Caring for a Covered Bridge

Today the quaint Cornish-Windsor covered bridge (winner of this year’s Outstanding Civil Engineering Merit Award) doesn’t look much different than it did when it opened to horse-and-buggy traffic in 1866. But modern materials and engineering allow it to carry standard loading—the moving weight of two, 15 ton tractor trailers plus two, 12 ton trucks. The 458 ft. long, two-span bridge, listed on the National Register of Historical Places, crosses the Connecticut River to link Cornish, N.H. and Windsor, Vt. It’s the longest covered bridge in the U.S., a National Historic Civil Engineering Landmark, and the only remaining Town lattice-truss bridge in the world. A 1966 engineering study showed that the bridge’s western span sagged by 17 in. The eastern span sagged by 12 in. Tension (with help from rot) was spreading apart the pieces in the lower chords. Wind had tilted the bridge upriver and had caused each span to bow 9 in. The bottoms were broken or twisted. One of the bottom chords was patched in several places and had an unrepaired split about 6 ft. long. Before repairs began in spring 1988, a unique suspension system was designed to temporarily raise the bridge to avoid winter ice floes and to support both sides of the pier at the same time so crews could replace the continuous top chords. To maintain the bridge’s appearance, yet meet the new design load requirements, engineers replaced rotting wood with glulam timbers and installed a bolster-beam support system.