As is so often the case with civil engineering disasters, a combination of factors coincided to cause the collapse of the westbound Interstate 35 motorway bridge in Minneapolis on 1 August 2007. The bridge across the Mississippi collapsed under live traffic at 6.05pm in the evening, killing five people and leaving eight unaccounted for.
The bridge was a three span continuous truss with a 139.6m steel truss arch and 81m side spans carrying a six lane motorway through central Minneapolis. It was only 40 years old when it collapsed.
The official report highlighted a lack of attention to design detail, construction work on the bridge and previous modifications to the structure as key contributors to the collapse.
The report, published by the US National Transportation Safety Board (NTSB), said that gusset plates used to connect load bearing columns to trusses had inadequate load bearing capacity. This struck a chord with British engineers who said that too often, structural failures resulted from a lack of attention to the small details of a design rather than because of problems with main structural elements.
In all there were 24 underdesigned gusset plates on the structure. The report accused consultant Sverdrup & Parcel & Associates of failing to ensure that the appropriate main gusset plate calculations had been carried out. It also said that federal and state transportation officials had carried out an inadequate design review.
NTSB acting chairman Mark Rosenker said: “Bridge designers, builders, owners, and inspectors will never look at gusset plates quite the same again, and as a result, these critical connections in a bridge will receive the attention they deserve in the design process, in future inspections, and when bridge load rating analyses are performed.”
The NTSB report also said that “substantial increases in the weight of the bridge, which resulted from previous bridge modifications” put further strain on the inadequate gusset plates. Concentrated construction loads on the bridge on the day of the collapse as a result of works being carried out added to the problem, it said. Inspections carried out had not identified the problem because gusset plates had been largely ignored.
To make matters worse, annual bridge inspections carried out in the 12 years before the collapse carried ominous, but largely unheeded, warnings about the condition of the bridge. In 2000, hinge joints were reported to have locked in full expansion, and fatigue cracks had been located.
More fatigue cracks were discovered in 2004.