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One Year After Bridge Fell, New Span Opens

Motorists make their way across the new Interstate 35W bridge over the Mississippi River Thursday morning Sep. 18, 2008 after a slow procession lead by state troopers and state highway trucks marked the opening of the new bridge in Minneapolis. The old bridge collapsed on Aug. 1, 2007 killing 13 people and injuring 145. The new bridge took 11 months to construct and cost about 234 million dollars. (AP Photo/Dawn Villella)
AP Photo/Dawn Villella
The new Interstate 35W bridge opened for traffic early Thursday, a little more than a year after the last one collapsed into the Mississippi River.

Traffic was heavy as a mix of cars, motorcycles, trucks and buses lined up to cross the new bridge, which reopens a major artery leading in and out of Minneapolis. Many vehicles honked their horns and a few motorists waved flags as they made the first trip across the span.

The old bridge fell Aug. 1, 2007, killing 13 people and injuring 145 others.

The new $234 million bridge contains hundreds of sensors that will collect 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 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 bridge was completed on budget and more than three months ahead of the Dec. 24 deadline. That means the contractors 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.

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 into 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," she 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 has also focused on the weight of construction materials that were on the bridge for a resurfacing project.
By Associated Press Writer Steve Karnowski