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New I-35W Bridge Spans Technological Divide

Sat October 11, 2008 - National Edition
Steve Karnowski - ASSOCIATED PRESS

MINNEAPOLIS (AP) More than a year after a key Minneapolis bridge collapsed and killed 13 people, state troopers prepared to lead motorists in a slow procession Sept. 18 morning across the new span that reconnects Interstate 35W over the Mississippi River.

Crews were scheduled to remove barricades that have stood since the old bridge fell Aug. 1, 2007, reopening a major artery leading in and out of Minneapolis that carried 140,000 trips a day.

The new bridge contains hundreds of sensors that will collect a stream of data. 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 of 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 $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 also are 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. They also will watch for corrosion from road salt.s

A system of sensors and cameras will feed data on traffic flow — including speeds, accidents, stalls and other disruptions — to a management center. Other sensors will activate an anti-icing system when necessary, and security sensors are meant to detect intruders in unauthorized areas, such as the hollow concrete box girders.

The data will feed into computers in a control room near the bridge, Phipps said. From there, engineers at the Minnesota Department of Transportation and researchers at the University of Minnesota can download it for analysis.

Catherine French, a civil engineering professor at the University of Minnesota, has worked with the developers of the system and will be among the researchers analyzing the data. The number of sensors on the bridge, and the fact that they were installed from the start, make this project stand out, she said.

“It is kind of on the cutting edge,” French said.

The main value will be the insight the system provides for building future bridges. Engineers will be able to compare the bridge’s behavior to models they’ve developed, she said.

The National Transportation Safety Board has scheduled a hearing in November to discuss its investigation into what caused the old bridge to collapse. In January, NTSB Chairman Mark Rosenker pointed to a design error in the plates that helped connect the bridge’s steel beams as a “critical factor.”

The NTSB also has focused on the weight of construction materials that were on the bridge for a resurfacing project.

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