November 16, 2008 | Mark Paradies

NTSB: Minnesota Bridge Collapse Due to Design Flaw

Here’s a link to the NTSB press release:

Here’s an excerpt:


Washington, DC – The National Transportation Safety Board has determined the probable cause of the collapse of the I-35W bridge in Minneapolis, Minnesota, was the inadequateload capacity, due to a design error by Sverdrup & Parcel and Associates, Inc., of the gusset plates at the U10 nodes, which failed under a combination of (1) substantial increases in the weight of the bridge, which resulted from previous modifications, and (2) the traffic and concentrated construction loads on the bridge on the day of the accident.

Contributing to the design error was the failure of Sverdrup & Parcel’s quality control procedures to ensure that the appropriate main truss gusset plate calculations were performed for the I-35W bridge and the inadequate design review by federal and state transportation officials.

Also contributing was the generally accepted practice among Federal and State transportation officials of giving inadequate attention to gusset plates during inspections for conditions of distortion, such as bowing, and of excluding gusset plates in load rating analysis.

“We believe this thorough investigation should put to rest any speculation as to the root cause of this terrible accident and provide a roadmap for improvements to prevent future tragedies,” said NTSB Acting Chairman Mark V. Rosenker.  “We came to this conclusion only through exhaustive efforts to eliminate each potential area that might have caused or contributed to this accident.

“Bridge designers, builders, owners, and inspectors will never look at gusset plates quite the same again, and as a result, these critical connections in a bridge will receive the attention they deserve in the design process, in future inspections, and when bridge load rating analyses are performed.  By addressing all three areas in our recommendations, we are hopeful that industry and government bodies will take appropriate action and the American people can continue to have confidence in the safety of our nation’s bridges,” he added.

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