>>So far, investigators say they have ruled out nothing and will consider everything from the expansion and contraction of the bridge in the extreme weather conditions of Minneapolis to the possible corrosive role of bird droppings.
You gotta be kidding me!<<
Look at the oval panels in the compression chords—the ovals used to be open, but now they’ve been retrofitted with a perforated screen. Why? Pigeons were roosting in them.
When I was a kid I climbed the Northern Pacific railroad truss bridge, about 3 miles upriver from the 35W bridge, with similar oval-holed compression chords. You could climb right up the diagonal beam like a ladder using those holes. All was well until I scared up a bunch of pigeons that just about hit me in the head as they were bailing out, good thing I was hanging on tight...
I’m thinking 30 years of pigeon dung and urea packed in around your welds and rivets is not a good thing for a bridge.
How did those birds know all those cement trucks were going to park there?
The discussions of the “kingpost” and lower chord near the pier being an insignificant members are off track. As alluded to in some other posts, the truss is a three-span continuous structure. (See the general plan and elevation on the previously referenced MnDOT page) Tension in the top chord and compression in the bottom chord over the piers. Reverse that as you progress toward mid-span. The tension diagonals on either side of the "kingpost" are also significant members (as are almost all members in a truss). Those diagonals transmit the accumulated load to the “kingpost” which in turn transmits the reaction to the bearing.
The speculation regarding the SE bearing is interesting. I have yet to develop an opinion that it is either a cause or a consequence.
I do have a suggestion. Think in more than one plane. Most of this discussion has focused on the primary truss mechanics. Look again at the general plan and elevation from the MnDOT web site. Note the Section drawn near Pier 7 in the lower left corner. I have been wondering if there was a failure in a floor truss or the lateral bracing which allowed the trusses over the south pier to roll. The floor trusses are the smaller trusses which span between and cantilever over the main trusses supporting the deck stringers.
The term cantilever is being overused. The only cantilevers I have noticed are the single panel cantilevers at the north and south ends of the deck truss which catch the ends of the multi-girder approach spans and the cantilevers of the floor trusses outside the main trusses to support the deck.
Some have wondered about differences in the bearings over Piers 6 and 7. According to the drawings, the bearings over pier 7 (north) are fixed with expansion bearings over pier 6 (south). Hence the toothed rollers in the Pier 6 bearing assemblies.
Way over my 2 cents worth here. Thanks for tolerating. Yes, I am a bridge engineer.