WASHINGTON - Mystified about how a simple human error occurred four decades ago, leading to the deadly collapse of the Interstate 35W bridge, federal safety officials recommended new rules Friday to prevent such error in the future.
In the end, the National Transportation Safety Board attributed the failure of the downtown Minneapolis span to a math calculation that wasn't made. The board found that the designers of the doomed bridge failed to perform some necessary calculations on the critical steel beam gusset plates that eventually gave way.
That failure, combined with inadequate oversight by state and federal regulators, led to the construction of a bridge that would not withstand all the weight that was added over its 40-year life, which ended with a total collapse on Aug. 1, 2007, killing 13 rush-hour motorists.
"It's intriguing to me that an error of omission of that magnitude would have been made," said Steven Chealander, one of five board members who signed off unanimously on the final report. "It's inexplicable."
The board's long-awaited findings, released Friday in summary form, were accompanied by a recommendation for a nationwide bridge design quality assurance program. Federal and state officials vowed immediate action.
The NTSB's final conclusions are likely to provide fodder for a spate of lawsuits that have been filed this week by relatives of the dead and about 145 others who were injured in the accident. While the NTSB concluded that the primary cause of the bridge collapse was its under-designed gusset plates, it also highlighted the heavy construction loads that were placed on the bridge the day it fell.
The design flaw was pinned in large part on a now-deceased engineer working for Sverdrup & Parcel, which was contracted to design the 35W bridge in 1962. The firm was bought by Jacobs Engineering a decade ago.
NTSB investigators who pored over 10,000 pages of documents from Jacobs Engineering and state highway officials from the 1960s found no evidence of completed calculations for the design of the gusset plates, which tie the bridge's steel beams together.