The following is an update on the National Transportation Safety Board's investigation of the I-35W bridge collapse in Minneapolis, Minnesota on August 1, 2007.
A wreckage removal effort is still underway. Several NTSB investigators remain on-scene and the agency expects to remain in Minnesota until November.
"The Safety Board is continuing coordination of the removal process in an expeditious manner but still ensuring that all relevant components of the superstructure are not destroyed during the removal and are developing a layout plan for critical structural elements recovered from the river," said NTSB Chairman Mark V. Rosenker. "Additionally, investigators in Washington are sifting through materials that have already been gathered," he added.
The NTSB is also examining the de-icing system that was installed on the bridge in 1997 and the chemical used in this system and what type of corrosive properties it might have.
"The finite element model of the bridge is being revised based on the measured deck thickness from core sections and physical examination of the bridge structure. All structural elements have been incorporated in the model; however, some aspects of stiffness, weight and connections between elements are being modified to match the condition of the bridge the day of the accident," Rosenker said.
Safety Board investigators have received records from the bridge designer and will assess the design calculations. Furthermore, investigators have begun developing a list of the various construction projects and modifications that were performed to the bridge after the original construction.
The NTSB has received 184 calls on the witness line. Two callers identified themselves as people who were on the bridge and escaped immediately prior to the collapse. In total, 314 witnesses have been contacted and or interviewed.
Currently, 25 construction workers and drivers who delivered construction materials to the bridge for the construction project that was underway when the bridge collapsed have been interviewed. The weights of the various construction materials that were delivered to the work site between 11:00 a.m. and 2:30 p.m. on August 1 have been verified at about 383,000 pounds. The combined weight of the loads and construction vehicles is about 575,000 pounds.
"Through interviews, we have refined the location of these materials and vehicles and have verified this information by means of a pre-collapse photograph taken by a passenger on an airplane that was departing from Minneapolis on the afternoon of the collapse," Rosenker said.