Rebuilt bridge uses sensors

Rebuilt bridge uses sensors

MINNEAPOLIS (AP) — A stream of data will flow from hundreds of sensors on a new bridge this morning when commuters on Interstate 35W drive across the Mississippi River for the first time since the old span collapsed more than a year ago.

The purpose of the “smart bridge” technology isn’t to warn of another impending disaster; it’s to detect small problems before they become big ones, said Alan Phipps, design manager for the project with Figg Engineering Group Inc. of Tallahassee, Fla.

“What these sensors are for, it’s like going to your doctor for your health checkup,” Phipps said. “It’s to ensure you’re maintained in top shape so you never get close to having a serious problem.”

The new bridge is due to open early today when crews remove barricades that have stood since the old bridge collapsed Aug. 1, 2007, killing 13 people and injuring 145. State troopers will then lead a slow procession of motorists across the bridge in both directions to reopen a major artery that carried 140,000 trips a day.

The $234 million bridge was completed on budget and more than three months ahead of the Dec. 24 deadline. That means the contractors — led by the team of Flatiron Construction Corp. of Longmont, Colo., and Manson Construction Co. of Seattle — should get a bonus close to the contract maximum of $27 million, though the actual amount hasn’t been determined.

There are also more visible differences between the new bridge and old. The new bridge is concrete instead of steel and is built with redundant systems so that if one part fails, it won’t collapse. The old bridge, finished in 1967, was called “fracture critical,” which meant that a failure of any number of structural elements would bring down the entire bridge.

Within the concrete of the new bridge are embedded 323 sensors that will generate a record of how it handles the stresses and strains of traffic and Minnesota’s harsh climate. The data will help engineers maintain the bridge and advance the art of bridge design, Phipps said.

The sensors will measure how the bridge handles loads and vibrations and how it expands and contracts as Minnesota alternates between frigid winters and steamy summers, as well as watch for corrosion from road salt.