Earlier this month, Simpson Gumpertz & Heger (SGH) presented on the design and fabrication techniques that resulted in the final product of the Boston Harbor Island Pavilion. This was particularly exciting in terms of resolving doubly-curved surfaces in concrete formwork to produce a stable structure with a desired finish quality.
As the first permanent structure on the Greenway, the goal of the Pavilion is to promote the Boston Harbor Islands, through the eye-catching form of the pavilion, which covers art installations and a visitor’s center, which provides further information on the islands. The form of the pavilion provides cover for the new facilities while also acting as a drain for itself. Water drains from the upper canopy to the lower and eventually into the catch basin at ground level, providing irrigation for the surrounding planted area.
The combination of the formwork and concrete mixture determined the level of resolution in the final pavilion. A thin profile and smooth finish (without touch-ups) drove the experimentation process. Various digital fabrication techniques were tested for the formwork, including CNC-milled foam block and bent plywood. The chosen formwork technique for this project was composed of plywood ribs and bent plywood sheets. Each unique rib was fabricated for a specific location with a CNC router, which drew from the same model used for steel fabrication, ensuring alignment upon construction. With ribs in place, a plywood “skin” was set in place. With 400 uniquely shaped panels, the precision of the curved form was achieved. These panels, mostly quadrilaterals, some with curved edges, were glued and screwed to the ribs. Specifically placed, counter-sunk screws have a heavy influence on the finish of the pavilion. The controlled nature of this pattern reveals itself as a designed feature.
A thin slab was desired while still preserving the structural integrity. In order to read the slab as thin as possible, the edges were thinned to three inches while the depth throughout was closer five inches; SGH compared this technique to the look of the Macbook Air, where the edges define a thin reading thickness while the actual thickness more substantial. Still a very thin slab, special attention was paid to the selection and installation of reinforcement. Thinner rebar (3/8-inch diameter) was used in order to get adequate cover above and below. The placement of the reinforcement still posed a challenged in achieving the complex curvature. This was the one place within the project where manual techniques were favored. In this case, workers bent each bar manually to achieve the curvature in the thin concrete slab.
Following suit of the formwork, the steel structure was developed through digital modeling to develop the varied bent steel ribs, supporting and highlighting the curvature of the canopies while keeping the impact at the ground level as minimal as possible.
The resolution of complex curvature within the Boston Harbor Pavilion is inspiring for the use of material innovation in the future. Smaller scale structures often act as a test bed to larger scale use in the future.
Curved canopy for draining to catch basin
Paul Kassabian, project manager, presents on the project