[jeffers]

Excellent catch from that first pic. I had seen that, but before one i posted earlier, and did not catch the much better view of the buckled kingpost.

Good eye!

Important to note, the bottom chord of a cantilever usually isn’t in tension. Think teeter-totter. Raise the seat board with a vertical strut (kingpost), then run a diagonal from the fulcrum to each end of the seat board (topchord). The top chord should remain in static tension, while the kingpost and both diagonal braces are in compression.

I’m sure you alread know this, but for others, a cantilever is basically a balancing act. From the center of one span to the center of the next is just an upside down triangle, balanced on the piers in its middle. Sometimes, the shore end of a cantilever is weighted or fixed by a large concrete counterweight. Sometimes, each end is just attached to the floatinjg end of another cantilever.

Something’s been bugging me about that first pic, and I just realized what it is. The road deck shoreward of the pier is intact, while towards centerspan, it’s vaporized. Starting to speculate here, that the trigger could well have been right in that area, just north of the southeast pier.

Take a look at the bottom chord and bridge foot assembly on the west side cantilever in your second image.

You see what I see? Corrosion, new paint, patchwork steel, something, almost all the way out to the next strut? I’d expect the cantileverassewmblies to weather and age in pairs. If the other side looked like that before the collapse...

[jim_trent]

I will have to find some complete “before” pictures too look at, but it looked like a conventional truss type bridge to me. The only thing that is unusual about it is that the road deck was on top rather than along the bottom like normal. I am unsure what your use of the word “cantilever” means in this case.

Anyway, I have no expertise in this particular bridge, just engineering observations. After the final report on the collapse, we may all be wrong. BTW, I do agree that the failure was probably near, but not at, one of the supports.

[wagglebee]

I agree, those are excellent pictures and great explanations.

As far as the conspiracy theorists go, it’s time that they acknowledge that this couldn’t be anything other than massive and catastrophic structural failure.

[spunkets]

Thanks.

The road bed is supported by trusses. Those trusses are used instead of plates to reduce weight. A truss is essentially a plate that contains no shear. The bridge has fixed points at both ends where it hits the earth, so it's not free at the ends. A cantilever is free to move up and down at one end. The top and bottom chords of the bridge can only move in expansion and contraction, so this is a mix of fixed ends and simple support. I included this fig to illustrate the forces just for the thread.

"The road deck shoreward of the pier is intact, while towards centerspan, it’s vaporized."

That came from the road bed flexing back and forth considerably when the SE side fell. The rebar from that is all drawn out and extends into the river. Concrete isn't any good in tention, so it just crumbled. What didn't go up in the air as dust slid down the embankment into the river.

Yes, both sides at the pad had notable rust stains.