[Lijahsbubbe]

The problem began on the right side of that picture.
Pier footing, pier or steelwork.
Bank on it.
I don’t say that very often, but pictures don’t lie.

And couldn't a drought cause shifting in the earth or footings? I still think the heat has something to do with it.

[jeffers]

No, on closer inspection, the mainspan piers appear to be intact.

Heat will expand steel, but good design takes this into account. An expanding deck, for example is not permitted to shift verticals out of plumb.

Something else did that here:



Either the diagonals between the kingposts failed, allowing them to deflect sideways under load, or else the next pier/truss system in the sidespan (behind the word "kingpost" in green) failed, transferring considerable gravity load to the mainspan pier/truss system and it couldn't take it.

Thanks to Straight Vermonter for the underlay image and the notion of sidespan pier failure.