A number of key challenges had to be met in designing a replacement structure for the existing William H. Dalton Memorial Bridge in Massachusetts. To help meet the logistic and aesthetic needs, designers created a three-span structure consisting of New England Bulb Tee (NEBT) girders that is being constructed in phases to keep traffic moving.
Among the criteria for the new design, which replaces a five-span structure composed of concrete T-beams with steel widening, were the need to minimize traffic disruptions and provide an aesthetic design that maintains the character of the memorial bridge. Designers took those a step further by reducing the number of piers in the narrows and providing long-term durability.
“We like precast concrete, especially over salt water,” says Marcia Kelly, senior structural engineer for transportation projects at AECOM. “It was definitely the best way to go.” The NEBT girders were used due to the long spans the designers wanted. “For long spans, the NEBTgirders are the most popular shape available in New England.”
The design features two end spans of 10 112-foot-long NEBT 1600 girders, with a center span of 10 125-foot NEBT 1600 girders. The bridge is 70’-9” wide, allowing the contractor to maintain two lanes of traffic in each direction while the other lanes were demolished and replaced. “It was a really tight staging, but we had to keep traffic flowing,” she says. Average daily traffic on the bridge is approximately 25,400 vehicles.
The precast concrete design also allowed for the replication of the granite facings used on the original structure’s substructure elements. “Massachusetts requires granite cladding over salt water for protection, and we wanted to keep the new bridge in character with the original bridge, which is a memorial structure,” she explains.
The designers selected traditional New England gray granite and had 6-inch-thick slabs shipped to the precaster. “Because we were able to use thinner pieces than full blocks, we were able to provide a more economical design that also was easier to ship,” she notes. The precaster laid the slabs with stone anchors into forms prior to pouring the 8-inches of concrete to form an integral panel. They were then set on footings inside the cofferdam and the core was filled.
The project is progressing through five stages of construction to provide continuous traffic access. Construction also has to take into account periods of water restrictions to avoid interfering with fish breeding. The work is expected to be completed in 2014.
