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.
The I-35W bridge was 40 years old and it was considered "structurally deficient" because of a relatively low rating of its superstructure.
"The Safety Board seldom rules out any potential causes of an accident during the initial stages of an investigation until we have had the opportunity to thoroughly investigate all potential causes," NTSB Chairman Mark V. Rosenker said. "Much of the bridge superstructure is still underwater and there is still considerable work remaining to determine why it collapsed."
The deck truss bridge design is now considered obsolete and newer bridges no longer use this design because of the inherent lack of redundancy in the structure. The bridge is composed of steel beams held together by flat gusset plates and a failure in one of the gusset plates could have catastrophic consequences. The Safety Board has not recovered all of the gusset plates yet, but investigators have observed damage in some gusset plate locations that warrants further investigation.
Bridge work was taking place at the time of the accident and 287 tons of construction materials and equipment were on the span. The Safety Board is interested in this additional loading and will conduct a very detailed finite element analysis of the structure and the loading of each component.