[Ramius]

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.

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.

[jeffers]

The deck can act as a membrane for lateral stability, but that adds greatly to the dead load. In mechanical effect, you have a zigzag series of braces to transfer the lateral loads doing it that way, whereas a very few diagonals connecting the cantilever truss systems accomplish the same objective.

This is what the pictures tell me: