** FILE ** Divers search for victims near the Interstate 35W bridge which collapsed over the Mississippi River in Minneapolis, in this Aug. 2, 2007 file photo. When 13 people died last summer in the collapse of the Interstate 35W bridge in Minneapolis, the National Transportation Safety Board found structural problems within days but expected a full investigation to take more than a year. The NTSB chairman, Mark V. Rosenker, plans to update reporters on the agency's progress Tuesday Jan. 15, 2008. (AP Photo/Morry Gash, File)
WASHINGTON (AP) -- Undersized gusset plates in the Interstate 35-W bridge in Minneapolis are being described as the "critical factor" in last summer's deadly bridge collapse.
National Transportation Safety Board chairman Mark Rosenker says the plates were roughly half the thickness they should have been, because of a design error. He says investigators found 16 fractured gusset plates from the bridge's center span.
The Minneapolis bridge was a steel-deck truss bridge that opened in 1967. Rosenker said it wasn't clear how the design flaw made it into the bridge because investigators couldn't find the design calculations.
He says once the plates made it into the completed bridge, there was little chance they would be noticed by inspectors.
There are about 465 other steel-deck truss bridges around the country. Rosenker says the safety board has no evidence that the deficiencies go beyond the Minneapolis bridge.
The collapse in August killed 13 people and injured 100.
(Copyright 2008 by The Associated Press. All Rights Reserved.)