In Rockingham, Vt., there were two bridges over the Williams River that were falling into disrepair. They were built in the 1960s and were not designed to withstand the increasing flow of traffic in the region. The state of Vermont ultimately chose an innovative precast concrete design using pretensioned bulb-tee beam segments that are post-tensioned together.

The final design features two distinct bridges with individual substructures and continuous girders. The new bridges are wider than the existing structure, with two 12-ft travel lanes, a 4-ft left side shoulder, and a 10-ft breakdown lane. Both bridges 24N and 24S have abutments and pier substructures supported by footings on piles, with the exception of one pier on the southbound bridge bearing on bedrock.

The main prestressed concrete beam segments range in weight from 144,920 lbs to as much as 186,920 lbs. “The final design was based on J. P. Carrara’s capacity to fabricate and ship the elements to the job-site,” French says. The girders maxed out the lifting capacity at the Middlebury, Vt., plant, and Carrara used professional drivers and trucks with steerable trailers to safely transport the girders along the winding roads that led to the job-site.