Right. The E side broke loose first. I think within one second after the intial break the west side went.
I can't make out most of these still pics, because the resolution's so low. I looked for the bearing plates, but couldn't positively ID them. The stills on this page don't indicate much corrosion. There's just some shots that show rust where the king post rests on the pylon and rust stains dripping down from above on that same post. MAybe you can make things out from the stills on that page, because your familiar with it. The SW king post appears to be intact, and on the ground. It appears to have broke free from the road bed.
I couldn't make out anything under the bridge before the fall. There's a curious fog under the brige where each of the members of the truss and the posts between them meet the road bed. There's some kind of lateral support bracket at those locations. If that's a fog there, and not just camera effects, the truss, at least on the east side, appears to have sheared longitudinally from the road bed.
"This had to be a catostrophic underpinning/super-structure failure. Either they chipped away too much concrete around the anchor bolts at the piers, or they created a point load in the wrong place."
I don't think they were working on the peirs yet. There just seems to have been cutouts in the concrete on the one side where traffic was one lane. That from the MNDOT camera. It showed them and some equipment parked along the other side. that equipment could weight much. I think it was just and air compressor and a ~1000gal water truck. I think one of the truss members was already essentially failed and the roadbed itself(the pins at the bearing plates) was allowing it to stand a bit longer. Is it possible the foreman, or any of the workerswould note an odd opening at the expansionjoint for this weather?
I am not sure but I think I saw two front discharge concrete mixers on one of the fallen roadways, I drive a concrete mixer that fully loaded is near 69.000 lbs and those type of mixers can weigh much more.Looking at the videos its just a bit strange how it fell, more like a controlled demolition. I would ask some of those demolition experts if it could be done without using explosives.