Design flaw blamed for bridge collapse
Stress of 287 tons also contributed to Minn. accident
WASHINGTON - Undersized steel plates and the stress of 287 tons of stockpiled construction material were singled out yesterday as reasons for last year's collapse of a highway bridge in Minneapolis that killed 13 people and injured 145.
Federal safety investigators said the Aug. 1, 2007, collapse was unavoidable once gusset plates in the bridge's center span failed, dragging other sections and rush-hour commuters into the Mississippi River. The plates helped connect the bridge's steel beams.
Members of the National Transportation Safety Board, during a two-day hearing, criticized Minnesota transportation officials for allowing storage of construction material on the Interstate 35 West bridge.
Investigators told the board that Minnesota had no policy on how much construction material to allow on the bridge, so state officials did not object when contractors began storing the material a month before the collapse.
"That was a very high load on the bridge that day," board member Debbie Hersman said. She said she did not want states to use the absence of national guidelines on the issue as an "escape clause" when blame is assigned.
But Bruce Magladry, director of the NTSB's office of highway safety, stated flatly, "Had the gusset plates been properly sized, this bridge would still be there."
Added the board's acting chairman, Mark Rosenker, "We know at this precise moment that this terrible tragedy began some 40 years ago with the inadequate design of a gusset plate or series of gusset plates."
Investigators also ruled out any preexisting cracking or corrosion as factors in the accident.
The hearing quickly focused on the gusset plates, steel plates that are commonly fused to intersecting beams to reinforce the connection. The safety board as far back as January had identified design flaws in the plates as a critical factor in the collapse.
The bridge, finished in 1967, 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 that the nation's roads and bridges were crumbling.
Board members, though, 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," said board member Robert Sumwalt.
In January, Rosenker, 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 Representative 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.