Preliminary results on 35W bridge collapse

A design error in the Minneapolis bridge that collapsed Aug. 1 prompted Minnesota officials to look at the original plans of 59 similar bridges.
The $500,000 re-evaluation follows Tuesday’s preliminary report blaming gusset plates that connect bridge beams for the Interstate 35W bridge collapse.
Bridges Minnesota officials gave priority status for the re-evaluation include U.S. 61 over Mississippi River at Hastings, U.S. 63 over Mississippi River at Red Wing and the Blatnik Bridge in Duluth. Bridges throughout the state will be examined.
State transportation officials said inspectors most likely will not have to physically examine most of the bridges, but consultants will re-evaluate their design to make sure they were properly designed. Bridges whose design is being examined were built anywhere from 1889 to 1987.
No problems were found in the 56 bridges similar to the one that collapsed, but State Bridge Engineer Dan Dorgan said problems such as occurred on the 35W bridge would not have been discovered during an inspection.
Chairman Mark Rosenker of the National Transportation Safety Board said in Washington that some 35W gusset plates were too thin for the 35W bridge; they were a half-inch thick instead of an inch like they should have been.
Dorgan said gusset plates generally are the strongest part of a bridge and inspectors don’t look at their thickness once a bridge is built. However, he said, had a re-evaluation like now is being done on the 56 bridges been done on the 35W structure, the problem would have been discovered.
The problem was in the bridge’s design, Rosenker said, and there is no evidence other bridges have the same problem.
“The design process led to a serious error,” Rosenker told a news conference. “The bridge inspections would not have identified the error in the design of the gusset plates.”
The NTSB will take several more months to complete its investigation.