Category Archives: Engineering

NTSB Says Design Flaw Found in I-35W Bridge

For those of you who don’t know, my full time job is practicing Structural Engineering at a local Engineering firm in the Chattanooga area. If you recall the I-35W bridge that collapsed in Minneapolis last year… On January 15, 2008, the National Transportation Safety Board (NTSB) announced their findings and it doesn’t look good. Below is a summary that I received from my American Society of Civil Engineers (ASCE) news updates. You will also find a link to the NTSB report in PDF. Given the consecutive popping noise that people heard while they were on or near the bridge, I would agree that it was most likely one gusset plate after another that failed once the first one gave way. This would be comparable to someone unzipping a zipper. Once the first plate went the rest had no chance of carrying the weight…

Dear ASCE Member:

The National Transportation Safety Board (NTSB) today announced that a serious design flaw was found in some of the gusset plates on the I-35W bridge in Minneapolis, which collapsed on August 1, 2007. As a result, the NTSB issued a recommendation that the Federal Highway Administration (FHwA) require bridge owners to verify that the stress levels in all structural elements, including gusset plates, remain within applicable requirements whenever planned modifications or operational changes may significantly increase stresses. This would apply to all non-load-path-redundant steel truss bridges within the National Bridge Inventory.

The collapse of this bridge led to 13 deaths, 145 injuries and serious disruption to the lives of thousands of motorists and residents in the region. ASCE again extends it condolences to the victims of this tragedy. As a profession, it is our obligation to learn all that we can from such events. ASCE will continue to monitor the NTSBÂ’s investigation and will incorporate the recommendations and lessons learned into our future technical and professional policies and standards.

ASCE urges its members to review the NTSBÂ’s report and recommendations.

NTSB Safety Recommendation Letter

FHwA Turner-Fairbank Highway Research Center Interim Report

Adequacy of the U10 & L11 Gusset Plate Designs for the Minnesota Bridge No. 9340 (I-35W over the Mississippi River)


David G. Mongan, P.E., F.ASCE