[jeffers]

Southack
wrote:
Why did both sections land flat with no twisting or turning?

Cantilever or pier/footing failure results in a straight vertical drop?

The left sidespan dropped like half a teeter-totter drops when it breaks in half in the center. It lost its counter balancing weight when the mainspan sheared off over the left (east?) pier.

The mainspan seems to have sheared at both ends and dropped straight down, but the images don’t guarantee that so far. I saw some video footage that seemed to show lateral (downstream) deflection on that end of the main span, side span, and supporting steelwork.

Right mainspan pier or associated steelwork, or possibly a right side approach span pier or deck. Either that mainspan pier failed on it’s own, or else something transferred many times design load to that pier very rapidly and it structurally vaporized.

I believe that’s the west end of the bridge, but some confirmation would be helpful as left and right are ambiquous, depending on POV.

[Southack]

"The mainspan seems to have sheared at both ends and dropped straight down, but the images don’t guarantee that so far. I saw some video footage that seemed to show lateral (downstream) deflection on that end of the main span, side span, and supporting steelwork.
Right mainspan pier or associated steelwork, or possibly a right side approach span pier or deck. Either that mainspan pier failed on it’s own, or else something transferred many times design load to that pier very rapidly and it structurally vaporized."

Structurally vaporized. Mainspan sheared at both ends and drops straight down. You heard it here, first.