise ships. Symonds recalls thinking, "If …. cruise ships can bump into them, but they rebound without damage from impact, then a refned version of this technology could work for Brookfeld with the trafc and ice loading." Te initial ideas for a pontoon-based buoyancy design came from research engineers Olivia and Xenia Rofes at UMaine's Advanced Structures and Composites Center (ASCC). "UMaine came up with the initial cross-sectional shape and helped us out as material property experts as we moved forward with fnal design," says Josh Olund, bridge engineer at T.Y. Lin's Falmouth, ME, ofce. Although concrete pontoons were considered, concrete was ruled out because it would have required dredging along the shore to accommodate what would have been 3m-deep pontoons. Specialized trucks and cranes also would have been necessary for transport and installation. "We managed everything inside the right-of-way with minimal environmental impacts," Fitch says of Preserving the past for the next century Reportedly the world's frst foating fber-reinforced polymer vehicle bridge, the Brookfeld Bridge preserves the character of the timber structure it replaces, but undergirds it with a buoyancy system designed to last for a century. (See more photos of the new bridge and some vintage shots of the 1978 structure it replaced in the online version of this article noted in "Learn More.") Source | Brookfeld