Collapse mystifies officials, experts

The 40-year-old bridge was considered a bit of a marvel when in 2001 it became the first of its kind in the U.S. to be equipped with an anti-icing system. A computerized system sprayed an anti-icing chemical based on sensors that collected data on pavement temperature, ice and moisture. About 140,000 vehicles traveled over the bridge each day.

The U.S. Department of Homeland Security said the collapse does not appear to be terrorism-related.

Kent Barnard, of the Minnesota Department of Transportation, said at the time of the bridge collapse, workers were doing some repair on the bridge decks, which was supposed to be completed this September.

"We were not redoing the bridge," he said. "I want to make that very clear. We were not rebuilding the bridge. We were doing some concrete repairs and we were doing some resurfacing. There was nothing dealing with the structure of the bridge."

The bridge was designed with a single 458-foot-long steel arch to avoid putting piers in the water so as not to interfere with river traffic. Gov. Tim Pawlenty said there were no reasons to think the bridge had any serious flaws:

"It's a somewhat unique structure in the way that it was designed," Pawlenty said. "It was inspected both in 2005 and 2006. There were no structural deficiencies identified in the bridge according to MnDOT. There were some cosmetic or minor repair items that needed some attention but no structural defects or deficits identified in the bridge."

Pawlenty said MnDOT told the state that any major improvements to the bridge would not be needed for many years to come.

"They notified us from an engineering standpoint the deck may have to be rehabilitated or replaced in 2020 or beyond, but no immediate or noted structural problems with the bridge," Pawlenty said.

In 2001, several years before the latest inspections, MnDOT commissioned a study to evaluate the stresses on the bridge. Researchers at the University of Minnesota Department of Civil Engineering released a report on the I-35 bridge based on a new way to analyze fatigue cracking.

Researchers monitored the trusses while dump trucks of known weights crossed the bridge in normal traffic. They also monitored the strains on the bridge over several months.

The report determined that the bridge should not have any problems with fatigue cracking in the foreseeable future. The report said as a result, MnDOT did not have to replace the bridge sooner than expected because of fatigue cracking.

Of the three evaluations in the past six years, none concluded there were problems with the bridge's structural integrity.

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