May 18, 1998 Writer: Aaron Hoover, email@example.com
Source: Max Sheppard, (352) 392-1570
Editor's Note: PHOTO AVAILABLE
GAINESVILLE, Fla. --- It's quiet, invisible and maddeningly complex.
Underwater erosion around bridge piers, a leading cause of bridge failure, has so confounded engineers they spend millions of dollars overbuilding bridges just to be safe.
Now, a University of Florida researcher is about to conduct large model tests to test equations aimed at cheaper, safer bridges. Coastal and oceanographic engineering Professor D. Max Sheppard's equations also could slash the cost of maintaining nearly 17,000 bridges nationwide thought at risk from the natural process known as "scour," experts say.
"We are anticipating some big things coming out of the studies," said Shawn McLemore, state drainage engineer for the Florida Department of Transportation (DOT) in Tallahassee. "The numbers that we're looking at for large structures we expect to be able to reduce by maybe a half, maybe down to a quarter of what we're used to, which you could imagine saves a significant amount of money."
Using research dollars from the Florida DOT and the Federal Highway Administration, Sheppard is conducting the tests at the nation's largest manmade water channel in Massachusetts on a scale never before attempted. While scour experiments have been conducted in small laboratory channels, Sheppard's work at the federally-owned flume in Turners Falls will rely 1,000 tons of sand, a 3-foot-wide artificial bridge pier and nine 50-horsepower outboard motors -- crucial to testing his equations for the most massive bridges.
"Good laboratory data exists for small scale structures, and there is a clear trend towards less relative scour depths for larger structures, but we will not know for sure until these tests are completed," Sheppard said.
The constant flow of water in rivers and coastal waters tends to erode or "scour" sand or soil away from bridge piers. Without support, the piers lose their ability to support the structure, increasing the risk the bridge will collapse under its own weight or fail to withstand a collision with a ship or barge.
In 1987, scouring created by a major flood caused a spectacular collapse of the then 33-year-old Schoharie Creek bridge near Albany, N.Y., that left 10 motorists dead. More recently, officials in Florida and North Carolina have closed bridges while workers made emergency scour-related repairs. At one bridge in Florida, divers discovered a hole so deep one pier was completely undermined, Sheppard said.
Recognizing the danger posed by scouring, the FHA more than a decade ago began requiring builders to use "conservative" equations to predict scour depth, Sheppard said.
While those equations have lead to safer bridges, they have added greatly to the cost of bridges throughout the country, Sheppard said. The equations he is testing at the Turners Falls flume are intended to more accurately predict scour with the goal of scaling back construction and improving safety, he said.
"Where these existing predictive equations are very conservative, you can save literally millions of dollars," he said. "In some of these bigger bridges, you could probably save at least $1 million on one bridge, because these bridges are very, very expensive."
The U.S. Geological Survey-owned test flume in Massachusetts is 21 feet deep, 20 feet wide and 125 feet long. For some tests, researchers plan to sink the 3-foot-wide, 19-foot-long pile into 6 feet of sand and observe scouring depths using video cameras and acoustic measuring devices, Sheppard said. To test the equations in fast-moving waters, they will reconfigure the flume to form a closed "race track" shape and accelerate the flow of the water using three rows of three outboard motors each.
If successful, Sheppard's equations will lead to savings not only for new bridges, but also on existing bridges, experts say.
Based on the highway administration's current equations, 16,998 bridges nationwide are thought at risk from scouring should a major storm arise, while 65,689 bridges have a potential to be at risk, federal figures show. One hundred fifty-nine Florida bridges are thought to be at risk, while 180 more could be at risk, state officials said. Improved equations could result in the removal of some bridges from the list, sparing the expense of monitoring or improving them, said Richard Kerr, a bridge inspection and evaluation engineer with the Florida DOT.
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