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. An engineering study around 2015 determined that the most cost-effective solution was to replace both bridges.
When the owners put out a call to engineers for proposals, HDR’s team consider five possible design options, laying out the benefits and challenges of each. They ultimately chose an innovative precast concrete design using pretensioned bulb-tee beam segments that are post-tensioned together.
“A high-level cost analysis of each option showed that the four-span, spliced precast concrete girder alternative was the least costly, and best met their means and methods,” says Thomas French, senior project manager and office leader for HDR. He notes that the girders are designed for a 100-year life span, exceeding the typical 75-year standard service life for bridges in the area. The design includes using high-performance 9000-psi concrete, and stainless steel for any projecting steel.
The project owner agreed that precast concrete was the best choice. “Precast, prestressed concrete provides great long-term durability as well as reduced maintenance efforts over its lifetime versus a steel superstructure,” says Carolyn Cota, project manager for the Vermont Agency of Transportation (VTrans).
The spliced precast concrete girder option brought many ancillary benefits to the project. It allowed a reduction in superstructure depth and weight, will require minimal maintenance, and has a smaller “coefficient of expansion,” reducing the need for multiple large bridge joints.
This is a major issue for bridges in the region, says project engineer David Peterson. “VTrans has found that failing bridge joints are one of the largest factors in reducing the life of a bridge and increasing the need to perform maintenance and repairs, especially in colder climates where road salt is heavily used during the winter,” he says.
A Long and Winding Road
The winning 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 lb to as much as 186,920 lb. “The final design was based on J. P. Carrara’s capacity to fabricate and ship the elements to the jobsite,” 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 jobsite.
Once demolition was complete, the bridges were constructed in two phases with one bridge built at a time. Highway crossovers were used so that one lane of traffic in each direction was maintained on the existing bridge while the new bridge was constructed adjacently, preventing the need for total road closure.
Due to the large size of the girders, cranes with 440-ton capacity were required to install them. The contractor built a temporary trestle across the river that had enough capacity to hold the weight of the cranes as well as the picks. The resulting bridges were completed on schedule in the fall of 2020, and the owners are thrilled with the results.
“The continuity between the superstructure and substructure provides an aesthetically pleasing bridge,” Peterson says. “Vermont is a small state, so when we are giving the opportunity to provide a unique bridge for the public, we take pride in making that happen.”