When Massachusetts Department of Transportation officials decided to replace the existing deteriorated bridge over the Assabet River on SR85 in Hudson, Massachusetts, they wanted to minimize disruptions while providing a minimum service life of 75 years. To achieve that, they specified precast concrete Northeast Extreme Tee (NEXT) beams that were erected in only two days.
The new bridge, with a single span of 85 feet, replaces a three-span granite stone arch bridge. It features seven NEXT 40D beams supported by cast-in-place concrete abutment pile caps on steel-cased micropiles. The new pile-cap abutments are placed behind the existing abutments. It also includes 12 10-inch-thick precast concrete approach slabs that are 7’4” wide and 15 feet long, as well as eight 12-inch-thick precast concrete closure-wall panels that are 5 feet tall and 13’1” long at their maximum.
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Beams Expedite Construction
“NEXT beams were used to expedite construction and to minimize the closure of the bridge to vehicular and pedestrian traffic,” says David Massenzio, structural engineer in the Bridge Department of Jacobs Engineering, which designed the project. The NEXT beams offered other benefits, including no requirement for painting, minimal maintenance, pre-installation of utilities and supports, and beam flanges (8 inches thick) that were used as the bridge deck. “The lengthy duration of cast-in-place concrete deck installation was not required with the NEXT 40D beams.”
Due to the required span and width of the new beams, a concrete compressive strength of 10,000 psi was used to meet AASHTO and MassDOT prestressed-beam design requirements. To provide continuity of the transverse flange reinforcement between adjacent NEXT beams and allow the bridge to act as a rigid structure, longitudinal ultra-high performance concrete (UHPC) closure-pour joints were required. The project is one of two projects using UHPC in Massachusetts. Ultra-High Performance Concrete Connections for Prefabricated Bridge Elements (PBES) is one of the innovations highlighted in FHWA’s Every Day Counts (EDC) Four Initiative.
Site Restricted Cranes
The beams were erected in only two days despite a variety of site restrictions, including overhead utility wires and nearby storefronts that limited crane movements. Two cranes with capacities of 500 tons and 450 tons were required to erect the beams. The cranes had to retain a sufficient distance from the newly installed abutment pile caps to ensure loading under the crane mats did not impact the abutments. Prior to construction, an adjacent building was demolished which allowed for additional area for construction-staging, but other nearby retail outlets remained open during construction.
Total construction time, from demolition of the existing three-span masonry-arch bridge until opening of the new bridge is projected to take less than 3 ½ months. “Pre-installation and testing of micropiles at the south and north abutments prior to closing the bridge was beneficial in reducing the duration of the bridge closure,” says Massenzio. Ornamental light poles were incorporated into the design to help integrate the bridge with the adjacent downtown area.
