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NTSB Reveals Design Flaw on I-35W Bridge

Mon January 28, 2008 - National Edition
Construction Equipment Guide

WASHINGTON (AP) A design error led to undersized gusset plates used in the Interstate 35-W bridge in Minneapolis, which collapsed last year, killing 13 people and injuring 100, the National Transportation Safety Board said Jan. 15.

The agency stopped short of saying the gusset plates caused the collapse.

NTSB Chairman Mark V. Rosenker said the gusset plates, which connected steel beams, were roughly half the thickness they should have been.

Investigators found 16 fractured gusset plates from the bridge’s center span, he said.

Rosenker said the agency’s investigation found no evidence that cracking, corrosion or other wear “played any role in the collapse of the bridge.” They also found no flaws in the steel and concrete material used in the bridge.

The Minneapolis bridge 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.

There are approximately 465 other steel-deck truss bridges around the country. Rosenker said the safety board had no evidence that the deficiencies in the Minneapolis bridge design “are widespread or go beyond this bridge.”

But he cautioned that states and contractors should look at the original design calculations for such bridges before they undertake “future operational changes.”

About a week after the Aug. 1 collapse, in which the bridge plunged into the Mississippi River, the NTSB said it had found issues with the collapsed bridge’s gusset plates.

Meanwhile, Transportation Secretary Mary Peters is expected to issue an advisory later Tuesday urging states to check the gusset plates when modifications are made to a bridge — such as changes to the weight of the bridge or adding a guardrail, said a federal official with knowledge of the plans. The official spoke on condition of anonymity because Peters had not yet made the announcement.

Currently, such calculations are done for the entire bridge, but not down to the gusset plates, the official said.

Last August, Peters advised states to consider the additional stress placed on bridges during construction projects. An 18-person crew was working on the bridge when it collapsed.

Nearly three months later, Peters told a gathering in Washington of a “working theory” of a poorly designed gusset plate and a heavy load of construction materials.

Meanwhile, state lawmakers announced plans last month to spend up to $500,000 to hire legal counsel to aid in a legislative inquiry into the Minneapolis collapse.

The bridge was deemed “structurally deficient” by the federal government as far back as 1990.

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

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