MINNEAPOLIS — Ten weeks into their probe of the Interstate 35W bridge collapse, National Transportation Safety Board investigators have intensified their inspection of a long-corroded gusset plate that was located in the section of the bridge that fell first.
In addition, authorities are analyzing what role the 91- degree heat on Aug. 1 might have played in increasing stress on the already-weakened L-11 gusset plate, which connected four steel beams located near the bridge’s south end.
In 1993, a state inspector found that the half-inch gusset plate had lost nearly half of its thickness in some spots because of corrosion, but no repairs were ordered, according to Minnesota Department of Transportation records.
Two structural engineers who have viewed the wreckage said this week that the L-11 gusset plate is one of three closely situated connections that could hold the secret of what caused the bridge to collapse. All three joints appear to have been damaged by some primary force — not from secondary impacts sustained during the collapse, the engineers said.
The three damaged gusset plate connections are clearly visible along the secured West River Road site, where federal experts have laid out the bridge parts most critical to their investigation.
Within a week of the bridge disaster, NTSB Chairman Mark Rosenker announced that stress on gusset plates may have been a factor in the collapse. A month later, he said that “a failure in one of these plates could have catastrophic consequences.” Now it appears that of more than 100 gusset plates on the bridge, the NTSB is focused on just a few.
One of the structural engineers who has viewed the wreckage and knows the design of the bridge said a runoff of salt and de-icing chemicals from the bridge deck could have contributed to the corrosion in the L-11 gusset-plate connection. That’s because a diagonal, H-shaped beam running into the joint could have acted to channel the liquid toward the gusset plate, the engineer said.
Both structural engineers interviewed by the Star Tribune said the issue of fatigue cracking is not currently at the forefront of the NTSB’s investigation. Rather, they say federal authorities are examining whether intense heat on Aug. 1 triggered a chain reaction of force that overpowered gusset plates in crucial locations — such as the one at L-11.
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