Officials at the Vermont Agency of Transportation required a 100-year service life for the new twin bridges carrying I-91 over VT-44 and Mill Brook in the Town of Windsor. To achieve this goal in the most economical way, designers specified 157-foot precast concrete bulb-tee girders for the four-span bridge.
The existing bridges were structurally deficient, having been built as part of the highway’s creation in the early 1960s. They had met their 50-year design life, and officials wanted to ensure the new structures offered longer durability and lower maintenance needs. Designers were given a choice between precast concrete and high-performance steel, and both were evaluated.
“Pricing was not conducive for the steel option,” explains John Wilson, senior project manager for Jacobs Engineering Group, which worked on the design-build project with Lane Construction Corp. The temporary supports required during construction for the staging to support the two-span steel option added a significant cost to the project, he says. “We created a precast concrete design, which not only provided a simpler construction method but will reduce maintenance and inspection needs for the owner.”
Each of the two bridges, one northbound and one southbound, contains 20 beams, four lines of five beams, with all beams cast 157 feet long. “We needed some of the beams to be 157 feet to span the river in the most effective way to avoid the federal floodway restrictions,” he says. “We decided to design all of the beams to the same length to simplify the design and reduce costs.” Northeast Prestressed Products LLC provided the superstructure elements. The beams are supported by cast-in-place piers and abutments.
Designers used software programs to evaluate the girders for the 100-year design life to create the best combination of concrete mix and cover for different types of strands and rebar. The software took the input variables and calculated the time before reinforcement would corrode in each combination. “We adjusted and analyzed the mix designs and amount of cover, and we added a corrosion inhibitor to the mix to enhance durability to achieve the desired life cycle.”
Stainless-steel reinforcing was used for all pieces protruding from the girders into the deck, while epoxy-coated reinforcing was used for the rest of the rebar. Black-steel strand was used to prestress the beams. Two inches of cover were provided for the black-steel reinforcing, as it was used in areas not easily accessible to salt, with increased cover used in other areas more susceptible to salt, he explains.
Delivering the 157-foot beams from the precaster’s Pennsylvania plant created some challenges, due to varying restrictions between states on delivery requirements. The beams couldn’t be transported on weekends, with only two transports allowed over the same stretch of road per day. In addition, work was underway to reconstruct a bridge along the delivery route, requiring that deliveries wait until the bridge was structurally completed before trucks could proceed.
The precaster delivered and the contractor erected one span per week during construction, with deliveries occurring Wednesday through Friday and the erection taking place the next Monday and Tuesday.
Construction is being phased over two construction seasons, with the southbound bridge replaced in 2013 and the northbound bridge scheduled for replacement in 2014.