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Plates In Collapsed Bridge Were Too Thin, NTSB Says

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By FREDERIC J. FROMMER, The Associated Press

Published: January 16, 2008

WASHINGTON - Steel plates connecting beams in the Interstate 35-W 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 Tuesday.

The connectors, called gusset plates, were roughly half the 1-inch thickness they should have been because of a design error, the 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. It is the critical factor that began the process of this collapse. That's what failed," Rosenker said.

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

The Minneapolis span was a steel-deck truss bridge that opened in 1967. Rosenker said it wasn't clear how the design flaw made it into the bridge because investigators couldn't find the design calculations.

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

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

However, the NTSB couldn't discount the possibility of similar errors in similar bridges, he said, and cautioned that states and contractors should look at the original design calculations for such bridges before they undertake "future operational changes."

Rosenker said 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 if 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 to determine, he said.

Rosenker said there was little chance that bridge inspectors would have noticed undersized gusset plates.

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