The National Transportation Safety Board on Friday, Nov. 14, announced that it has determined that the probable cause of the collapse of the Interstate 35W bridge in Minneapolis was the inadequate load capacity of the gusset plates, due to a design error by engineering firm Sverdrup & Parcel and Associates.
NTSB said the gusset plates at the U10 nodes failed under a combination of substantial increases in the weight of the bridge, which resulted from previous modifications, and 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, as well as inadequate design review by federal and state transportation officials, NTSB ruled.
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, according to NTSB.
“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.”
On Aug. 1, 2007, the eight-lane 1,907-foot-long I-35W highway bridge over the Mississippi River 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, NTSB said it learned that 24 underdesigned 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.
“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,” Rosenker said. “By addressing all three areas in our recommendations, we are hopeful that industry and government bodies will take appropriate action and that the American people can continue to have confidence in the safety of our nation’s bridges.”
According to NTSB, on the day of the collapse, roadwork was under way on the I-35W bridge, and four of the eight travel lanes — two outside lanes northbound and two inside lanes southbound — were closed to traffic. In the early afternoon, construction equipment and construction aggregates — sand and gravel for making concrete — were delivered and positioned in the two closed inside southbound lanes. The equipment and aggregates, which were being staged for a concrete pour of the southbound lanes that was to begin about 7 p.m., were positioned toward the south end of the center section of the deck truss portion of the bridge near node U10 and were in place by about 2:30 p.m.
Shortly after 6 p.m. a lateral instability at the upper end of the L9/U10W diagonal member led to the subsequent failure of the U10 node gusset plates on the center portion of the deck truss. Because the deck truss portion of the I-35W bridge was considered nonload-path-redundant, the total collapse of the deck truss was unavoidable once the gusset plates at the U10 nodes failed.
NTSB said it 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, 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 at NTSB’s website under “Board Meetings.” The board’s full report will be available on the website in several weeks.