The covered bridges still in use around the country, some built more than a century ago, bear eloquent testimony to the durability of wood. That's one reason researchers at Pennsylvania State University figure wooden bridges are due for a comeback.
Working with the Pennsylvania Transportation Dept., the researchers designed a sleek oak bridge that opened to traffic in November near the campus, on Route T-330. Its 34-foot-long deck is a lamination of boards 8 to 12 feet in length, a parquetlike technique that could span up to 90 feet, says Harvey Manbeck, professor of agricultural engineering. The $260,000 structure proved it could support modern traffic when two trucks, each weighing 37 tons, were driven onto the bridge. If the lumber is treated with creosote, adds Manbeck, wood bridges should outlast steel or concrete designs by as much as 20 years. Moreover, the Northeastern states that produce most of the nation's oak, maple, and poplar are growing twice as much timber as is being harvested, so the raw materials are in abundant supply.