Investigators from the National Transportation Safety Board looking for the cause of the Minneapolis bridge collapse are focusing attention on a corroded gusset plate located in the section of the bridge that fell first, according to a report in the Star Tribune.
Investigators from the National Transportation Safety Board also are analyzing what role the 91-degree heat on the day of the collapse might have played in increasing stress on an already-weakened gusset plate, which connected four steel beams located near the bridge's south end.
Two structural engineers who've spent time at the wreckage site said that the gusset plate in question is one of three closely situated connections that could be key to what caused the bridge to collapse. Those three joints appear to have been damaged by some primary force, not from secondary impact during the collapse, the engineers said. The newspaper said the engineers spoke on the condition they not be identified.
Officials from NTSB and the Minnesota Department of Transportation did not immediately respond to requests for comment from The Associated Press.
Within a week after the bridge collapse on Aug. 1, NTSB Chairman Mark Rosenker said stress on gusset plates may have been a factor in the collapse, and a month later he said that "a failure in one of these plates could have catastrophic consequences."
One of the structural engineers, who knows the design of the bridge, said runoff of salt and de-icing chemicals from the bridge deck could have caused corrosion in the gusset-plate connection in question. A diagonal, H-shaped beam running into the joint could have channeled the liquid toward the gusset plate, the engineer said.
The engineer also said two of the three damaged gusset plates that appear to be of primary interest to the NTSB are half an inch thick. The thickness of gusset plates used in the bridge varied between half an inch and 1 inch, which could be an important issue because a consulting firm hired by the state has said some half-inch gusset plates may not have been strong enough to hold the bridge up.
The structural engineers said federal authorities are examining whether intense heat on Aug. 1 triggered a chain reaction of force that overpowered gusset plates in crucial locations.
The I-35W bridge was designed to flex, to handle expansion and contraction in extreme heat and bitter cold. But roller bearings designed to accommodate such flexing may not have been working correctly because of corrosion and buildup of debris.
A consultant's report in 2006 noted that, "The bearings are not allowing the structure to move linearly with changes in the ... temperature."
The NTSB has stressed it could take 12 to 18 months to reveal a probably cause for the bridge collapse, which killed 13 people and injured more than 100.
(Copyright 2007 by The Associated Press. All Rights Reserved.)