The NTSB intensified its investigation on the I-35W Bridge looking into a corroded gusset plate located at the section of the bridge that fell first. Section L-11, as the plate is known, is where four steel support beams were joined. Two other connections next to L-11 also appear to have been damaged before the collapse and may have caused the collapse.
In addition, authorities are analyzing the effect of the 91-degree heat on August 1 on increasing stress to the already weakened L-11 gusset plate. In 1993 inspectors noticed that the half-inch gusset plate lost nearly half of its thickness in some spots because of corrosion but did not order any repairs, according to Minnesota Department of Transportation records.
NTSB chair Mark Rosenker had said stress on gusset plates may have been a factor in the collapse and, a month later, emphasized that "failure in one of the plates could have catastrophic consequences." Now the NTSB is focusing on only a few gusset plates, out of more than 100, as the focus of the bridge collapse. Intense heat on August 1 may have triggered a chain reaction of force that overpowered the strength of gusset L-11. The gusset could not withstand the weight as it became more pliable in the heat, according to engineers.
Two engineers who studied the wreckage said the L-11 gusset plate is one of three connections that could have caused the bridge collapse because all three joints appear to have been damaged by a primary force, not from secondary impact sustained during the collapse.
One engineer said the runoff of salt and de-icing chemicals from the bridge deck could have contributed to the corrosion in the L-11 gusset plate. The two damaged gusset plates under scrutiny are only half-inch thick, a factor that may not have been strong enough to hold up the bridge without repairs, according to a MnDOT consultant.
The bridge was designed to flex and handle expansion in extreme heat and contraction in extreme cold; that design assumed that the roller bearings, mounted on top of the piers to support the bridge, would move accordingly. Examination of the bearings showed that they lacked marks of wear, and engineers said they may have locked because of corrosion and debris.
The NTSB has laid out bridge components along a side of the Mississippi until at least mid-November and sent less important pieces of the wreckage to a gravel pit owned by the state.
Work in the Mississippi River has been completed.
On Wednesday, October 17, three Minnesota congressional members introduced a stand-alone bill to expedite the $195 million to replace the bridge. The funds have been stalled in Congress while Democrats and the White House engage in a budget fight. Minnesota's Democrats were asked to sign on to the bill before it was filed, but none of them signed. However, the White House said it supports the bridge funding and assured state officials that reconstruction funds will be made available with or without congressional action.