National Transportation Safety Board officials today announced serious flaws in the size of several gusset plates used in the main truss of the I-35W bridge.
NTSB Chairman Mark V. Rosenker said the investigation into the collapse is ongoing and would not confirm whether the agency believes the undersized plates caused it, but did say the design flaw was “a critical part” of the bridge’s failure.
A final report on the cause is expected by the end of the year.
Rosenker said undersized plates were found at eight of the 112 joints on the main truss of the bridge. The 16 plates, two at each joint, were about half the required thickness and were too thin to provide the margin of safety expected in a properly designed bridge.
“Although the board’s investigation is still ongoing and no determination of probable cause has been reached, interim findings in the investigation have revealed a safety issue that warrants attention,” Rosenker said during a press conference in Washington, D.C. “During the wreckage recovery, investigators discovered that gusset plates at eight different joint locations in the main center span were fractured. The board, with assistance from the FHWA, conducted a thorough review of the design of the bridge, with an emphasis on the design of the gusset plates.”
The NTSB today also issued a safety recommendation to the Federal Highway Administration (FHWA) stating, “for all non-load-path redundant steel truss bridges within the National Bridge Inventory, require that bridge owners conduct load capacity calculations to verify that the stress levels on all structural elements, including gusset plates, remain within applicable design requirements, whenever planned modifications or operational changes may significantly increase stresses.”
The I-35W bridge collapsed into the Mississippi River Aug. 1, killing 13 people and injuring 145.