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Continued: Bridge's de-icing system coming under scrutiny

Christine Beckwith, a MnDOT research engineer, wrote an article on the state's bridge anti-icing systems published in February by the American Public Works Association (APWA) in advance of a spring conference in St. Paul.

According to Beckwith, Minnesota had nearly a dozen anti-icing systems. Beckwith said the April APWA conference was to include a tour of the I-35E Lexington Bridge, which also uses potassium acetate to de-ice the span.

"It has been working very well since installation and requires surprisingly little maintenance and intervention," she said. Beckwith could not be reached for comment Wednesday.

Federal investigators are compiling a list of modifications made to the bridge and have contacted more than 300 witnesses.

Weight load wasn't unusual

The 575,000-pound weight of loads and construction vehicles on the bridge at the time of the collapse was the equivalent of about seven fully loaded semitrailer trucks that regularly went across the bridge.

The weight didn't appear to be near the loads a bridge that size would be designed for, said Kent Harries, an assistant professor of structural engineering and mechanics at the University of Pittsburgh. How the weight was distributed on the bridge could make a difference, Harries said, but he added: "One would not expect it to be distributed in a way that would cause a problem."

MnDOT metro construction engineer Terry Zoller estimated that debris removal at the bridge site could take one to two months. Debris that Safety Board investigators want to analyze is being taken downriver to the Bohemian Flats near the University of Minnesota, while other debris is being disposed of, Zoller said.

State officials also spoke about Tuesday's temporary closing of a northwest Minnesota bridge that crosses the Red River to North Dakota. As part of an inspection directive issued by Gov. Tim Pawlenty after the collapse, a crack was discovered in a support bracket for a girder in the Hwy. 11 bridge in Kittson County.

The crack was not in a fracture-critical area, officials said. It was a weld crack that officials were monitoring and decided to fix after noticing that it had spread, said Dan Dorgan, MnDOT bridge engineer.

The Associated Press contributed to this report. plouwagie@startribune.com • 612-673-7102 dbrowning@startribune.com • 612-673-4493

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