November 18, 2008

Undersized steel plates cited in failure of Minnesota’s Interstate 35W bridge

WASHINGTON

The metal plates holding the bridge together were simply too small, and then workers piled on nearly 300 tons of construction material.

It all collapsed and 13 people died.

Safety investigators have singled out the undersized steel plates as the chief cause of last year’s deadly collapse of a highway bridge in Minneapolis.

Contractors working on the bridge had also stockpiled material on the centre span over the Mississippi River, and that additional weight contributed to the collapse that killed 13 and injured 145.

Federal investigators told the National Transportation Safety Board that the collapse of the Interstate 35W bridge on August 1, 2007, was unavoidable once gusset plates in the centre span failed.

Investigators focused on the U-10 gusset plates, which were designed at only half the required thickness.

They also discussed the construction materials on the centre span over the Mississippi River.

“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.

Board members criticized Minnesota officials for allowing 287 tons of construction materials to be stockpiled on the bridge’s centre.

During rush hour that evening, the bridge shuddered and then dropped into the river.

Investigators told the board that Minnesota didn’t have a policy on weight added to bridges for construction projects.

Minnesota’s transportation commissioner, Tom Sorel, attended the hearing and told reporters: “We’ve changed our specifications to make sure that doesn’t happen again.”

In a statement, Minnesota Gov. Tim Pawlenty, who had come in for some criticism on bridge upkeep, noted that the board ruled out corrosion or cracking.

He said the design flaw “was unrelated to subsequent inspections or maintenance of the bridge.”

Board member Debbie Hersman said a Minnesota transportation official had noticed bowing, or bending, of the U-10 gusset years before the collapse, and the state took no action.

Associated Press

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