This single-family Nantucket residential compound uses a variety of materials and structural systems. Architect Workshop/APD’s vision for this project centered around creating a private subterranean world, completely hidden from public view. One of the most compelling structural challenges was the design of an underground basketball court, uniquely positioned with its playing surface adjacent to the ocean and situated below sea level. Realizing this bold concept demanded innovative structural thinking and the resolution of numerous complex engineering challenges inherent in building below the waterline.
Comprised of three buildings above ground connected by a single 8,000-square-foot basement, the primary goal was to maximize usable floor area while respecting the strict zoning requirements of the island.
The basement contains an 18-foot-tall, 1,300-square-foot sports hall, located below sea level.
Excavation of the deep basement was facilitated by a sheet pile cofferdam that was installed down through groundwater to sea-level elevation. A 5-foot-thick tremie slab seals the excavation and counteracts buoyancy underneath the foundations.
Another key element of the vision was the creation of an expansive, 2,000-square-foot, column-free garage to house a collection of classic automobiles that required a structural span of 45 feet.
To achieve the garage roof span, a post-tensioned concrete slab with a tapered top surface for drainage was introduced, allowing for an open, unobstructed space beneath a heavily landscaped terrain. One hundred and fifty high-strength steel tendons (shown in blue) were draped through the depth of the slab with curved profiles for optimum post-tensioning of the concrete slab.
One of the ensuing challenges of the ambition for a subterranean world was to create light and airy spaces below grade. The integration of a sunken garden and windows in large punched openings in the reinforced concrete walls supporting the suspended gardens above allows natural light to percolate into these spaces.
Inside, the two-story gable masses have open gabled ceilings that required ridge beam spans of up to 40 feet.
Use of 3D coordination software through design and during construction allowed for bespoke framing solutions such as around this retractable TV that is housed in a chamber hanging from the floor framing.
This spiral staircase with integral guardrail stringers of curved steel plate was designed in collaboration with stair fabricator, EeStairs. The central helix behaves like a column, providing structural strength and stiffness from foundation to second floor. Lateral restraints at each floor provide stability only, relieving these wood-framed floors of supporting the stair.
Delivering this highly customized home to fruition required precise coordination across all disciplines, with CRAFT on site to observe execution of the critical structural details. The project stands as a testament to creative thinking and technical expertise, pushing the structural engineering of single-family residential construction to new levels.
PROJECT TEAM
Architect: Workshop/APD, General Contractor: Reid Builders, Foundation Subcontractor: Toscana Corporation, PT Slab Subcontractor: Turner Brothers, Stair Fabricator: EeStairs, Landscape Architect: Miroslava Ahern Landscape Design Studio
