Couple of things - first, say a prayer for the families and friends affected by this collapse - 7 dead at last check, but lots of missing folks right now.
The bridge is actually on I-35W, both north/southbound in Minneapolis over the Mississippi River. I-35 splits into I-35W and I-35E in the Twin Cities to go to both Minneapolis and Saint Paul. The bridge was carrying about 140,000 vehicles per day.
The bridge was constructed in 1967 and was undergoing rehabilitation of the 8 lane deck at the time of the collapse. Work had been ongiong for about six weeks to repair the delmainations in the deck. Lots of speculation about the deck rehab and its interaction with the failure.
For those unfamiliar with the area, the bridge crosses the Mississppi River just downstream of lower Saint Anthony Falls near downtown Minneapolis. As a result, there is often a huge amount of moisture in the air in the area, which has caused corrosion problems. The bridge was equipped with an automatic deicing system that applied potassium acetate liquid deicer to the bridge deck during winter conditions. Prior to the deicing system, black ice was a huge problem on this bridge in the wintertime.
The bridge was constructed with a 400+ foot main span to cross the river without piers to avoid impacts on river navigation. The bridge passed its last inspections in 2005 and 2006. Several reports studying this specific bridge design have been conducted in the past. The bridge contains what are called "Fracture Critical Members". This describes parts of a bridge that are subject to tension loadings that if they fail will likley result in the complete failue of the bridge. Fatigue cracking and failure is a major issue in these systems.
Its going to be a while before we know what happened here. No real use in speculating at this point - the real answer will eventually come out.
(I used to cross over this bridge a minimum of 2x daily in my previous consulting life. I'm a civil engineer by trade - though my specialty is NOT bridges/structural engineering - but I remember enough to be dangerous!).
