WASHINGTON, D.C.—The National Transportation Safety Board (NTSB) determined that the probable cause of the collapse of the I-35W Bridge in Minneapolis was inadequate load capacity because of a design error by Sverdrup & Parcel and Associates, Inc. (now part of Jacobs Engineering Group), of the gusset plates at the U10 nodes. The plates failed under a combination of substantial increases in the weight of the bridge that resulted from previous modifications, and 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 inadequate design review by federal and state transportation officials. Also contributing to the failure 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. "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."

During its investigation, the Safety Board learned that 24 under-designed gusset plates, which were about half the thickness of properly sized gusset plates, escaped discovery in the original review process and were incorporated into the design and construction of the bridge.

The NTSB examined other possible collapse scenarios—such as corrosion damage found on the gusset plates at the L11 nodes and elsewhere, fracture of a floor truss, pre-existing cracking in the bridge deck truss or approach spans, temperature effects, and shifting of the piers—and found that none of these played a role in the accident.

As a result of its investigation, the NTSB made nine recommendations to the Federal Highway Administration and the American Association of State Highway and Transportation Officials dealing with improving bridge design review procedures, bridge inspection procedures, bridge inspection, training, and load-rating evaluations. Recommendations include developing and implementing a bridge design quality assurance/quality control program, and modifying approved bridge inspector training to address inspection techniques and conditions specific to gusset plates.

A synopsis of the NTSB’s report, including the probable cause, conclusions, and recommendations, is available online at www.ntsb.gov under the "Board Meetings" link.