Friday, November 14, 2008
NTSB: Gusset Plate Failure And Excess Weight Caused I-35W Bridge Collapse
The Interstate 35W bridge collapsed when a too-thin gusset plate ripped along a line of rivets, initiating a collapse that killed 13 people and injured 145.
That conclusion from a structural analyst is the first revelation from a two-day hearing in Washington, D.C. of the National Transportation Safety Board. The analyst, Jim Wildey, also ruled out corrosion, fatigue cracks and other wear-and-tear as factors in the collapse.
The bridge collapsed during rush hour Aug. 1, 2007, the nation's deadliest bridge collapse in 30 years. More than 100 cars were on the bridge at the time, and 79 different agencies and an estimated 100 or more citizens came to help.
The NTSB is likely to issue findings of probable cause -- essentially, why the bridge collapse -- at the conclusion of the hearing. The board also plans to discuss the impact of tons of aggregate and construction equipment that was on the bridge at the time of its collapse.
But the hearing is focusing on what are known as U10 gusset plats -- large connecting plates located over the main span of the river near their piers and located along the top of the steel truss that held the roadway up.
"Much of what we will (talk) about in this investigation will focus on the U10 gusset plates," said Mark Bagnard, investigator in charge for the NTSB's Office of Highway Safety. Those plates had previously been underdesigned by the NTSB. Most plates on the bridge are one-inch thick, but U10 plates were just a half-inch thick.
A full-size image of a U10 plate is hanging in the NTSB's hearing room. Wildey said there was no significant corrosion on those plates.
The bridge, finished in 1967, was called "fracture critical," which meant that a failure of any number of structural elements would bring down the entire bridge.
Safety board investigator Jim Wildey said there is "nothing inherently dangerous" about this type of bridge so long as each structural element is designed to withstand the expected stress loads.
Is the blood of 13 dead and hundreds injured on his hands?
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