Bridge Collapse

MINNEAPOLIS (AP) ― A stream of data will be flowing from hundreds of sensors in the new Interstate 35W bridge Thursday morning when commuters drive across it for the first time since the old bridge collapsed into the Mississippi River 13 months 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 at 5 a.m., when crews remove the 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 new 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 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 an extensive 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. They carry technical names that include:

--"Strain gauges" and "accelerometers" to measure how the bridge handles the loads and vibrations that the heavy traffic will put on it.

--"Linear potentiometers" at the expansion joints and bearings to measure how the bridge expands and contracts as Minnesota alternates between its frigid winters and steamy summers. Phipps likened them to "electronic rulers."

--"Chloride penetration sensors" in the bridge deck that will watch for corrosion. They will monitor how deeply deicing salt penetrates into the deck over time by measuring how much corrosion the salt causes in "sacrificial" steel bars implanted at various depths, so that engineers can better determine when the deck needs replacing.

A system of sensors and cameras will feed data on traffic flow -- including speeds, accidents, stalls and other disruptions -- to a traffic management center. Other sensors will automatically activate the bridge's anti-icing system when weather and temperature conditions are ripe for the formation of ice. And security sensors are meant to detect intruders who might try to get into unauthorized areas such as the access doors to the bridge's hollow concrete box girders.

During construction, sensors were also placed in the fresh concrete to monitor the curing process to help determine when it was strong enough to proceed to the next steps. Phipps said knowing that allowed the work to proceed faster.

Phipps said these sorts of monitoring technologies have been used on limited scale elsewhere, mostly on existing bridges. He predicted that more and more bridge owners will want them as a way to more effectively maintain the health of their structures.

The data will feed into a bank of 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 over the long term. She said the large number of sensors on the bridge, and the fact that they were installed from the start, make this project stand out.

"It is kind of on the cutting edge," she said.

French said the main value will be the insights the system provides for building future bridges. Engineers will be able to compare the bridge's actual behavior to the models they've developed and refine those models accordingly, she said.

To calibrate the sensors, crews drove eight 25-ton MnDOT sand trucks onto the bridge Sunday night, concentrating as many as 200 tons in various locations so the instruments could measure how the structure responded, Phipps said. Engineers were pleased to see that even the gauges deep in the foundation elements were able to detect the loads, he said.

Because the four hollow box girders that support the traffic lanes are big enough to walk through, it will be simple enough to upgrade the monitoring systems in the coming decades by mounting new sensors inside. That's important, since the bridge is designed to last more than 100 years.

"I'd be disappointed if a hundred years from now it's still the same technology," Phipps said.

The National Transportation Safety Board has scheduled a public 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 gusset 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.

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The original I-35W bridge over the Mississippi River opened in November 1967 and was 1,907 feet in length. The replacement bridge opened in September 2008 and measures 1,216 feet in length.

 

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