Thursday, August 09, 2007
I-35W Bridge Design Flaw Identified
Earlier this week, construction crews who had been working on the I-35W Minneapolis Bridge reported that they had felt the structure “wobble” in the days leading up to the disaster. Now, the NTSB is taking a close look at the design of steel connecting plates called gusset plates that were used on the Minneapolis Bridge. Gusset plates are usually fabricated from lower quality steel. When the Minneapolis Bridge was built in the mid 1960s, such plates were usually welded to trusses. This type of welding makes the plates more prone to fatigue. Standard bridge design today calls for gusset plates to be bolted to trusses.
The Minneapolis Bridge was being resurfaced prior to the collapse. Traffic on the 8-lane bridge had been reduced to four and several large trucks were parked on the structure during the construction. At a news conference yesterday, Minnesota Department of Transportation (MNDOT) construction engineer Liz Benjamin said that workers had dumped large sand and gravel piles on the I-35 W Bridge at 2:00 p.m., just hours before the disaster. The sand and gravel piles could have weighed as much as 100 tons. Even so, they still would have been lighter than the trucks parked on the Minneapolis Bridge.
The NTSB would not say exactly what type of design flaw investigators were looking at, only that it involved the gusset plates. An unidentified source told the Minneapolis Star Tribune that the NTSB is looking at the thickness of those plates. Despite Wednesday’s alert, the NTSB has said that the gusset plates are just one area that the agency is investigating, and has not ruled out other causes of the Minneapolis Bridge collapse.
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