CHICAGO — Federal investigators on Thursday placed the blame for last year's deadly Minnesota bridge collapse on engineering design flaws that led steel plates to buckle under the weight of construction equipment and supplies, rather than on corrosion or a lack of upkeep.
The testimony came on the first day of a National Transportation Safety Board hearing in Washington, after more than a year of investigation into the Aug. 1, 2007, tragedy that killed 13 people and injured more than 100.
After the collapse of the Interstate 35W bridge, which crosses the Mississippi River in Minneapolis, investigators focused on the gusset plates that connect the support girders.
NTSB officials had raised concerns about the design of the steel plates months earlier, questioning whether they were too thin to hold up under the increased traffic and added weight of infrastructure improvements made since the bridge opened in 1967. The modifications, which included thickening the driving deck, further strained the bridge's weaker spots, investigators said.
According to Thursday's testimony, nearly 300 tons of construction equipment and materials being used to repair sections of the roadway were piled on top of the steel plates at the time the bridge gave way, dropping traffic into the Mississippi.
The workers had started putting gear and supplies onto the bridge that summer, but by Aug. 1 the pile stretched out over the plates, which were less than half the thickness required -- given the current traffic and other stresses -- and did not meet engineering specifications at the time the bridge was built.
"The collapse was the result of a serious design error," said Mark Bagnard, the NTSB's lead investigator in the case.
Several victims and relatives of those killed in the accident filed lawsuits Thursday against two firms the state had hired to do inspections and repair work on the bridge, claiming officials had failed to spot early signs of corrosion.
During testimony in Washington, NTSB investigator James Wildey told the board that corrosion and cracks were found on other parts of the bridge during regular inspections, but not on the gusset plates. Investigators, he said, discounted corrosion as the cause of the collapse because "there simply was no corrosion to identify in any areas associated with the fracture."
NTSB investigator Carl Schultheisz told the board that if the plates had been of adequate size, the bridge "would have been able to safely sustain these loads, and the accident would not have occurred."
The five-member board is scheduled to issue a final ruling on the investigation today, including guidance on how to avert similar problems.
The report could add fuel to the survivor lawsuits, which allege that URS Corp. -- a consulting firm hired by the Minnesota Department of Transportation to evaluate the bridge -- had failed to tell state officials about the structure's "dire condition" and that Progressive Contractors Inc., the company that had been repaving the bridge the day of its collapse, was negligent in how it stored its supplies on the span.
URS officials could not be reached for comment. Lawyers for Progressive Contractors issued a statement saying the construction firm wasn't responsible for the collapse. "Given the problems with the gusset plates, it was only a matter of time before the bridge collapsed. PCI just happened to be there when it did," the statement said.