Feb 1, 2008 12:00 PM

Undersize gusset plates used in the I-35W bridge in Minneapolis MN were “the critical factor” in the August 2007 bridge collapse that took 13 lives and injured 100, according to the National Transportation Safety Board.

Mark Rosenker, NTSB chairman, said that due to a design error, the plates that connected steel beams were about half the thickness they should have been. Sixteen broken gusset plates from the bridge's center span were discovered by investigators, he said. The steel-deck truss bridge opened in 1967.

Rosenker said it remains unclear why flawed plates were used in constructing the bridge. Investigators could not locate the design calculations. About 465 other steel-deck truss bridges are in use nationwide.