“Those are the vehicles that wheel the bridge into place.”
Oh, OK. It appears that shoring should have been in place before those mobile jacks were removed, which would have been a good use of a couple of traffic lanes. Hindsight, but would have been obvious even to a child in advance if they’d thought about it.
Clearly the truss was designed to support its own weight after the vehicle jacks were lowered and shoring would not be needed. As a structural engineer said above: materials error; design error; workmanship error.