Steel is called key to bridge collapse

WASHINGTON - Steel plates connecting beams in the Interstate 35W bridge in Minneapolis were too thin by half and fractured - "the critical factor" in the collapse that killed 13 people and injured 145, the National Transportation Safety Board said yesterday.

The connectors, called gusset plates, were roughly half the one-inch thickness they should have been because of a design error, NTSB Chairman Mark Rosenker said. Investigators found 16 fractured gusset plates from the bridge's center span.

"It is the undersizing of the design which we believe is the critical factor here," Rosenker said at a news conference. "It is the critical factor that began the process of this collapse. That's what failed."

What caused the bridge to collapse during rush-hour traffic in the early evening of Aug. 1 - "the straw that broke the camel's back," as Rosenker put it - was not yet known, he said. A final report by the NTSB was expected this fall.

The Minneapolis span was a steel-deck truss bridge that opened in 1967. Rosenker said it was not clear how the design flaw was incorporated in the bridge because investigators could not find the design calculations.

The bridge was called "fracture critical," or lacking redundancies, meaning that a failure of any number of structural elements would cause the entire span to collapse.

Rosenker said the safety board had no evidence that the deficiencies in the Minneapolis bridge design "are widespread or go beyond this bridge."

However, the NTSB could not discount the possibility of similar errors in other like bridges, he said, and cautioned that states and contractors should look at the original design calculations for such bridges before they undertook "future operational changes." The NTSB issued a safety recommendation to the Federal Highway Administration suggesting that the agency require bridge owners to do so.

U.S. Transportation Secretary Mary Peters called on states to calculate how changes in bridge weight and capacity or evolving bridge conditions would affect gusset plates. "With a few calculations, we can help reassure travelers that our bridges remain safe," she said in a statement.

Rosenker noted that structural weight had been added to the Minneapolis bridge in two major renovations, in the 1970s and 1990s. "When they added the weight, they didn't realize they were bringing the margins of safety down to where they didn't exist anymore," he said.

Rosenker said that construction materials on the bridge the day it collapsed, which were part of a resurfacing project, added about 300 tons and were on the same side where failure of the bridge began.

Asked whether the construction was the tipping point, Rosenker said: "I'm not ruling it in, and I'm not ruling it out." That will be left to the final report, he said.

Rosenker said there was little chance that bridge inspectors would have noticed undersized gusset plates. "No one recognized that you could undersize a gusset plate," he said.

The Minneapolis bridge was deemed "structurally deficient" by the federal government as far back as 1990, and the state's maintenance of the structure has been questioned. But Rosenker said the NTSB investigation found no evidence that cracking, corrosion or other wear "played any role in the collapse of the bridge." Investigators also found no flaws in the steel and concrete material used in the bridge.

Minnesota is reviewing 23 state bridges with truss designs to make sure their current load ratings fit with the gusset design. Officials hope to complete the state reviews by June at a cost of $500,000.

Late last year, President Bush signed a spending bill that included $195 million to help replace the bridge. That came on top of the $178.5 million the federal government already had given Minnesota for the project.

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