Some Bridges Seen Able to Take More Weight

Weight limits on some rural bridges can be increased 25% or more without compromising safety or inflicting damage, allowing heavy vehicles to use the spans and cut their operating costs, according to a civil engineer from the University of Missouri at Columbia.

Michael Barker said that, based on computerized testing he’s developed over the past year, many small bridges are conservatively posted — a fact known and discussed by truck operators and other highway engineers for years. Low weight limits cause trucks and school buses to make long detours around the bridges.

Mr. Barker said his system, using sensors, lasers, computers and other devices to measure and record bridge deflections, can prove that bridges can safely take higher loads. Part of the justification for raising load limits on four smaller bridges he tested is strength added by curbs and railings.

Deflection, along with lateral and longitudinal weight distribution, are among the eight factors measured and recorded by his system.

It can take as little as 90 minutes to run a deflection test on a bridge. It begins with attaching strain gauges on the girders and setting up laser drives. Then these detection devices are connected to a strain transfer box, which in turn is wired to a data acquisition box and then to the data acquisition vehicle. This is a converted motor home containing computer equipment, parked up to 200 feet away.

A brick hauler truck with a crane is loaded and weighed, then rolls at a crawl speed across the bridge as the electronic equipment measures and records deflections the truck causes. These data are later used in the mathematical equations Mr. Barker developed.

He said his testing system would work equally well with combination trucks, including tractor-trailers.

Mr. Barker spent more than a year to develop, build, test and fine-tune his system. The four bridges tested since last January are smaller county bridges and tests on another are planned.

Finding the money to finance additional testing is not easy, he indicated. State highway departments would rather spend funds on new construction than on repairs. But because of the potential economic benefits of less highway travel for their trucks, the lead mining industry in southeast Missouri helped finance tests in that area.

He has passed on his data to the Missouri Department of Transportation, which decides whether to raise posted limits. The day after his presentation he planned to present his findings to the state Legislature.

Several other states, including Delaware and Florida, have conducted similar tests, Mr. Barker said. But his aim is to develop a standardized testing system that can be used on different size bridges of various designs with one, two or more lanes.

For the full story, see the Dec. 21 print edition of Transport Topics. Subscribe today.