Composite bridge among the firstBy Liz Crumbley
Spectrum Volume 19 Issue 12 - November 14, 1996
Blacksburg will be the site of one of the first bridges in the U.S. constructed with composite materials to carry traffic.
The low-cost, low-maintenance bridge, scheduled for installation in June 1997, is the joint effort of Virginia Tech, the Town of Blacksburg, Virginia Department of Transportation (VDOT), Virginia Transportation Research Council (VTRC), Morrison Molded Fiberglass (MMFG) of Bristol, Virginia; and Virginia's Center for Innovative Technology, which is providing research funds.
Richard Weyers, a Virginia Tech civil engineering professor, said the composite structure will replace an existing bridge over Tom's Creek that has been posted and load limits lowered because its main supporting steel beams have corroded.
As designed by Weyers, Jack Lesko of the Tech engineering science and mechanics department, Jose Gomez of the VTRC, and Ernesto Villalba of VDOT, the new bridge will rest on the existing concrete foundations. The bridge's roadway, or deck, will be constructed of transverse glulam wood panels treated with creosote as a preservative and will be overlaid with a new waterproofing system.
Composite beams, developed by MMFG and Georgia Tech and supplied by MMFG, and weighing about 200 pounds each, will replace the current 420-pound steel beams. Weyers said municipal crews can handle the light-weight beams themselves, saving the expense of hiring extra crews and large equipment. A long-term cost-saving factor of the composite beams is the fact that composite materials, unlike steel, do not corrode. Also, these beams will never have to be painted, making them safer for the environment as well as having a lower life-cycle cost than steel beams.
The wooden deck will be paved with two layers of asphalt. Weyers and Joe Loferski and Dan Dolan of the Tech wood science and forest products department have developed the waterproof system of asphalt, waterproof membrane, and asphalt-wearing surface. The pre-formed, self-adhering membrane of woven fibers placed between the two layers of asphalt will keep water from reaching the wooden deck. When the bridge needs repaving, Weyers said, the top layer of asphalt can be quickly removed and replaced.
Once the components of the bridge are ready, Weyers noted, construction should take only about a week, which will be a real savings in work time and inconvenience to the public.
Lesko emphasized that the project is interdisciplinary, combining civil-engineering structural knowledge with the implementation of composite-materials technology. He also cited the project as an example of cooperation between researchers and state and local representatives, including Malcolm Kerley, chief bridge engineer for VDOT, and Randy Formica and Adele Schirmer of the Town of Blacksburg.
The need for such departures from traditional bridge design is widespread. The federal government defines bridges as structures of at least 20 feet in length. In the United States, there are about 590,000 of these bridges and the Federal Highway Administration currently classifies 32 percent of them as structurally deficient or functionally obsolete. Weyers said bringing them up to current standards would cost an estimated $70 billion. In Virginia, for example, 29 percent of the 12,400 bridges more than 20 feet in length are rated as sub-standard.
The problem is worse than those figures indicate, however. There are about four times as many bridges less than 20 feet in length, like the 17-foot bridge over Tom's Creek. Weyers said that 40 to 50 percent of the shorter bridges probably are sub-standard.
Weyers said that the composite design should work for bridges of up to 30 to 40 feet in length. Beyond that length, he said, the materials are too flexible to provide the same stiffness as bridges constructed with steel beams.
Lesko said the bridge project also provides an opportunity to assess the durability of composite materials in a real infrastructure environment. "Municipalities are not going to implement these new materials if they cannot be sure of their durability," Lesko noted.
The new Tom's Creek bridge will be in service for 10 to 15 years before removal for road-widening, which, Lesko said, makes it a safe durability test site for the composite beams. "This effort will bring about more confidence as time passes and we are able to glean new information about how composite materials perform."