Monitoring Prestressed Concrete Box-Beam Bridge for Superloads

Monitoring Coeymans Creek Bridge in New York State for superload permit trucks is discussed. The superloads were boiler modules carried on 16-axle trailers, driven by one or two tractor-power units during the bridge crossing. The bridge is an integral abutment structure consisting of 11 prestressed concrete box beams with a composite concrete deck. Approval of the superload permits was based on an engineering analysis that recommended crossing of the bridge in two configurations: the first required that a crabbed trailer be towed across the bridge; the second called for a trailer to be driven in a diagonal crossing fashion. Crabbing, that is, locking trailer wheels to remain parallel to the bridge centerline during the crossing, was recommended for moves of gross weights equal to or exceeding 1,775 kN (400 kips), and diagonal crossings were recommended for those lighter than 1,775 kN (400 kips). The low rating of the structure and the unusually heavy loads motivated the need for investigating actual stress levels in the bridge beams during the superload moves. There was also interest in comparing the two recommended crossing patterns and investigating the level of fixity provided by the integral abutments. The beams were instrumented, and strain data were collected during four of the moves. Analysis of the collected data indicated that the stresses in the beams remained below what would have caused cracking and that, for this bridge, crabbing had no clear benefits over diagonal crossing. The analysis also indicated that the bridge has good transverse load distribution and a significant level of end fixity provided by the integral abutments.