By Frederic J. Frommer and Joan Lowy
Associated Press
WASHINGTON — A federal safety board is expected to decide what caused last year’s deadly collapse of a Minneapolis highway bridge, a day after pointing to design flaw as the origin.
The National Transportation Safety Board also was to make recommendations today aimed at avoiding a repeat of the collapse, which killed 13 people and injured 145.
On Thursday, the first day of a two-day hearing, investigators said the Interstate 35W bridge collapse was unavoidable once gusset plates holding the center span together failed.
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When that happened, the span buckled, dragging other sections of the bridge and rush-hour commuters into the water.
The bridge was called "fracture critical." That meant a failure of any number of structural elements would bring down the entire bridge.
While investigators homed in on the U-10 gusset plates, which were designed at only half the required thickness, they also discussed the construction materials stockpiled on the center span over the Mississippi River.
On the day of the collapse, Aug. 1, 2007, additional construction materials were stockpiled on the bridge’s center, bringing the total to 287 tons.
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.
