The National Transportation Safety Board dispatched a Go Team on August 1 to investigate the collapse of the I-35W bridge over the Mississippi River in Minneapolis, Minnesota. The NTSB today provided the following update.
The Safety Board's investigative team of 19 was led by Investigator-in-Charge Gary Van Etten; NTSB Chairman Mark V. 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 Federal Highway Administration, 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.
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. They have identified four empty vehicles that can be removed to allow them better access to the wreckage. Those vehicles might be removed later today. NTSB is monitoring this effort so that removal of major pieces of wreckage can commence, which will occur once the Hennepin County Sheriff's office has determined that it has searched all it can under current conditions.
The NTSB is working with the Federal Highway Administration 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, Virginia. 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 first day, the NTSB has been conducting 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 Board 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. NTSB investigators will conduct a closer look at the superstructure on the north end when it becomes more accessible.
Chairman Rosenker said that NTSB investigators have observed a design issue with gusset plates at particular locations (gusset plates are steel plates that tie steel beams together). 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.
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.
The Board has obtained the original security camera video equipment and footage provided by the Army Corps of Engineers that shows a portion of the bridge collapsing. The Board is in the process of reviewing the entire contents of the system in our laboratory, and is providing detailed imagery back to the accident site to help guide investigators in their on-scene activity.
A Safety Board representative from the Office of Transportation Disaster Assistance has been briefing family members each evening about the progress of the NTSB's investigation.
Further information on the progress of the Safety Board's investigation will be released as it develops.