Wooden Bridges Stage Comeback

WOODEN bridges are coming back into vogue with technology that trades big logs for hundreds of flimsy wood strips glued together into laminated beams. Trus Joist Corporation, a manufacturer of specialty construction products in Boise, Idaho, has supplied the laminated bridge beams for 24 new wooden bridges in the Northwest in the last four years and hopes to sell 20 this year.

Spurring the market has been a United States Forest Service program that gives rural highway districts grants for new bridges if they use wood.

The Forest Service's goal is to promote stability in timber towns by finding new uses for wood products, while helping communities replace aging bridges.

``We're hoping it will be a catalyst to get people in the industry to get involved with some of the materials,'' says Clare Mitchell, a spokesman for the Boise National Forest.

At Trus Joist, executives are hoping to capture a fast-growing market once the US begins replacing aging bridges. So far, timber bridges are a $400,000-a-year business for Trus Joist. Company salesmen predict it could generate $10 million a year.

``This country has ignored the aging bridge problem for the past 30 years,'' says Rick Thomas, a Trus Joist salesman. ``These poorly maintained bridges are a problem for the counties to get goods to market and people to their job.''

He says his marketing efforts are aimed at small towns like those in Idaho, Montana, and Washington, where budgets are tight and bridge needs great.

``There's a tremendous number of bridges on secondary roads that need to be replaced,'' Mr. Mitchell says. ``In a lot of cases it is a lot more cost-effective'' to use wood. Mitchell says about 240,000 steel and concrete bridges nationwide could be replaced with wooden bridges.

Trus Joist appears well positioned to tackle that large market. Its parent company, TJ International, had $327 million in sales last year and operates manufacturing plants in eight states and two Canadian provinces.

Trus Joist's 2-foot-tall ``T'' shaped bridge beams are manufactured much like plywood.

Douglas fir saw logs are ``peeled,'' using a saw that cuts the tree into long ribbons of wood about one-tenth of an inch thick.

Those strips are smeared with glue and then pressed together at high temperatures and pressures to make the shapes and sizes needed. Trus Joist calls the material Micro-Lam.

``In plywood, each sheet's grain runs counter to the next one,'' Mr. Thomas says. ``In Micro-Lam, they all run parallel.''

By overlapping the various pieces, defects in the wood can be eliminated, providing a lightweight, high-strength material.

Micro-Lam has also been used in diving board cores, downhill ski cores, and for planks on high scaffolds.

When Trus Joist gets a bridge order, it custom-manufactures the beams at its Boise plant and then delivers them by truck.

While wooden bridges aren't always cheaper than concrete and steel, the Forest Service says they have several advantages, particularly in remote areas. While road salt corrodes steel and concrete and weakens bridges in 15 years, wooden bridges are expected to last 50 years. Wooden bridges are also easier to install, since they are lighter and require less heavy equipment to lift beams into place.

If a concrete casting plant is nearby, concrete can be cheaper. But at about $50,000 delivered, a 50-foot long two-lane wooden bridge can be just the ticket for a logging road in the mountains or a developer looking for a more attractive bridge.

The wooden spans are designed to handle loads up to 100,000 pounds, which is about the maximum that can be carried on an 18-wheeler truck.

For the timber industry, the laminated bridge beams are a way to stay in the business of supplying large lumber despite the shortage of large saw logs.

``You'd be hard pressed to find trees you could cut big timbers out of,'' says Russ Stoddard, communications manager for Trus Joist. ``With old-growth, it's a dwindling resource and a good amount of what's remaining is locked up.''