In public testimonies on the I-35W Bridge over two days, investigators said the main flaw was gusset plates designed too thin for the amount of weight supported. In addition, construction equipment and materials equivalent to the weight of a 747 airplane appeared to produce the breaking point on August 1, 2007, when the bridge collapsed.
Investigators told the National Transportation Safety Board in Washington, D.C. that the age of the bridge, 40 years, had nothing to do with the collapse. However, the 287 tons of equipment and materials on the bridge for a repaving project, in addition to rush-hour traffic, was the largest load the bridge ever held.
Several government witnesses told the five-member board that if the gusset plates were of proper size, the bridge would be standing today. The lone designer has died and the engineering firm was bought by a California business about 10 years ago.
Construction loads were not considered
Minnesota officials and contractors working on the bridge did not consider the added weight when they began the repaving project. Records and testimony revealed no national standards or policies for Mn/DOT to follow when handling increased loads on a bridge.
The construction equipment and materials were set directly above the U-10 node gusset plate, the first to fail, because workers mixed the quick-drying concrete on site and in that spot. A construction foreman asked a Mn/DOT inspector about setting up the equipment in the middle of the bridge and received verbal approval.
During testimony, Mn/DOT told NTSB investigators the contractor should have made a formal written request, which would have been denied. A federal investigator said state officials told him Mn/DOT has no specific policy on stockpiling equipment and materials on bridges. Another NTSB member expressed frustration that Mn/DOT officials "are not reading from the same page." But the new commissioner of Mn/DOT, Tom Sorel, who attended the hearings, said the policy on stockpiling equipment would not be allowed since he was appointed to the position.
Failure to perform necessary calculations
An NTSB engineer pointed out the corrosion and pre-existing cracks on the bridge prior to the collapse and said they were "a source of concern" although they did not contribute to the collapse.
The U-10 gusset plate nodes showed signs of sudden facture associated with excessive loads, according to the engineer. Bridge modifications on top of the bridge in the 1970s and 1980s added to the heavier loads. Records showed more inspections on the I-35W Bridge than federal guidelines required.
In the final report, the board found that designers of the bridge failed to perform some necessary calculations on the steel beam gusset plates. In addition, state and federal regulators did not have adequate oversight of the structures, which led to a construction that could not handle the necessary weight over a 40-year life.
Among key recommendations of the NTSB report is a call for greater attention to the effects of rust and corrosion on the "structural integrity of bridges … but the accident was not the result of corrosion and aging infrastructure. It was the tragic result of an inadequately designed gusset plate."
Mary Peters, secretary of U.S. Department of Transportation, said federal officials are acting immediately to work with states to improve quality controls during the bridge design process. Sorel said his department will move forward without waiting for national standards.