Since 1820, Brookfield, Vt., has had a unique claim to fame. Its Sunset Lake features the only floating bridge east of the Mississippi River. The Brookfield Floating Bridge originally hovered at the waterline as a creative system of timber over floating barrels. However, the roadside attraction has needed repeated updates to keep it afloat. In 1978, the bridge was reconstructed for the seventh time using a series of 380 plastic, foam-filled floats that supported a single traffic lane and pedestrian walkways.
In time, those floats also began to fail, and the bridge sank lower and lower, leaving only the most determined drivers to cross the watery path. In 2008, the bridge was shut down entirely to vehicular traffic.
But the Vermont Agency of Transportation was not ready to give up on the historic tourist attraction. In its plans to bring the bridge back to life, the agency had a very clear idea of what the updated structure would need to accomplish – and it determined that composite rafts would be the best option to meet its strict requirements.
In the early planning stages, the engineer of record, T.Y. Lin International in Falmouth, Maine, considered concrete as a familiar and relatively inexpensive solution for keeping the historic bridge afloat, but a cost-benefit analysis revealed GFRP as a clear winner. Among other reasons, concrete rafts would require significant dredging to the site to accommodate their expected 10-foot depth and the necessary eight feet of draft. GFRP rafts, on the other hand, would need only a 3-foot depth with one foot of draft and, unlike the concrete rafts, would feature flotation foam within to keep them afloat even in the event of a leak. In addition, GFRP allowed for offsite construction, which offered big benefits.
“Concrete would be pretty massive and would have to be built onsite with significant impact to the surrounding [environment]. It would also be a bit more time consuming,” says Josh Olund, P.E., bridge engineer for T.Y. Lin. GFRP would allow for the use of smaller cranes at the cramped installation site and would be comparatively inexpensive to transport. Durability also was a factor. The engineers predicted a 100-year design life for the GFRP rafts, with only biennial inspections required.