Note the beams still connected together in the "Vibration problem" photo. The center connection plate appears to be the one that used to sit on the back pier. This assembly seems to have rolled towards the other pier. Since the video of the collapse shows that the center sections fell as a unit, and this beam assembly is still in one piece, it suggests that the initial failure was one or two sections north of these piers, close to where the construction equipment was located. Could the point load of the construction equipment and the vibrations from jack hammering have been a precipating factor? Could expansion due to the relatively high temperatures and corrosion of the pins have put unexpected
stress on components? By carefully looking at the beams in the pictures it is possible to determine where those components were located in the truss assembly. It is interesting to note where rivits/bolts are now missing, and where the beams tore away as the structure came down.