Possible design flaw found in collapsed bridge

MINNEAPOLIS - A week after a deadly bridge collapse, U.S. Navy divers cut through tangled debris with underwater torches and saws on Wednesday in the search for victims, while investigators identified a possible flaw in the 40-year-old span’s design.

At the dive site, two large cranes were ready to go. But they sat idle as divers returned to the water about 7 a.m., doing “a very meticulous, hand-over-hand search of the scene,” said their spokesman, Senior Chief Dave Nagle. Officials said they expected removal of heavy debris to begin later than expected to give the divers time to do their work.

Navy and FBI dive teams are trying to go deeper into the debris of the Interstate 35W bridge than the local dive teams that have worked since the Aug. 1 collapse, police Capt. Mike Martin said. He said he expects it to be at least a week before cranes start regularly hauling out large pieces of debris.

The FBI team had to abandon the use of the larger of its two unmanned submarines, agent Paul McCabe said Wednesday. The remote-controlled vehicle — equipped with a camera, sonar, lights and a grabbing arm — was too big to maneuver amid the unstable, twisted bridge wreckage and vehicles in the cloudy water, he said.

Instead, FBI divers will try their smaller sub, a shoebox-size vehicle equipped only with lights and a camera. Its smaller thrusters make it more difficult to navigate the Mississippi River’s stiff river currents.

Debris removal had been expected to begin this week. The State Patrol said 88 vehicles have been located at the collapse site, including those in the Mississippi River.

Potential design flaw found
Meanwhile, the company that was working on the bridge rejected a report that one of its workers had noticed unusual swaying of the bridge in the days before the collapse. Progressive Contractors Inc. had said that it didn’t believe any of its work contributed to the bridge failure, but the company hadn’t responded directly to claims of wobbling.

“We have now met with every single worker who was on the bridge when it collapsed,” Tom Sloan, vice president of the company’s bridge division, said in a statement. “None of them observed or reported any unusual swaying.”

The National Transportation Safety Board has said reports of wobbling will be part of its investigation. The agency issued a brief update of its work Wednesday, saying helicopter observations had found several “tensile fractures” in the superstructure on the north side of the bridge, but nothing that appeared to show where the collapse began.

Also Wednesday, NTSB investigators said they found a potential design problem with gusset plates, steel plates that tie together angled steel beams of the bridge’s frame.

Investigators are trying to verify loads and stresses on these plates at specific locations as well as the materials used to construct them.

Construction weight an issue?
One possible stress may have been the weight of construction equipment and materials on the bridge when it collapsed, the U.S. Transportation Department said in a statement on Wednesday.

More bridge collapse news Interactive map: Check your bridges Heavier vehicles take toll on roads E-mails show officials’ bridge reactions College offers tuition for collapse ‘hero’  |       Video: 'Bridge doctor' tries to prevent disaster FirstPerson: Share your images, video |   Gallery   Discuss your reactions to the collapse

Safety board and transportation officials said the increased focus on steel plates is preliminary. They would not say exactly where they were located on the bridge or whether their failure alone would have caused the collapse.

“We are continuing to make progress on this investigation, and each area of inquiry gets us closer to ultimately determining the cause of this tragedy,” safety board Chairman Mark Rosenker said in a statement.

Federal transportation officials were concerned enough with the NTSB finding to issue an advisory to states to “carefully consider” the weight of construction equipment used in bridge projects.

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