National Transportation Safety Board investigators probing the Aug. 1 collapse of an Interstate highway bridge in Minneapolis have found that some of the gusset plates in the bridge's main trusses were about half as thick as required and too thin to provide the expected "margin of safety," NTSB Chairman Mark V. Rosenker said. In response to these "interim findings," NTSB recommended Jan. 15 that the Federal Highway Administration require states and other bridge owners to conduct load-capacity calculations of non-load-path-redundant steel truss bridges.
Rosenker said that the board has not yet finished its investigation and has not determined a probable cause for the bridge collapse, in which 13 people were killed and 145 injured. But he said that investigations found fractured gusset plates at eight of the 112 joint locations in the bridge's main trusses. A review of the bridge design, with a focus on the gusset plates' design, "discovered that the original design process of the I-35W bridge led to a serious error in sizing some of the gusset plates in the main truss."
NTSB found that 16 plates–two at each node–were about half the required thickness, "and too thin to provide the margin of safety expected in a properly designed bridge."
The board also said it has found no evidence suggesting the design deficiencies are widespread, or occur on more than the I-35W bridge.
U.S. Transportation Secretary Mary E. Peters responded quickly to the NTSB recommendation, directing FHWA to issue a technical advisory to state agencies, to go beyond current standard practices and include possible effects of changing weight and other conditions on gusset plates in their load-capacity calculations for the 13,000 steel truss bridges around the country. "We want to take every possible step to address the latest update from the NTSB," Peters said.
The NTSB added that a probable cause for the accident would be determined in its final report, expected to be sent to the board before the end of the year.