National Transportation Safety Board Chairman Mark V. Rosenker testified today before the U.S. House of Representatives Committee on Transportation and Infrastructure on Structurally Deficient Bridges in the United States. During his testimony, he discussed the Safety Board's history of investigating bridge accidents and the Board's investigation into the August 1, 2007, I-35W bridge collapse over the Mississippi River in Minneapolis, Minnesota.
"Forty years ago, following the bridge collapse in Point Pleasant, West Virginia, we issued safety recommendations to the Federal Highway Administration to establish national bridge inspection standards for locating, inspecting, evaluating and correcting bridge deficiencies," Rosenker said. "Currently, Safety Board staff is still on-scene in Minnesota investigating the tragic accident that claimed 13 lives and involved 133 injuries."
A month after the bridge accident, much of the bridge superstructure is still underwater and there is still considerable work remaining to determine why the structure collapsed.
During this initial stage of the investigation, the Safety Board has learned the following:
- The bridge was 40 years old and that it was considered "structurally deficient" because of a relatively low rating of its superstructure.
- 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 that 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. Once a sequencing study is completed investigators will be able to determine the earliest identifiable fracture area or areas.
"We will determine the probable cause of the bridge collapse; make recommendations, and help restore public confidence that this kind of accident will not happen again-that is our mission," Rosenker said.
Last year, Congress turned to the Safety Board to investigate the collapse of ceiling panels in the Big Dig tunnel in Boston. As a result of the NTSB's investigation, there are radical changes in the thinking in the highway construction industry about the long-term structural properties of epoxy in the overhead applications as they relate to epoxy creep.