Federal investigators told the National Transportation Safety Board that the collapse on Aug. 1, 2007, of the Interstate 35W bridge was unavoidable once gusset plates in the center span failed. When that happened, it dragged other sections of the bridge and rush-hour commuters into the water. The plates helped connect the bridge's steel beams.
Board members criticized Minnesota transportation officials for allowing the storage of 287 tons of construction materials for lane-widening on the bridge. The materials were stored above the gusset plates that fractured. But board members said it was not possible to determine if the materials alone - or factors such as weather and traffic, combined with the added weight - pushed the plates to a breaking point.
"Had the gusset plates been properly sized, this bridge would still be there," said Bruce Magladry, director of the NTSB's office of highway safety.
Investigators said the half-inch thick plates were inadequate to handle traffic and other stress factors and did not meet engineering guidelines when the bridge was built in 1967. The safety board, as far back as January, had identified design flaws in the plates as a critical factor in the collapse.
The board's final ruling was expected Friday.
The bridge was called "fracture critical." That meant a failure of any number of structural elements would bring down the entire bridge.
Safety board investigator Jim Wildey said there is "nothing inherently dangerous" about this type of bridge, as long as each structural element is designed to withstand the expected stress loads.
From the start, the investigation has been laced with politics.
Democrats in Minnesota heaped criticism on the state's Republican governor, Tim Pawlenty, and Democrats in Congress said the accident showed the nation's roads and bridges were crumbling.
Board members pledged to keep politics out of their deliberations.
"We are here, not to protect other agencies or other organization, we are not here to point fingers or to lay blame, or find fault. ... We are not here to push personal agendas. We're here to seek the truth," board member Robert Sumwalt said.
In January, Rosenker, who is a Republican, said that design error was a "critical factor" in the collapse. He also said there was little chance that state bridge inspectors would have noticed undersized gusset plates.
Pawlenty took that as a measure of vindication because the initial focus had been on his administration's program for maintaining bridges.
Rosenker's comments angered Rep. Jim Oberstar, a Minnesota Democrat who heads the House Transportation and Infrastructure Committee. Oberstar said the early pronouncement committed the safety board to a finding that might not bear out with further investigation.
In St. Paul, Minn., a group of collapse survivors gathered at a National Guard armory to watch the NTSB presentation via the Internet. Michele McLane, 41, said the hearing was "the last door to close for me."
McLane was able to drive her car safely off the northern end of the span. But she said she was emotionally traumatized. "I finally get it now," she said. "I finally understand."
During Barack Obama's campaign for the White House, he cited the bridge collapse and called for spending more on crumbling highways, bridges and tunnels.
In July, the House passed legislation authorizing an additional $1 billion next year to rebuild structurally deficient bridges on the national highway system. The bill would require states to come up with repair plans for troubled bridges.
The Senate has yet to act on the bill. If no action is taken during a lame-duck session that starts next week, lawmakers would have to start anew on the legislation in January.
