Investigators Focus on 'Design Issue' with Gusset Plates of Fallen Bridge

In its ongoing investigation of the I-35W bridge in Minneapolis that collapsed Aug. 1 over the Mississippi River, killing at least five people, the National Transportation Safety Board's 19-member team is now looking closely at a potential design flaw in the bridge's gusset plates, which are steel plates that tie steel beams together, connecting girders. NTSB said Aug. 8 that the team is in the process of verifying the loads and stresses on the gusset plates in particular locations, as well as the materials used in constructing the gusset plates.

"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," NTSB Chairman Mark V. Rosenker said.

NTSB's team includes highway engineers, survival factors specialists, and the Board's senior metallurgist. Parties to the Board's investigation are Federal Highway Administration, Minnesota Department of Transportation, and Progressive Construction Inc.

In a statement issued late Wednesday, Secretary of Transportation Mary E. Peters said, "Given the questions being raised by the NTSB, it is vital that states remain mindful of the extra weight construction projects place on bridges."

NTSB is working with FHA to conduct a structural analysis of the bridge, using computational Finite Element Analysis methods. Within days of the collapse, development of the computer model based upon the original design drawings began at the FHWA's Turner Fairbank Highway Research Center in McLean, Va. 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.

Since the day of the collapse, the team has conducted inspections of areas of the bridge that are accessible. On Monday, using a gyro stabilizer, high-resolution camera mounted on a state police helicopter, the team looked at the superstructure on the north side of the bridge. Several tensile fractures were observed, but nothing that looked to be the initiating location. Investigators will conduct a closer look at the superstructure on the north end when it becomes more accessible.

NTSB has also 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 team 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. Investigators have also obtained the original security camera video equipment and footage provided by the Army Corps of Engineers that shows a portion of the bridge collapsing. NTSB is in the process of reviewing the entire contents of the system in its laboratory, and is providing detailed imagery back to the accident site to help guide investigators in their on-scene activity.

With the investigation still months from completion, recovery divers are working 18 hour shifts from 4 a.m. to 10 p.m. to continue searching for victims. No additional victims have been recovered, NTSB said.

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  • OHS Magazine Digital Edition - June 2021

    June 2021


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