I-35W investigators cite undersized gusset plates in bridge collapse

An error in the original design of the I-35W bridge contributed to the I-35W bridge collapse this summer, the National Transportation Safety Board said today.

Investigators found fractured gusset plates in eight different locations on the main trusses of the bridge, according to the NTSB. The undersized gusset plates, which connected beams, on the bridge were undersized on the design drawings, as well. Sixteen gusset plates were half the thickness required to deem safe.

According to the report, state departments of transportation rely on bridge designers to perform accurate calculations and to check their work. Beyond the designer’s internal review, there isn’t a process in place to locate original design errors, the report said.

The NTSB recommended that owners of similar bridges review original design calculations before any modifications or operational changes are made.

On Aug. 1, 13 people were killed and 145 people were injured in the collapse. At the time, construction was being conducted on the deck truss portion of the bridge.

“The collapse of the I-35W Bridge was not an act of God, but a failure of Man. Our state and country have not invested as we must in roads, bridges and transit — and that lack of investment has serious consequences,” Mayor R.T. Rybak said in a statement.

Probable cause of the collapse will be determined when the NTSB receives a final report, which is expected to happen sometime before the end of 2008.