Steel Plates in Collapsed Bridge Inadequate from Design Error, NTSB Says

A design error that caused inadequate steel plates to be used may have caused the collapse of the I-35W bridge into the Mississippi River last year in Minneapolis, NTSB investigators announced Jan. 15. About 1,000 feet of the deck truss fell into the river, leaving 13 people dead and 145 others injured.

"Although the board's investigation is still ongoing and no determination of probable cause has been reached, interim findings in the investigation have revealed a safety issue that warrants attention," said National Transportation Safety Board Chairman Mark Rosenker. "During the wreckage recovery, investigators discovered that gusset plates at eight different joint locations in the main center span were fractured. The board, with assistance from the [Federal Highway Administration], conducted a thorough review of the design of the bridge, with an emphasis on the design of the gusset plates. This review discovered that the original design process of the I-35W bridge led to a serious error in sizing some of the gusset plates in the main truss."

Undersized gusset plates were found at eight of the 112 nodes (joints) on the main trusses of the bridge. These 16 plates, two at each node, were about half the thickness required -- too thin to provide the margin of safety expected in a properly designed bridge, according to NSTB, which announced the findings at a news conference.

The board issued a safety recommendation to FHWA that said "for all non-load-path- redundant steel truss bridges within the National Bridge Inventory, require that bridge owners conduct load capacity calculations to verify that the stress levels on all structural elements, including gusset plates, remain within applicable design requirements, whenever planned modifications or operational changes may significantly increase stresses."

NTSB officials said there is no evidence to suggest the deficiencies in the various design review procedures associated with this bridge are widespread or go beyond this particular bridge, which they said is the sole bridge failure of this type of which the safety board is aware. "The safety board has issued this recommendation, at this time, to ensure that the original design calculations for other bridges of this type have been made correctly, before any planned modification or operational changes are accomplished affecting such bridges and before any additional stresses are placed on them," Rosenker said.

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