The National Transportation Safety Board has closed its investigation into the I-35W bridge collapse. The board ruled that the gusset plates on the bridge were not designed properly and also found that MnDOT had not adequately overseen the bridge’s design.
The 35W bridge’s undersized gusset plates were staring bridge inspectors and engineers in the face for the 40 years of the bridge’s life. They inspected the bridge, watched it age and documented it with photos.
But inspectors weren’t trained to know that under-sized gussets posed a threat to the bridge. As NTSB Acting Chairman Mark Rosenker said, at the time, even computer modeling didn’t consider gusset plates.