The National Transportation Safety Board announced its findings late last week. It said the design error led to inadequate load capacity 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. The NTSB had previously reported that an estimated 99 tons of sand were putting stress on the bridge when it collapsed in August 2007.
Also contributing to the collapse, the NTSB said, 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."
About 6:05 p.m. Central Daylight Time on Wednesday, August 1, 2007, the eight-lane, 1,907-foot-long I-35W highway bridge over the Mississippi River in Minneapolis experienced a catastrophic failure in the main span of the deck truss. As a result, 1,000 feet of the deck truss collapsed, with about 456 feet of the main span falling 108 feet into the 15-foot-deep river. A total of 111 vehicles were on the portion of the bridge that collapsed. Of these, 17 were recovered from the water. As a result of the bridge collapse, 13 people died, and 145 people were injured.
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.
A synopsis of the Board's report, including the probable cause, conclusions, and recommendations, is available on the NTSB's website, www.ntsb.gov, under "Board Meetings." The Board's full report will be available on the website in several weeks.