In the report it states that the gusset plates should be stronger than the connected beams. Therefore, when bridges are reanalyzed it is standard practice to assume they are and not reanalyze the gussets.
Also, not unusual for an structural design problem to remain hidden for years before failure. The plant I work at had an original design error that let lose after being in service 35 years.
This graphic shows where the construction equipment was located, in the blue box. It appears to be in the area of the U-10 failure. (South side is on the left of the graphic.)
http://www.startribune.com/newsgraphics/13820902.html