From
USA Today ^ is a report from the NTSB blaming the gusset plates; used 1/2-inch thick as against 1-inch thick. Seems they didn't fully account for the increase of road lanes from four to eight against the original design weight loading:
Design changes in 1977 and 1998 added additional pavement and concrete barriers that increased the weight of the Interstate 35W bridge in downtown Minneapolis.
Thank you for your info above.