Wooden Bridges Carrying the Burden

The tree, man’s first way across stream or chasm, is making a comeback in the form of wooden highway bridges that may be cheaper and more durable than their steel and concrete cousins.

Motorists in places as diverse as county Highway S near Hayward, Wis., roads near Pueblo, Colo. and State College, Pa., are zipping across new wooden bridges. The spans are among 80 to be built in 30 states as part of a government-backed project.

There are about 575,000 highway bridges in the United States, and more than 40% of them need to be replaced, according to the Timber Bridge Initiative and Information Resource Center in Morgantown, W. Va.

The center is operated by the U.S. Forest Service under a $6.7-million program designed to find out if wooden bridges can carry their weight in the transportation system.

Those involved in the project say the signs are encouraging.

“Once the wood is treated using new, environmentally safe materials, these bridges can last more than 70 years without any maintenance,” said John Pasquantino at the center.

“These are not your old covered wooden bridges, but modern bridges capable of handling 18-wheelers,” he said.

While research is under way to determine whether such bridges would work on heavily traveled interstate highways, most of the emphasis has been on on secondary roads maintained by the states and counties.

“Canals, ditches and waterways in rural areas--that seems to be where the greatest need is,” Pasquantino said.

A bridge costs about $80,000 on average, he said, or $1,000 per linear foot of a two-lane span. The surfaces can be coated with concrete or asphalt or left natural, he said, and early indications are that salt and other chemicals used in winter to melt ice and snow do less damage to wood than to other materials.

“The other neat thing is that in terms of design, they are so sophisticated that it’s easy to put one together,” Pasquantino said. “They can be built and treated and then shipped to a rural area where local construction crews can put them together in less than two weeks.

“And, often you can use a local species of tree. We’re trying to use wood that’s right in everybody’s back yard. In the Northeast and central United States, that might mean hardwoods,” he said. “In the South and West, it might be Ponderosa (pine).”

The program could also expand the market for wood products and stimulate milling, fabricating and other related industries, backers say.

“Concrete was sexy,” said Ted Peterson of the University of Wisconsin, “but we’re finding out about wood all over again. The treating and proper design makes wood very competitive.”

The demonstration bridge near Hayward, Wis., was constructed by a crew of three in eight days, and cost 20% to 40% less than a steel or concrete bridge, he said.