[concretebob]

River beds are by definition “bedrock”.

depends on where they are geograpically.
Ideally, yes, the steel piles or auger cast piles (A hole drilled in the ground and filled with concrete, also called caissons), are taken to rock or "driven to refusal".
However, not all bedrock is lying close enough to the surface to do this. Thats why we have geo-structural engineers to determine bearing capacity and how big the cassions (Auger cast piles) have to be and how deep they have to go to hit adequate bearing.
This was not a foundation issue.
This was a structural/mechanical failure. Either anchor bolts failed or the brige was point loaded beyond design capacity, which is frankly irelevant due to the construction/repair work.It's referred to as "moments". Time and distance travelled. It may only be a fraction of an inch of travel and milliseconds of time, but it's going on constantly, and if it's interuppted for any reason, it can be catastrophic.

[bert]

The bridge structure is a truss with bolted connections. It is very likely that one bolted connection failed leading to progressive overloading of other connections.

Since the vertical truss components are still standing it is unlikely an anchor bolt failure.

[wagglebee]

Thanks for the info. Another thing that people need to remember is that Minneapolis routinely has temperatures below zero in the winter and highs in the nineties in the summer. These extremes lead to even more expansion and contraction than normal which will serve to weaken the steel.

This is an enormous tragedy, but it is not terrorism.

[brityank]

Thank you all for the educated discussion you've all provided, it is appreciated.

I just watched a blurb about the MNDoT Bridge List, and they showed some pictures of the structure of this one from (I believe) the 2005 report. [ It runs as "Bridge History" under KARE-11 OnLive ^ ] They showed missing bolts, severe corrosion, and misaligned members in the steelwork support structures.

I'm sure there are probably more than a single cause to the failure, just as there is a multiple cause for the dearth of maintenance to the infrastructures -- self-aggrandizing politicians, money-grubbing lobbyists, and an uninterested public.