Source: PCI Northeast
Citizens in Berkley, Massachusetts, couldn’t abide a two-year closure for the replacement of the Padelford Street Bridge, a key overpass. To alleviate that impact, the Massachusetts Department of Transportation reviewed its plan and decided to find an accelerated bridge construction (ABC) solution. As a result, the new bridge features 46 precast concrete substructure units and eight prefabricated bridge units (PBUs). Each bridge unit consists of a pair of structural steel beams cast compositely with a concrete deck.
The solution resulted from a base technical concept that proposed using prefabricated/precast concrete elements as much as possible. MassDOT also used performance-based specifications with a design-build delivery method as part of the objective to limit the bridge closure to six weeks.
The 204-foot-long overpass, the first in the state to use performance-based specs, was required to provide two spans of steel stringer construction, a specific clearance height of 16’9” over State Route 24, and a prescribed roadway profile. The bridge cross section features two 11-foot-wide vehicular travel lanes, two 5-foot shoulders, and a 6-foot sidewalk on the north side. CP-PL2 concrete traffic barrier with Type II protective screen was provided at the outside edges of the bridge deck.
The bridge initially was scoped to be a design-bid-build project, explains Andy Benkert, project engineer at design firm WSP Group. “The plan was to use conventional construction with cast-in-place concrete and steel girders in a traditional overpass,” he says. “But when the town learned of the two-year duration of construction for that approach, they had significant concerns about the time frame and requested MassDOT investigate alternatives to reduce the impact to the community.”
MassDOT, in its base technical concept, conceived a “heavy lift” construction option utilizing SPMTs to set the new bridge during weekend closures of Route 24. However, with the design-build process and performance specifications, prospective bidders were able to develop their own best-value option to meet the six-week bridge-closure limit. This allowed for innovation to achieve the specs at a lower cost.
The design-build team of MAS Building and Bridge and WSP met the challenge by designing with the precast concrete elements. The PBUs consist of 41-inch-deep, welded-plate girders with an 8-inch concrete deck. Cantilever abutments and center pier-bent substructure elements founded on spread footings also were used. The entire substructure, including pier cap, pier columns, abutment and wingwall stems, footings and backwalls, consists of precast concrete elements. The Fort Miller Co. fabricated all of the components.
The PBUs were designed with a unique gap-splice detail in the closure pour to avoid fit up conflicts seen on past PBU installations. The PBUs were preassembled at Fort Miller’s plant to ensure proper field fit-up and to mitigate any potential erection concerns. The precast substructure units were joined with grouted splice sleeves to speed construction on the $5.5-million project. Other innovative methods to speed construction included using tight precast concrete formwork tolerances, templating techniques, and field erection procedures to ensure the proper alignment of grouted splice sleeves and their mating dowels.
MAS and WSP developed a detailed and customized Quality Management Plan to ensure the high quality control the job demanded. “Our primary focus during the design and fabrication phases was to ensure proper fit and performance of the structure during the installation and erection of the prefabricated pieces,” says Benkert. “The construction schedule did not have sufficient float available should there be fit up issues.”
WSP worked closely with general contractor MAS and Fort Miller throughout all phases of the project. This included several site visits to Fort Miller before and during fabrication. WSP also developed design contingencies that gave MAS flexibility and increased tolerances during precast installation.
“We worked very hard to ensure there were no problems at the site,” says Benkert. “We thought through all the steps and potential issues.”
The most challenging aspect was setting and connecting the center pier, which was designed with a continuous footing in three sections, each connected with a closure pour. “Aligning all the pieces was a key issue, so tolerances were absolutely critical,” he says. The advance work paid off. “Once the precast components arrived at the site, they were 90% ready to be put in place immediately,” he says.
The bridge is an example of what can be achieved when timing is critical. “MassDOT is using precast concrete more often today to speed up the construction process,” Benkert says. “They’ve revamped their LFRD design-spec manual and encouraged the use of prefabricated components to reduce closure times.”