Soft Shape in a Sharp-Edged Context
In 1995 San Francisco Museum of Modern Art opened the doors of its new home on Third Street in a warehouse district called South of Market (SoMa). For many years, South of Market (SoMa) had been a derelict area. The neighborhood had long been eyed for commercial opportunities by outsiders, but had been beset by many abandoned or stalled development projects. The question that then faced SFMOMA and their architect, Mario Botta, was how to create a modern art museum within this context; one that could both be magnetic and withstand the grit of the neighborhood. Botta responded with a bold, muscular building that for the last two decades has served as a powerful anchor for the museum in the city and has helped transform the neighborhood into what it has become today: a dense urban and cultural center for San Francisco (Fig. C).
Snøhetta’s expansion to the museum in 2016 not only had to respond to much-changed surroundings, but also to SFMOMA’s different institutional approach toward public outreach (Fig. B). This time, the museum’s goal was to express greater generosity toward public space, offer new community programs, enhance physical and visual accessibility, and create an unmistakable sense of destination. The architecture of the expansion building was developed to express this reinvigorated mission and to be an outward-looking cultural hub of arts education. This aim is mirrored in the building’s distinctive rippled facade that draws inspiration from San Francisco‘s undulating topography and maritime climate and, as a positive gesture, makes an active contribution to its urban setting. A key driver was how the building’s exterior forms a set of relationships to the surrounding urban fabric. Given the dimensions of the planned increase in density the cramped situation presented a formidable challenge. This led us to explore a vertically stacked organization. Over 100 meters long and almost 23 meters wide, the expansion can be thought of as a horizontal skyscraper mediating between the low-lying surrounding warehouse district and the verticality of the encroaching towers of the financial district. Inside, three floors of administration are stacked above seven floors of gallery space. Yet in section and in plan the building is pulled back from the northeastern site boundary, bringing daylight down to a new entrance in the middle of the super block and allowing oblique views of the new facade from multiple vantage points around the city. The soft, doubly curved form of this facade that is freed from its neighbors gives the museum a new public face that emphasizes its physical presence in the city.
Sculpted by Wind and Sea
From the immensity of its size to the finely grained details of the facade’s texture, the exterior expression of the building negotiates multiple scales. The facade is meant to appear as if carved either by hand or honed overtime by the weather, and links a sense of permanence that is symbolic of the museum as an institution within the city, to the hand-wrought craftsmanship of stone sculpture. The expansion building incorporates something of the muscularity of Botta‘s building, while also adding geological components that reference a different time, place, and set of values.
Design-Assist Planning Procedures
Just as the facade’s form references natural phenomena in the Bay Area, the technical processes that first enabled it to be built are also rooted in this region. From the latest digital fabrication technologies and computer code that emerged mostly from Silicon Valley, to the pioneering use of fiberglass-reinforced polymer that originated in the West Coast boat building industry, a compelling narrative can be woven from the many threads that tie the design to the city and the geological, cultural, and industrial landscape from which it emerges. A key factor in realizing the facade was Snøhetta’s partnership with the Kreysler & Associates, a local firm of facade fabrication consultants, which through a seamless design and fabrication process significantly expanded the area in which fiberglass-reinforced polymer (FRP) can be used. Bill Kreysler’s knowledge of manipulating composites like FRP is derived from over 30 years of building racing boats in the Bay Area. Kreysler & Associates have positioned themselves as leaders in CNC technology, with an in-house team of engineers that leverage the latest 3D software and use computer code to help realize various small projects for artists. In the SFMOMA expansion building, engineers and fabricators were involved in the design process from the start of the project in order to ensure a seamless sequence of processes that would allow nearly every step of the design, fabrication, and construction process to be optimized. Snøhetta developed the design surface in Rhino, Grasshopper, and a custom Python script. Kreysler used the same 3D model directly for FEA engineering, shop and fabrication documents, and ultimately to run the CNC machines that made the molds for laminating the panels. This direct line of communication between the designer and the fabricator enabled the complex design of the building envelope to be transferred to the facade panels with essentially no loss of detail, apart from those due to typical machine and material/fabrication tolerances (Figs. G – N).
Plastic instead of Concrete
Early on, the design team looked into precast concrete, ultra-high-performance concrete (i.e. ductal) and GFRC (glass fiber reinforced concrete). However, it was found that the lightweight characteristics of FRP with only just 25 kg per m² facade area would allow the greatest reduction of building structure, which translated to significant cost savings. This reduced weight allowed lighter and smaller structural steel sections to be used and consequently refined the quality of the architectural design. The FRP was also lightweight enough to be married onto a standard, thermally insulated curtain-wall system (Fig. M). Twice as many of the panels, which measured up to 1.50 × 9 meters in size, could be loaded onto a flatbed truck as would have been possible with precast concrete elements. The panels could be installed via a single pass with the crane, allowing up to 18 elements to be fitted in a single day (Fig. N). This obviated the need to erect scaffolding, which given the site constraints and the building geometry would have been extremely complicated. Multiple passes at assembly stage would have been cost-prohibitive, so using FRP maximized efficiency and minimized costs. Maintenance and cleaning of the facade is managed through a roof-mounted BMU (building maintenance unit) which can reach all areas of the facade.
Rough FRP Shell
The fiberglass build-up behind the finish coat is about 4.7 mm thick and consists of several layers of woven roving glass fiber and polyester resin. A c. 1.6 mm thick finish gelcoat of polymer concrete, which consists essentially of polyester resin, sand and additives to improve its UV stability and fire resistance, was applied to the outer face as weather protection. While the rippled geometry was developed to provide the FRP panels with a degree of self-reinforcement, they also have a light aluminum frame attached around the inside flange which provides a stable means of attachment to the unitized aluminum curtain-wall (Fig. I) To maintain visual continuity between the exposed FRP surfaces the stiffeners and the thermally insulated aluminum panels of the curtain-wall are not visible from the exterior. There are two ingredients contributing to the finished color of the facade: polyester resin and locally sourced sand from Monterey. As sand is a natural material that may vary in color, to ensure color consistency a single batch was purchased at the start of the project and was then mixed and stored. After the gelcoat finish had hardened, the surface of the FRP shells was roughened by means of sandblasting so that instead of the shiny resin the sand particles are exposed and produce the matt, sandy appearance that was desired (Fig. K).
Customization
Each of the 710 FRP panels is unique and was produced by using a specially made mold. While this degree of customization may initially seem unwarranted, especially given alternative materials like GFRC or metal, the fabricators were able to find ways to do this relatively economically. Since all the molds were used just once, they could be made out of standard EPS foam, which is very inexpensive, lightweight, machinable, and recyclable (Fig. G). A side benefit of the custom tooling is that once the panel was cast, the molds could be used as tailor-made handling cradles for transporting each panel to the construction site (Fig. L). In contrast to this elaborate customization the sizes and folds of the modular, thermally insulated panels of the aluminum curtain-wall to which the FRP panels are attached were limited to the minimum required to ensure simple and efficient weather seals (Fig. O).
Fire Protection Certification
The total surface area of the FRP envelope that consists of rippled panels and flat filler panels for the jambs and soffits amounts to around 5100 m². Given that this was the first application of FRP as architectural cladding at this great a scale, the design had to undergo rigorous fire testing. To evaluate flame spread a full-scale, two story mock-up of the modular curtain wall was exposed for a 30-minute period to an intense flame that came through a window opening. This facade was the first composite system to pass these rigorous fire regulation tests. These excellent results allow this facade construction to be used in the USA as external cladding to the upper floors of a high-rise building (from the fifth floor). The successful and efficient use of this material in the SFMOMA expansion building has certainly paved the way for making more frequent use of it in architecture projects in the future.
Looking Ahead
The development of such an efficient and unmistakable facade was possible only through early collaboration and iterations traded back-and-forth between the design team, the fabricators and the engineers. In this project the prevailing climate was one of innovation and cooperation that constantly developed further allowing a higher level of customization to be achieved in the design without, however, exceeding budgetary or feasibility boundaries. Today it is predominantly the factor of tool time that impacts production costs, which are largely unaffected by whether products are standardized and identical, or individually shaped. Fiber-reinforced polymers, which have been in existence since the 1950s (and therefore at least a decade longer than glass fiber concrete) have already been employed in applications far more elaborate than facade claddings. As FRP has an extremely long life it is frequently used in applications where durability under severe loads is critical e.g. acid storage tanks, wind turbine blades, salt water fish tanks (see the Monterey Bay Aquarium), and Antarctic research stations. The actual life span of such an FRP product is difficult to predict as it is basically inert and almost impermeable to water. It therefore seems clear that design concepts will increasingly exploit the adaptability of FRP along with its qualities as a building material and its suitability for customized forms, revealing new possibilities to create buildings that are ever more sensitive and compelling.
