Investigators have found what may be a design flaw in the 1-35W bridge that collapsed in Minneapolis, Minn.: in the gusset plates, the steel plates that tie steel beams together.
National Transportation Safety Board (NTSB) Chairman Mark V. Rosenker said that investigators have observed a design issue with gusset plates at particular locations. Safety Board investigators are in the process of verifying the loads and stresses on the gusset plates at these locations, as well as the materials used in constructing the gusset plates. This information has been shared with the other parties to the investigation, including the Federal Highway Administration (FHWA).
The NTSB and FHWA are working together to conduct a structural analysis of the bridge, using computational Finite Element Analysis methods. Data collected at the accident scene, with the help of the Federal Bureau of Investigation's 3-D laser scanning device, will be used in the computer model to further refine the model.
This work is expected to take several months, and officials in Washington said they were still working to confirm the design flaw in the gusset plates and what role, if any, they played in the collapse. Still, the potential flaw raises safety concerns for other bridges around the country. The FHWA responded by urging all states to take extra care with how much weight they place on bridges of any design when sending construction crews to work on them.
“Given the questions being raised by the NTSB, it is vital that states remain mindful of the extra weight construction projects place on bridges,” Secretary of Transportation Mary E. Peters said in a statement issues late Aug. 8.
Gary Peterson, Minnesota’s assistant bridge engineer, said the questioning of the gusset plates is a surprise to him, since he is unaware of any qualities that would distinguish them from those on other bridges. “I don’t know what this could be,” he said. “I’m frankly surprised at this point. I can’t even begin to speculate.”
The Safety Board's investigative team of 19 was led by Investigator-in-Charge Gary Van Etten; Chairman Rosenker accompanied the team. The team includes highway engineers, survival factors specialists and the Board's senior metallurgist. Parties to the Board's investigation are the FHWA, Minnesota Department of Transportation and Progressive Construction Inc.
"We are continuing to make progress on this investigation, and each area of inquiry gets us closer to ultimately determining the cause of this tragedy," Chairman Rosenker said.
The Safety Board has conducted interviews of eyewitnesses, vehicle occupants and construction employees, as well as with the crew of a dinner cruise ship that was in the lock near the bridge at the time of the collapse. It is reviewing construction records to determine the location of construction equipment and raw materials on the bridge at the time of the collapse, and to verify the weights of those vehicles and materials. The Board has obtained core samples of the bridge deck material to get a better picture of the deck thickness to help make an assessment about the amount of concrete on the bridge at the time of the accident.