A design flaw caused the Interstate 35W bridge in Minneapolis to collapse last summer, killing 13 people and injuring 100 in an accident that focused renewed attention on the safety of the nation's highways and bridges, according to federal sources familiar with the investigation.
The National Transportation Safety Board is expected to announce today that investigators have traced the failure to steel structures known as gusset plates that held together beams on the Minneapolis bridge, according to sources who spoke on condition of anonymity because they were not authorized to discuss the board's findings.
Some gusset plates on the bridge, which carried eight-lane Interstate 35W across the Mississippi River, snapped during evening rush hour on Aug. 1, causing the bridge to crumple, according to the sources.
Gusset plates are a common feature on steel bridges across the nation. They hold together angled beams on the bridge's frame.
The sources said the fault in the Minneapolis span stemmed from the bridge's design and would not have been discovered during detailed state inspections.
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When the bridge was constructed in the 1960s, its gusset plates were neither thick enough nor strong enough to meet safety margins of the era, the sources said. Over decades, renovations added weight to the span. It was undergoing a construction project with heavy equipment and material at the time of the collapse.
The sources said investigators were not sure what role those projects played in the incident. But investigators have speculated that the extra weight may have triggered the gusset plates' failure, two of the sources said.
During the construction projects, the sources said, state officials and contractors did not recalculate how the extra weight might affect the gusset plates. They said it was not standard procedure to do such studies.
The NTSB has not uncovered similar flaws in other bridges, the sources said.
The safety board is expected to recommend at a news conference today that federal and state authorities conduct more rigorous engineering studies of gusset plates before beginning renovation projects on bridges in the future, the sources said.
An NTSB spokesman said investigators could not comment on any recommendations or findings until they are publicly released.
A spokeswoman for the Minnesota Department of Transportation did not return a phone message seeking comment.
