The “block” lost internal integrity. It deformed diagonally, towards the downstream direction.
You lost a diagonal brace in there somehow, could have been overload, could have been a shoe collapsed, some crumbling of the piers (though they appear to be intact in the imagery) bolts connecting diagonals to uprights, etc.
Look at the picture in post #1404.
From the top of the nearest pier, follow steel vertical a short distance till you reach a diagonal that runs towards the top of the steel truss on the other near side pier. It’s purpose is to keep the trusses vertical over the piers and parallel to each other, so the deck can’t sway sideways, away from bearing directly over the piers.
From the available imagery, this appears to be where and how the failure occured.
Deliberate act has to remain a possibility, but my best guess says construction accident. On many jobsites, you have licensed and highly experienced engineers designing the final result, and much lesser mathematically inclined supervising the design of falsework and order of re-construction. Even if this level of management gets it all perfect, you still have the stiffs who may or may not carry out orders perfectly every time.
Gravity is a cruel mistress who tolerates very little error.
Thanks Jeffers.
A fair point. Though, from other reports my present understanding is that the construction work happening on the bridge was limited to merely resurfacing the road and that there was no structural work happening under the bridge at the moment.
Way upthread there was somebody (sorry, don't remember who...) that posited the idea that maybe it was the breakup of the roadbed that was the final straw that broke the bridge. This is an interesting idea.
The ribbon of asphalt and concrete that spans the bridge as the roadbed isn't usually considered to be among the structural supports. But it makes sense that this ribbon might spread the forces on the bridge and that some other structural problems on the bridge might be masked by this peripheral support. Then... when this ribbon of material is broken, it then of course cascades to expose the other weaknesses and perhaps break them.
Sheer guesswork at this point. The investigation will figure it out.
Crews had been ‘grinding’ out the old concrete, to prepare for pouring new.
There were concrete trucks (very heavy loads in concentrated areas) all over the bridge during bumper to bumper traffic (max vehicles on bridge).
Maybe somebody ground down tooooooo far. It would be like a row of dominos.