[spunkets]

This is all I can make out of that SE king post. The rest of the beams from the SE truss from that point are just in a pile and I can't see where they go from these pics. The bottom chord seems to have broke, or the the rivets/plate holding it gave. I think that was the beginning of the catastrophic event that brought the thing down. The king post buckled once that occurred. The truss section containing the SW king post is intact and popped off the post. That road bed was just under a lot of tension when the bottom cord popped loose at that resting pad. The top chord would have gone from compression to tension very quickly at the top of the SE king post.

Once the king post buckled a little, that expansion joint above would tear loose, popping pins across the joint, from SE to SE very fast. That would result in the fall seen in the video and reported as "the center falling, or dropping out". So the fatigue was in the bottom chord, or the elements of the chord attachment at the SE pad.

Here's the before for ref.

[spunkets]

"popping pins across the joint, from SE to SE very fast."

That's SE to SW...

[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...

[bvw]

Nice annotations and analysis.

[spanalot]

That works - the video makes it appear that the top chord failed first but that could have been secondary to the pin failure.

BTW - lots of corrosion on the bottom chord at the pier in the before picture.

[Abigail Adams]

Great pics with notes. Thanks.

[editor-surveyor]

That chord should be in compression in a cantilever; the top is the tensor on both sides of the support bay.