The experiment simulates the massive loads that tractor trailerstractor-trailers carry across American bridges every day. After about two million up and down cycles--one per second for three weeks--dangerous fatigue cracks have appeared on the concrete beam. Eventually, a strand of 7/8-inch diameter reinforcing bar snaps inside, and engineers analyze the point of failure.
Studying why bridges fail--and exploring possible cures for America's ailing bridge infrastructure--is a hot topic for researchers in the College of Engineering and Information Technology.
"Everybody knows the infrastructure is deteriorating," said Kent Harries, an assistant professor in the Department of Civil and Environmental Engineering. "We're spending too little on upkeep in this country, and it's starting to bite us."
An interstate bridge near Milwaukee buckled earlier this year, and a span fell from another in Connecticut several years ago. Wholesale replacement of dangerous bridges is expensive: in the United States the tab is estimated at $10.6 billion per year for 20 years. Meanwhile, one third of South Carolina's 9,000 bridges are considered structurally deficient or obsolete.
But finding better ways to fix bridges is doable, and Harries, Michael Petrou, and Dimitri Rizos are working to do just that. Their civil structural engineering lab has the proper tools, including the state-of-the art equipment for modeling environmental exposure such as a freeze/thaw cabinet that simulates winter and spring conditions. The department also has ample room for such research in its 300 S. Main St. facility.
In January 2002, a huge section of the I-85 northbound bridge over Cherokee Bridge Creek (near Gaffney, S.C.) will be disassembled and carried to USC for analysis. The research project, sponsored by the National Science Foundation and the state Department of Transportation, will examine the integrity of the 1961-era bridge and study ways such aging structures could be repaired in the future.
"Bridge repair is a big issue because replacement is often not feasible," Harries said. "In South Carolina, we have a lot of logging trucks and crane trucks hitting bridge overpasses. We need something that will make a good rapid repair."
In fact, a chunk of steel girder from an I-77 bridge north of Columbia is in the civil engineering lab because a logging truck struck the bridge and the follow-up inspection revealed insidious cracks. As for finding a material that will make rapid repairs, Harries and other USC researchers are bonding different types of carbon fiber cloth to concrete bridge beams.
"Carbon fiber increases the durability of concrete and load-carrying capacity of the bridge, but it doesn't really appear to help with fatigue," Harries said.
Another USC bridge project, this one sponsored by the Federal Highway Administration and state Department of Transportation, will use a glass fiber material to replace traditional concrete decking on a new bridge being erected north of Spartanburg. The short bridge, which crosses a railroad cut, will be used to demonstrate the lightweight, high-strength capacity of the decking material, which installs faster with less heavy equipment than concrete decking.
"If you can reduce the weight of the bridge decking, the bridge supports can support more load capacity and greater traffic loads," Harries said.
The project should get underway later this year and will be the first of its kind in South Carolina. Because of the higher cost of glass fiber bridge decking, the material likely won't replace traditional concrete except in special situations such as historic bridge repairs.
As the United States' 1950s-era interstate system continues to age, the search for methods to repair the aging bridge reinforcements inventory grows more urgent.
"There's no need for white knuckles every time you drive over a bridge--drivers talking on cell phones are 1,000-times more dangerous than most bridges," Harries said. "But in the next 10 or 15 years, we're probably going to see more bridge failures. Of course, now is the time to fix things."
