During the wreckage recovery, investigators discovered that gusset plates at eight different joint locations in the main center span were fractured.
Undersized gusset plates were found at eight of the 112 nodes on the main trusses of the bridge. These 16 gusset plates - two at each node - were roughly half the thickness required and too thin to provide the margin of safety expected in a properly-designed bridge, says the NTSB.
A recommendation to address the design issue has been made to the Federal Highway Administration (FHWA) and states: "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."
NTSB's safety board has emphasised that there is no evidence to suggest that the deficiencies in the various design review procedures associated with the I-35W bridge are widespread or even go beyond this particular bridge. "The Safety Board has issued this recommendation, at this time, to ensure that the original design calculations for other bridges of this type have been made correctly, before any planned modification or operational changes are accomplished affecting such bridges and before any additional stresses are placed on them," said NTSB Chairman Mark Rosenker.
The probable cause of this accident will be determined when the final report is presented to the board. The current estimate is that this will be before the end of the year.
