I’ve gone through just about all the information on the bridge. Construction began with a metal framework incorporating the span and canopy. Then cement was poured first into the span and then the canopy. The movement of the structure was done very carefully and it sure looks like everything was executed according to plan. Once the structure was landed on the two supports (South support and Tower landing) and any temporary supports removed, the full weight of the structure (950 tons) would be evenly divided between these two posts. It would be evenly divided if both posts were at the exact same elevation and vertical. Now, let us suppose that the tower post (the smaller of the two supports) moved ever so slightly either down or away from the bridge. In either case, the forces on the tower post increases, and pushes away from the structure. Under this scenario, the accident begins with the tower post moving slightly down or off vertical and then is unable to support the north end of the bridge. In rapid succession, the north end becomes disconnected from the tower and drops to the roadway, and the rest of the bridge is pulled down as a result.
The tower post seems to be located in a canal which runs along 8th Street. The base of the tower, could be the weakest point in the design during this initial installation. The rebar framework within the post, really provides the strength and resilience of the post and it could have been deformed during the loading of the span. There is no easy way to know if the tower post could support 475 tons of weight (or more if it had moved), until it was in position.
The alternative scenario is that the bridge buckled in the middle, because the structure wasn’t built strong enough. As the center of the bridge came crashing down, it pulled free of the north tower and then this section crashed to the roadway. So, it is a question of which crashed first, the center or the north side.
Certainly, a lot of investigation will be on the stress testing, to be conducted on the day of the accident and if temporary supports were removed prematurely. Also, the decision not to block off traffic during testing will be part of the investigation.
Please note, that I am not a bridge engineer, nor have any training or education in this area. So all of the above is pure speculation. I do not plan to post anything further until a final report is published. The real bridge experts are on the scene with full access to all the necessary information.