Designers on the Alloy Residence phase of the large Assembly Row mixed-use community in Somerville, Massachusetts, faced key challenges in creating the facility’s 122 high-end condominiums. Those goals included erecting a 13-story building with underground parking garage through a New England winter while substantially completing the building’s exterior as they went. To help achieve this goal, designers specified precast concrete hollowcore for the structural flooring systems.
The Assembly Row complex, developed by Federal Realty Investment Trust, features several segments that will add up to more than 600,000 square feet of retail, 1,800 residential units, and 2.8 million square feet of office space. Now underway are segments called Block 5A, the 13-story Alloy Residences & Autograph Collection Hotel, and Block 6, a 20-story, 273-unit apartment building along with a six-story, 174-unit wooden structure and a seven-story, 650-car precast concrete parking structure.
“Hollowcore was specified for the structural flooring systems for a number of reasons for the Block 5A project, several related to the harsh winter conditions that would be prevalent during construction,” explains Jake McManus, project superintendent and Brian R. Knox, senior project manager, both of PROCON Inc., the design-builder on the Block 5A project.
“Hollowcore could be fabricated in a temperature-controlled environment for the Block 5A project, allowing for the erection of the structural planks during the winter months,” they noted. “That led to faster on-site erection time, with quicker floor turnover to the interior/exterior framing contractors. It also provided low floor-to-floor heights. The total floor-assembly depths are far less than could be achieved with conventional structural steel methods.”
Some 1,240 pieces of hollowcore, encompassing 206,000 square feet, were erected for Assembly Row. The hollowcore is 8 inches thick and 8 feet wide, which accommodated a change to the design to angle the wings in the L-shaped building.
“Our 8-foot-wide planks could accommodate the skewed geometry without the need for custom solid wet-cast components,” explains Joe Carrara, director of sales and marketing at JP Carrara & Sons, which fabricated the hollowcore. “The structure relies on the hollowcore to support the exterior perimeter wall system, eliminating the need for steel spandrel beams” he notes. The structural system also uses hollowcore to transfer horizontal diaphragm cord and shear forces via additional prestressing strands and heavy embed-plate assemblies to the structural steel-framing systems.
Proper Spacing Required
A key challenge for the Block 5A project was the need to maintain proper spacing between planks so the joints could be adequately grouted, McManus notes. “Those can always be tough, and this project had some areas with steel top flanges that weren’t wide enough to allow for proper minimum bearings on both sides while maintaining a 1-inch joint.” A 1.5-inch gypsum-cement underlayment topping slab will be applied to the hollowcore, with slight variations to allow for tolerances and camber.
The hollowcore was installed in four months on the Block 5A project, from September 2016 to January 2017, with occupancy expected in early 2018. The work is progressing quickly despite the large amount of construction in the area, Knox reports. “Overall, the project has gone relatively smoothly, but staging the planks and trailers for site deliveries has been challenging,” he notes. Close communication with the precaster and traffic authorities have kept deliveries running well.
Similar hollowcore components are being used for the Block 6 housing units, also supplied by Carrara. It features 300,000 square feet of hollowcore in 1,683 pieces. The hollowcore was installed in five months in mid-2016. Here too, the structural system relies on the hollowcore to transfer horizontal diaphragm cord and shear forces via additional prestressing strands and heavy embed-plate assemblies to the steel-framing system.
The projected was completed in 2018.