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Video, computer model to aid investigators

MINNEAPOLIS -- It will take months to identify hidden defects suspected in the catastrophic collapse of Minnesota's busiest bridge, but the emergence Thursday of a dramatic security video and a computerized model of the span ultimately "will tell...

MINNEAPOLIS -- It will take months to identify hidden defects suspected in the catastrophic collapse of Minnesota's busiest bridge, but the emergence Thursday of a dramatic security video and a computerized model of the span ultimately "will tell us where the failure began," the leader of the investigation said.

The chilling video, captured by a security camera at an industrial site, shows the Interstate 35W bridge breaking apart under tremendous force and crashing into the Mississippi River during Wednesday evening's rush hour.

It took only four seconds for the massive steel-arched bridge to buckle and fall.

Dozens of vehicles plummeted 64 feet into the Mississippi River when the eight-lane bridge gave out. Divers resumed the dangerous search Thursday for bodies trapped under tons of twisted debris. The official death toll stood at four victims, but 8 people remained missing, officials said. About 80 survivors were treated for injuries.

Meanwhile, federal investigators said one focus of their investigation is whether vibrations and the weight of heavy equipment during bridge repairs that were being conducted Wednesday contributed to the worst U.S. bridge accident in almost 25 years.

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In addition, the second-guessing began over whether the 40-year-old bridge should have been closed several years ago amid signs of serious corrosion that may have put excessive stress on the structure. Early signs of problems emerged in 1990, officials at the Minnesota Department of Transportation reported Thursday. Annual inspections found the bridge to be "structurally deficient," but safe.

The emergence of the video, which was provided to the Minneapolis Police Department, "is the equivalent of recovering the cockpit flight recorder" from a downed plane, said Mark Rosenker, chairman of the National Transportation Safety Board, which is leading the investigation.

Authorities also were buoyed by the news that a Federal

Highway Administration employee had produced an exact computer software model of the Interstate 35W bridge when he was a Ph.D. candidate at the University of Minnesota. The model will be used in a failure-analysis study to map out every edge and surface of the 1,900-foot-long bridge structure, with the goal of pinpointing what went wrong, officials said.

The analysis involves removing selected parts of the bridge structure until the bridge falls down, revealing the components that failed, safety board officials said.

"What we have found today is going to help us significantly to speed up the analysis by months," said Rosenker, who sounded decidedly more optimistic than he did earlier in the day before the bridge-collapse video and computer model were made available to the 19-member investigation team.

Minnesota Gov. Tim Pawlenty announced that he is calling in outside experts, separate from the official investigation being led by the NTSB, to decide, "Was it an appropriate recommendation to keep the bridge open?"

State transportation officials disclosed that serious corrosion of bearings and steel inhibited the bridge from moving as freely as it was designed to under the forces of vehicles, river currents and wind. Despite the decay and fatigue cracking found in the deck truss and other places in the structure, officials deemed the bridge fit for service and decided to continue with annual inspections.

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Some independent experts said the evidence of buckling indicates fatigue of the steel or a corrosion-related crack, which is common in older bridges and difficult to detect during inspections.

"The bridge must have been near a state of collapse for some time, and the construction might have contributed to its failure," said Zdenek Bazant, a professor of civil and environmental engineering at Northwestern University. Bazant said he suspects there may have been a hairline crack or fatigue in the steel joints near bridge supports, leading to the buckling.

He said bridge inspections are not nearly as complete or sophisticated as the inspections that aircraft undergo.

Meanwhile, a former NTSB chairman said he was intrigued by a 2001 University of Minnesota study that found signs of "fatigue cracking" in the bridge supports, though he said that a later report apparently concluded that the bridge was in no immediate danger and did not need major repairs.

"I think that decision is going to come under new scrutiny," former chairman Jim Burnett said.

According to Federal Highway Administration data for 2006, Minnesota ranks relatively well for bridge deficiencies when compared to other states, Washington, D.C., and Puerto Rico.

The data show that 10 percent of about 13,000 bridges in the state were rated deficient, ranking 38th.

Just 3 percent of the 1,657 National Highway System bridges in Minnesota were rated deficient last year, and 10 percent of the Non-National Highway bridges were deficient. Minnesota ranked 40th and 41st, respectively, in those categories.

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The Minneapolis Star Tribune contributed to this report.

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