While the material advantages were fairly clear-cut, one significant challenge to the plan stood out. “From an FRP standpoint we really didn’t have much for a design code to go by,” Olund says. There are no composite-specific design guidelines for bridges and little information on general structural applications for composites, the engineer found. “It seemed there were a lot of manufacturer-written manuals about how they go about design and how they would suggest implementing those designs. Still, to take that from a global perspective and to actually turn that into a project was difficult,” Olund says.
T.Y. Lin applied the “LRFD Bridge Design Specifications” from the American Association of State Highway and Transportation Officials to develop load requirements for the updated bridge. It also relied on the manufacturing expertise of partner Kenway Corp., a custom fiberglass manufacturer based in Augusta, Maine. For its part, Kenway used the “Pre-Standard for LRFD of Pultruded FRP Structures,” developed by ACMA and the American Society of Civil Engineers, to calculate analysis requirements and resistance factors.
The research was crucial because, despite the lack of available structural guidelines, the client had very strict quality and dimensional tolerances that the bridge needed to meet. “It was a 255-foot floating span, and they wanted it to come together within +/- ½-inch on that entire length,” says Jacob Marquis, P.E., the senior project engineer for Kenway who consulted on the project. Given that firm length tolerance, the manufacturer faced challenges in adjusting materials to account for the rafts’ expected expansion and contraction through different temperatures. “We had to adjust our parts and our molds in order to achieve the specified length after adjusting to a reference temperature of 40 degrees,” Marquis explains.
Once general contractor Miller Construction officially brought Kenway on board as its design-build partner, the manufacturer faced a new challenge. The fabricator designed and built the fiberglass-over-wood molds to measure 52 x 12 feet. “At that time it was the largest single infused part that we had to make,” Marquis says.
The end result was a series of five foam-filled GFRP rafts spliced together. Each raft comprises two pontoons bonded together, and each pontoon has eight watertight, foam-filled compartments.