Evidence indicates that the failure was due to the column failing in flexure and the Span 12 bearings becoming over-rotated to the extent that they became unstable and tipped over. For the bearings to over-rotate, the pier column had to deflect in the longitudinal direction of the bridge. Since the investigation revealed that the pier foundation did not move, the only plausible alternative is that the top of the pier was deflected horizontally by forces caused by loads and thermal movements from the superstructure that were transferred through the bearings. This is reasonable based on the geometrics of the failed condition, and the findings of laboratory inspections of the bearings.
Observed levels of rust build-up and dishing of the masonry plates created conditions which inhibited their free movement and allowed for the transfer of horizontal forces. Over an extended period of time, the bearings continued to transfer horizontal forces to the pier, gradually increasing the column deflection. This increased deflection caused the bearings to tip further to the north to a point where the Span 12 bearings became unstable and tipped over.
At about this point the pier column cracked, which enabled the column to deflect further from reduced horizontal loads. With the Span 12 bearings tipping, enough column deflection was produced to result in the Span 11 bearings also tipping over, which in turn produced even more column deflection.
The Span 12 girders fell onto the edges of the pier pedestals, shearing the pedestal concrete and ultimately rested on edge of the pier cap. The Span 11 girders fell onto the tipped over Span 11 bearings.