FAYETTEVILLE, Ark. – Bridges, both long and short, are a way of life for most Americans, who may cross more than a dozen bridges every day. Recent terrorist threats and a catastrophic accident have put bridges in the spotlight and raised questions concerning their safety. University of Arkansas researchers John Schemmel and Ernie Heymsfield have been trying to find the answers for some well-traveled bridges.
There are 587,755 bridges in the United States, according to the U.S. Department of Transportation. In 2000, 29 percent of these bridges were rated structurally deficient. In addition, between 1990 and 2000, vehicular traffic across bridges increased by 13 percent – increasing the load on some already stressed structures.
"We are looking at precast concrete bridges, which are very common in some rural areas," explained Heymsfield, assistant professor of civil engineering. "We have identified a dozen states with this type of bridge in use. In Arkansas alone there are 400 precast concrete bridges."
Heymsfield and Schemmel, professor of civil engineering and associate dean of the College of Engineering, studied short-span precast concrete bridges, which might be found crossing small streams or railroad tracks. In addition to examining the bridges in place, they conducted loading tests on beams to determine what is actually happening to the individual beams as traffic passes over the bridge.
States began installing these bridges in 1952 and they continue to be installed. In addition to Arkansas, precast concrete bridges are used in Mississippi, Missouri, Iowa, Nebraska, Indiana, Pennsylvania, Minnesota, North Dakota, South Dakota and Wyoming. Originally designed for a 50-year usage life, many of these bridges are nearing that age.
"In an ideal world, many of these bridges would be replaced, but each state may have several hundred bridges. They simply don’t have the resources to be able to replace them all," Heymsfield said. "We are trying to determine what should be done to maintain their safety."
The precast concrete beams are shaped like an inverted letter "U" and several beams are joined together to form the bridge decking. Because the primary stress occurs at the bottom of the precast beam, steel reinforcing bars are built into each of the "arms," which extend downward when they are installed.
"One of the major problems with these bridges is that the concrete is crumbling away at the bottom, leaving the reinforcing bars exposed. Because they are made of metal, the bars rust," explained Heymsfield. "We wanted to find out if this problem was just cosmetic or presented serious structural issues."
The researchers examined 20 beams from 4 bridges and classified them as either new, good, average, poor or repaired, according to the condition of the concrete and reinforcing bars. Because they were built at different times, some of the precast concrete beams had reinforcing bars with greater concrete cover than others. In those cases, not much of the reinforcing rod was exposed.
"We found that in most cases, the problem may not be as serious as one might initially think from viewing these beams. Shear failure was found to be the typical failure mode for these beams from loading tests," said Heymsfield. "Fortunately, when the beams were manufactured, they were made using a higher concrete material strength than what was initially specified, providing additional shear strength. Our future research will include assuring that the reinforcing steel in these beams is adequately embedded and of sufficient strength."
The researchers are developing guidelines for field evaluation of these precast-concrete channel beams. This will help highway engineers determine the proper load limits for the bridge and its remaining life expectancy, as well as making decisions about on-site remedial improvements or replacement.
The study was conducted for the Arkansas Highway and Transportation Department.
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