This is the same thing that happened with the 35W bridge collapse in Minneapolis. The gusset plates after the disaster were examined and found to be only 1/2" thick when the original design called for them to actually be 1" thick. The bridge was a ticking time bomb since the day it was built in 1967.

As the years went on, the bridge's weight capacity was slowly eroded by subsequent construction projects like adding thicker concrete deck overlays, concrete median barriers and additional guard rail and other safety improvements. This was the second issue, lining up with the first issue of thinner gusset plates.

The third issue that lined up with the other two was the day of the bridges failure. There were approximately 300 tons of construction materials and heavy machinery parked on two adjacent closed lanes. Add in the additional weight of cars during rush hour when traffic moved the slowest and the bridge was a part of a bottleneck coming out of the city. That was the last straw and when the gusset plates finally gave way, creating a near instantaneous collapse.