August 9, 2007 in Nation/World

Connecting plates may have played role in bridge collapse

Alan Levin USA Today
 

At a glance

Possible design flaw

» The steel-truss underside of the Interstate 35W bridge in Minneapolis was built with hundreds of gusset plates – metal plates of various sizes where vertical, horizontal or diagonal beams are tied together.

» The National Transportation Safety Board said Wednesday that its investigators “observed a design issue” with the plates but wouldn’t specify the location or explain the potential flaw.

» A source familiar with construction work being done on the 40-year-old bridge said investigators are questioning whether the plates should have been thicker in the original design.

Minneapolis Star Tribune

MINNEAPOLIS – Overburdened steel support plates may have played a role in the collapse of a highway bridge laden with tons of construction equipment, prompting the nation’s transportation chief to warn states Wednesday about excessive loads on bridges.

Citing questions raised by federal investigators about the stress levels on the Interstate 35W bridge, Transportation Secretary Mary Peters said in a statement that “it is vital that states remain mindful of the extra weight construction projects place on bridges.”

Peters cautioned that the National Transportation Safety Board (NTSB) hasn’t found a definitive cause of the bridge’s collapse Aug. 1 that killed at least five people. But stress on gusset plates “may have been a factor,” she said.

Gusset plates are flat steel pieces that connect steel beams together and play a key role in keeping girders in place. NTSB Chairman Mark Rosenker said Wednesday that investigators found a “design issue” with the plates.

There were thousands of gusset plates on the bridge, some as big as 5 feet, the Minnesota Department of Transportation said.

The NTSB statement did not specify the problem or where the flawed gussets were on the 40-year-old bridge. The safety board expects to take months to study the wreckage and analyze computer simulations before pinpointing the cause of the collapse. In addition to the confirmed dead, eight others remain missing and are presumed dead.

Flawed gussets could cause severe problems on a bridge, said University of California, Berkeley, civil engineering professor William Ibbs. “If you were to lose a gusset plate in this type of design, you could have an instantaneous failure,” Ibbs said.

The bridge roadway was being resurfaced and tons of construction material – gravel, sand, cement and trucks – arrived the day of the collapse. The safety board is still trying to estimate how much the material weighed.

Investigators have also found several fractures in the bridge using special high-resolution cameras. None of the fractures appear to have been where the failure began, the NTSB said.

Meanwhile, an influential House Democrat on Wednesday called for a massive federal effort to repair the nation’s aging bridges. Rep. James Oberstar, a Minnesota congressman who chairs the Transportation Committee, said he would sponsor legislation to spend $25 billion over the next three years on upgrading bridges.

The bill would fund the repairs with a 5-cent-per-gallon increase in the tax on gasoline, which has been opposed by the Bush administration.


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