Federal investigators announced Tuesday that a “serious design error” was a key factor in last summer’s deadly collapse of a Minnesota bridge, but they also said the mistake was not likely to have been discovered during routine state inspections.
National Transportation Safety Board Chairman Mark Rosenker said the Interstate 35W bridge had been built with gusset plates – the steel parts that connect girders, which support the bridge – that were too thin to hold up the bridge with increased traffic and the additional weight of infrastructure improvements.
The Aug. 1, 2007, collapse killed 13 people and injured more than 100.
Investigators in the ongoing inquiry have found 16 fractured gusset plates from the steel-deck truss bridge’s center section, Rosenker said.
“Basically, those 16 gusset plates were too thin to provide the margin of safety expected in a properly designed bridge such as this,” Rosenker said. “These gusset plates were roughly half the thickness that would be required – half an inch thick rather than an inch thick.”
Safety board officials said their investigation was far from over, and the exact cause of the collapse is still unknown. A final report on the rush-hour tragedy is expected to be released by NTSB by year’s end.
The latest findings evolved out of concerns raised by the NTSB in the days after the bridge collapse.
It is unknown how or why “there was a breakdown in the design review procedures that allowed a serious design error to be incorporated into the construction of the I-35W bridge,” Rosenker said.
Investigators haven’t been able to locate the original designer’s calculations, Rosenker said, so “we cannot determine whether the error was a calculation error, a drafting error or some other error in the design process.”
The bridge was designed by Sverdrup & Parcel, a Missouri-based civil-engineering firm that also designed and oversaw construction of the Louisiana Superdome in New Orleans and the Chesapeake Bay Bridge-Tunnel, which has been called “One of Seven Engineering Wonders of the Modern World.”
The company later was bought by Jacobs Engineering Group in Pasadena, Calif., which did not return calls for comment Tuesday.
Last summer’s tragedy followed years of warnings that the I-35W bridge needed expensive repairs.