I was wrong and you were both right about the end of the chord being in compression. In my haste I got stuck on simple support, rather than cantilever and never drew a diagram. I would have seen the torque about the top of the king post then. I'll be looking at the MNDOT site, but not right away.
I see that sheer facture in compression is consistent.
That little truss builder is an interesting program. Too bad it’s not running on a Cray, and...and...and...
Getting deeper into this...so far I see:
1. SE Kingpost probably buckled under load. Eyeball says it didn’t just fall over sideways and bend when it hit the ground. It died ugly.
2. SE span 7 bottom chord separated near the kingpost, SW span 7 bottom chord did not. It failed further out.
3. Per the 2006 inspection report, the main trusses cantilever the approach spans at the crossbeams/truss ends. The crossbeams bear the riverward ends of the approach span deck beams. The crossbeams are supported on rocker bearings by the main trusses. In 1986 the SE rocker bearing froze and had to be replaced. The whole bridge was closed and the “span had to be jacked up” to replace the pin and repair related damage.
4. In one part of the 2006 report (page 12), it says all four rocker bearings now function normally with “obvious signs of movement. In another section (page 30), it says “SW rocker bearing has no movement”. The SW rocker bearing may have been frozen as of the 2006 report.
5. The SE crossbeam suffered significant damage when the rocker bearing froze in 1986. This was repaired by adding plates, drilling cracks, and adding braces.
6. There is significant evidence of similar damage at the NE crossbeam around the rocker bearing, which has also required repairs.
7. Further south, on span two, there is a hinge joint which is frozen in the maximum expansion position. This has rocked pier one to the north.
8. The “roller nest” under the SE kingpost showed “no obvious signs of movement” as of the 2006 report, and the bearings showed “rust and corrosion”. It may well have been frozen as of that time.
The interplay between all these points is still somewhat hazy, but it’s clear that there were expansion problems, especially on the south ends of both trusses and the north end of the east truss.
It’s also clear that somehow significant loads had to be transferred to the SE kingpost, if it indeed buckled under load.
Supercat discussed failure of the kingpost and subsequent loads imposed on the bottom chord trending near infinite.
Post collapse imagery shows a buckled SE kingpost and a bottom chord fracture consistent with extreme compression loads.
I’m not yet able to to connect the bridge’s expansion problems, expansion related damage, potential effects of jacking the structure up, potential cantilever dynamics in the event of crossbeam failure, and the SE kingpost and bottom chord failures, but I’m starting to disbelieve the idea that these are unrelated coincidences.
In the big picture view, we have a known frozen expansion mechanism near the south end of the approaches, a historical frozen rocker bearing at the SE corner of the truss system, a potentially frozen bearing now at the SW corner of the truss assembly, and a potentially frozen roller nest under the SE kingpost. Additionally we have significant damage at both the northern and southern truss end/crossbeams. Finally we have imagery of a mangled SE kingpost and possibly spectacular compression failure of the heavy bottom chord box beam.
Speculation:
If expansion issues failed the southern crossbeam, (or just the eastern end of it), the truss assemblies there would be relieved of the counterbalancing weight of the approach span. That could induce a moment around the base of the southern kingposts, and could fold up the truss assembly where the cantilever transitions to a simple truss on the mainspan just north of the southern piers.
However, I don’t see how that could translate to high compression loads on the kingposts, of the kinds which are required to buckle them and make the compression loads on the bottom chords go infinite.
I also don’t see any rotation towards centerspan of either the SE or SW kingposts in the post collapse imagery. I do see a lot of “missing” road deck in that area, as discussed earlier.
The best (tenuous) connection I can come up with so far has the east end of the southern crossbeam failing, and the moment, instead of rotating the SE kingpost about its (possibly frozen) base, created enoug instability and interaction with adjoining members to cause it to buckle, which then compression loaded the bottom chord beyond its design strength.
If the SE kingpost did buckle, it could put all the horizontals and diagonals between the SE and SW kingposts into excessive tension and pull the SW kingpost over sideways as indicated by the post collapse imagery. In the process, the south end of the mainspan seperates from the kingposts and cantilever group, drops, and the rest is as previously discussed.
There’s weak circumstantial support for this. From a birdseye view, the SW kingpost (and organic truss panel) is rotated clockwise from where it would be if it simply leaned over to the east. If the SE kingpost buckled beginning at it’s base, the lower crossmembers tying it to the SW kingpost would tear losse first, possibly leaving the upper crossmembers attached long enough to pull the SW kingpost over laterally.
At the same time, with the eastern truss collpasing with the SE kingpost, the western truss near the south pier is sagging towards the river, trying to pll the SW kingpost riverward off its base.
This is consistent with (but by no means the only explanation for) what we see in the pictures, the rotated western truss pier panel.
I know it’s thin, I don’t like trying to turn a rotational moment from cantilever counterbalance loss, about the base of the SE kingpost, into enough compression to buckle it, but right now, it’s the best mechanism I have to connect historical and current expansion problems to what we see in the pictures.
Thoughts?