Timber helps build bridges from past to future
Dan Sikarskie slaps one of the rough-hewn beams that gird 30 feet of wooden bridge across the Au Sable River here.
"This bridge is strong enough to drive a tank over," says the US Department of Agriculture employee, standing where red-pine planks span one of the premier trout streams in the north woods. The structure's weathered look is aesthetically pleasing, he says, and by replacing the dilapidated stone and concrete culverts that were here, both stream flow and water quality have improved.
"What more could you want in a bridge?" he asks.
In an era when space-age materials are as ubiquitous as the cellphone and laptop computer, a new-old building technique is spanning the gap between nostalgia and the modern-day demand for high performance. Timber bridges are making a comeback.
Here in Michigan, close to 30 wooden bridges were constructed in the 1990s using a combination of high-tech designs. Nationwide, 2,472 timber bridges were built in the last decade according to the Federal Highway Administration. The retro trend may be developing at an opportune time.
Upwards of 40 percent of the nation's 575,000 bridges are in need of major repairs or replacement. Spans up to 60 feet long - considered ideal candidates for timber - make up nearly a quarter of all bridges, meaning some 75,000 crossings are timber-bridge possibilities.
"Timber bridges have established themselves as a cost effective and environmentally friendly alternative to steel and concrete structures," says Edward Cesa, program manager for the National Wood in Transportation Program. "The whole idea behind our program is to employ a renewable resource, in the form of underutilized local species of trees, to revitalize local economies."
To some extent, two federally sponsored programs designed to spur timber bridge building have worked. In Ida county, Iowa, northwest of the famous bridges of Madison County, five bridge decks have recently been completed that utilize native cottonwood, not generally known for structural applications. Here in Michigan, red pine is the timber of choice. Hemlock is favored in Maine and red maple in Pennsylvania.
Although federal research funding resulted in the successful structural certification of new species of wood for use in bridge building and design standardization, cost is still the overriding factor for highway engineers deciding on what material to use for bridge building. Indeed, low lumber prices in the early 1990s were one of the reasons the feds began pushing timber bridges.
But a funny thing happened on the way to the river crossing: Timber prices lurched upward in the last half-decade, making timber bridges less of a bargain. Wooden bridges can still be cheaper than their steel and concrete kin, but bridge builders have to work at the cost savings.
Mr. Sikarskie notes that some county road commissions in Michigan will harvest red pine themselves -- donated from state forest lands - have it milled and then use it to build timber bridges, sometimes only a few miles from where it had been growing. "You definitely save on the cost of materials when you can grow your own bridge," he says, only half-jokingly.
Another cost advantage timber bridges often enjoy is on labor. County road crewscan be used for much of the construction rather than contracting out for skilled labor.
Ashtabula County (Ohio) Engineer John Smolen keeps his crews busy in winter prefabricating timber deck panels indoors when there's no snow to plow. The same crews assemble the bridges in warm weather months.
Performance is another issue. Highway officials generally perceive timber to be the poorest performing bridge material when compared to steel and concrete. Many have spent years replacing the obsolete structures of old that often split and rotted with age and fell prey to termites.
Cesa acknowledges that one of the worst advertisements for modern timber bridges are old ones. "Engineers see some deterioration and think that's what they're getting into. Engineered-wood products have drastically changed the wood industry but they still don't fully realize the same is true for transportation applications."
While wood bridges are not new, their design and manufacture have gone hightech. The Au Sable bridge here uses glued-laminated timbers that are squeezed together, or "stressed," using iron bars. Preservatives now seal and waterproof the deck, which is already immune to the road salts. And timber doesn't require painting.
The result here is a structure that can handle the weight of a tank and last 50 years - with a rating of 80 tons, higher than bridges on a nearby interstate.
Timber-bridge proponents fear that just as their nascent movement is gaining a foothold its momentum is threatened. They complain of a backlash by steel- and concrete-bridge interests and blame them for the elimination of timber bridge funding and for lowballing bids to keep wood bridges at bay.
But Cesa is optimistic. "How do you attract others to look at wood, because when they do, they're usually hooked."
(c) Copyright 2000. The Christian Science Publishing Society