The use of precast concrete hollowcore plank in a girder-slab structural system, mixed with composite-slab deck construction, meets major building-height concerns and strong demand for high ceilings in this boutique apartment building in Boston’s rapidly-developing city core.
Gateway Boston is located at 900 Beacon Street, and sits on a 11,952-square-foot site in the Audubon Circle neighborhood on the MBTA’s Green line near Boston University and the famous Fenway baseball park. The five-story, mixed-use project contains 32 rental apartments, ground floor retail and restaurant space, plus 30 underground and 6 above-ground parking spaces. A major challenge for designers was to create an infill building that would knit together the existing urban fabric of this historic area site.
Precast Concrete Aids Numerous Design Challenges
The new building faced numerous design challenges, according to Laura Homich, AIA, LEED AP BD+C, Senior Associate at architects PCA and project architect/project manager. “It was a tight urban site requiring zero lot line insertion. There would be residential units over retail and restaurants and an underground parking garage surrounded by water. The responsibility of creating a beautiful compliment to the historic Audubon Circle context, and one challenge in particular, brought the design team to the decision to use precast planks for the majority of the building’s floor structural system” she recalls.
Explains Homich: ”Throughout the community and approval process with our neighbors and the BPDA, the building height was a significant concern. Listening to their concerns, we held our building at a particularly low floor-to-floor height, but still wanted to provide generous nine foot ceiling heights within our units. The precast planks enabled us to provide the desired ceiling heights while maintaining appropriate sound separation and fire ratings with just enough room for required MEP/FP systems.”
Specifically, the project utilized a girder-slab structural system—combining the precast concrete hollowcore planks with steel framing—combined with composite-slab/deck construction. “The majority of the third and fourth floor and the roof structure,” describes Homich, “is girder-slab with composite-slab/deck in the center at the utility cores. The ground floor and second floor are composite-slab/deck.”
In total, 108 precast concrete hollowcore planks 8-in-thick, 8-ft-wide with typical spans of approximately 30-ft were provided by the precast concrete manufacturer, J.P. Carrara & Sons.
“Long spans, the thinness of the precast concrete hollowcore planks and the ability to have the steel beams at either end of the slabs be able to be located at the top of the plank rather than underneath it,” says Homich, “enabled us to maintain our desired ceiling heights.”
The building’s foundation is cast in place concrete. The exterior façade is a combination of brick, fiber cement and composite metal panel with eight-foot windows.
Precast Concrete Aided with use of BIM Immersion Room
Selection of the efficient precast-plank/steel-frame construction system for the structure was facilitated by the collaboration and coordination of design team participants in a unique BIM 3D-modeling Immersion Room at ODEH Engineers. This facility seats 20 and features a 30-foot-wide high-resolution screen fed by 10 projectors. The process allowed the participants to model several structural variations, leading to the choice of the hollowcore plank system. The exercise also streamlined the integration of the different structural and MEP systems and reduced construction time.
“The entire team worked in Revit,” says Homich. “This enabled the team to visit ODEH, use their immersion room, and to see where we had conflicts. We were able to address or resolve them at the design stage not on the fly in the field.”
The project started on June 1, 2015 and completed November 2, 2016. Erection of the precast concrete hollowcore plank began in November 2015 and was completed in January 2016.