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Minneapolis Bridge Collapse Caused by Poorly Designed Gusset Plates, NTSB Says

Nov 14, 2008 | Parker Waichman Alonso LLP

Last year's Minneapolis Bridge Collapse, which killed 13 people and injured scores of others, was the result of design defects rather than a lack of maintenance.  According to the National Transportation Safety Board (NTSB), which met yesterday to issue a final draft report on the catastrophe, flawed gusset plates caused the I 35 West bridge to collapse under the weight of construction equipment that had been sitting on the structure at the time of the disaster.

The Minneapolis Bridge collapsed on August 1, 2007 at 6:05 p.m. It was the height of Minneapolis’ evening rush hour, and cars were lined up bumper-to- bumper across the span. At least 88 vehicles and hundreds of people fell 60 feet into the Mississippi River below. Thirteen people died and at least 100 others were injured. It would be nearly three weeks before the final victim of the Minneapolis Bridge collapse was pulled from the river.

Early on, the investigation into the Minneapolis Bridge collapse focused on the structure's steel gusset plates.  Gusset plates act like braces to hold bridge joints together.  Deck truss bridges, like the I-35 W Bridge that collapsed, are especially vulnerable if gusset plates fail. That type of bridge design has no back up features to prevent a collapse if one component of the structure fails. For that reason, bridges like the 40-year-old Minneapolis Bridge are rarely built today.

The LA Times is reporting that during testimony at the NTSB hearing yesterday  in Chicago, it was confirmed that the gusset plates on the Minneapolis Bridge were too thin.  According to NTSB investigators, the gusset plates did not meet the bridge's original design specifications, and were half the thickness required to carry the loads the bridge was subjected to when it collapsed.

The gusset plates were already strained as a result of increases in traffic volume since it was built in 1967.  In addition to being inadequate to hold up under the weight of  traffic it had been carrying, the gusset plates were also compromised by the weight of infrastructure improvements - including thickening of the driving deck - that had been made on the I 35 W bridge since it opened.   Finally, the Minneapolis Bridge was unable to withstand the strain of nearly 300 tons of construction equipment and materials that were piled on the bridge when it collapsed.  

According to the LA Times, NTSB investigator Carl Schultheisz said that had the gussets been designed properly, the bridge "would have been able to safely sustain these loads, and the accident would not have occurred."

There had been speculation shortly after the collapse that corrosion, as well as inadequate maintenance, had played a role in the disaster.  But  the LA Times is reporting that another  NTSB investigator, James Wildey, told  the hearing that although corrosion and other wear and tear  was present on other areas of the Minneapolis Bridge, the gusset plates showed no signs of such damage.

The NTB hearing into the Minneapolis Bridge collapse is scheduled to continue today,the board will likely make a final ruling on the collapse.  The board is also expected to issue recommendations on avoiding future disasters, the LA Times said.