NTSB cites bridge design errors

WASHINGTON, D.C.—The National Transportation Safety Board (NTSB) last week issued a safety recommendation that addresses a design issue with the I-35W bridge that collapsed into the Mississippi river on Aug. 1, 2007. In this accident, approximately 1,000 feet of the deck truss fell into the river and, as a result, 13 people died and 145 people were injured.

"Although the Board’s investigation is still on-going and no determination of probable cause has been reached, interim findings in the investigation have revealed a safety issue that warrants attention," says NTSB Chairman Mark V. 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 FHWA, 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 gusset plates (two at each node) were roughly half the thickness required and too thin to provide the margin of safety expected in a properly designed bridge.

NTSB made the following recommendation to the Federal Highway Administration (FHWA) and states: "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."

Nevertheless, the NTSB says there is no evidence to suggest that the deficiencies in the various design review procedures associated with the I-35W bridge are widespread or even go beyond this particular bridge. In fact, the Safety Board says this is the only bridge failure of this type of which it 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 explains.

The probable cause of the I-35W bridge accident will be determined when the final report is presented to the NTSB, currently estimated to be sometime before the end of the year.

The Board’s recommendation letter is available online at www.ntsb.gov/Recs/letters/2008/H08_1.pdf (60 KB PDF).