November 26, 2014

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Video shows bridge collapse took four seconds

By Jon Hilkevitch
Chicago Tribune

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 Highway 35W bridge, linking Minneapolis and St. Paul, breaking apart under tremendous force and crashing into the Mississippi River during Wednesday evening’s rush period.

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

Dozens of vehicles plummeted 64 feet into the Mississippi 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 up to 30 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.

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 surfacing 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 were also 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.

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.

Other engineering experts said that the 1960s design of steel-arched bridges did not contain structural redundancies, meaning that if one component fails, the whole structure is in jeopardy because the weight does not shift to other points on the bridge.

“We know that we would not build a bridge like this today,” said Kent Harries, an assistant engineering professor at the University of Pittsburgh.