The new expansion of the San Francisco Museum of Modern Art (SFMOMA), currently under construction, represents the first major use of FRP cladding on the exterior of a multistory building in North America. Designed by Norway-based practice Snøhetta, in collaboration with local firm EHDD, it rises 10 stories in a narrow space partially adjoining the existing red brick building. The east facade is a series of brilliant white curved surfaces, with a rippling horizontal texture recalling the waters of San Francisco Bay just a few blocks to the east. The north, south and west façades are largely opaque white glass, except for a fragment of the west facade where the glass seems as if scraped away to reveal white, rippled FRP texture.
You can see construction progress here. As of this writing, the building, constructed by general contractor Webcor, is still in the beginning stages of construction.
The cladding panels are being fabricated by Kreysler and Associates, a leading consultant in architectural FRP (and – full disclosure – sponsor of this blog). The FRP façade “rain screen” panels are being fabricated using recyclable EPS foam molds that are milled by a CNC hotwire machine for rough shape, and by a 5-axis CNC router for creating the final double curved surface. The approximately 700 panels are each about 5.5 ft wide, and range in length from 7 to 30 feet. The skin thickness is just 3/16 inch. In the finished east façade, about 54,000 sf, there will be visible joints, both for visual effect and to allow for natural (and possible seismic) building movement.
According to Bill Kreysler, President of Kreysler and Associates, “FRP was chosen because of its durability, its inherent characteristic of being formable into complex shapes, and its very high strength-to-weigh ratio leading to less steel and an overall shorter schedule.”
The minimal weight is especially valuable in a high seismic area such as San Francisco. The lighter the cladding, the lighter the support system necessary, the less vulnerable the building is in an earthquake. This is a bit counter-intuitive, because it is contrary to our personal experience: we are used to heavier things being harder to move. But when it’s the Earth itself that’s moving, nothing is too heavy. Anything attached to the Earth moves with it, anything unattached stays in the same position in space while the world moves around it. At that moment, the structure of a building determines how much it is attached, and how much it isn’t. The greater the building’s mass that is set in motion, the greater its ability to rip itself apart.
If this façade had been molded in panels of 6-inch thick concrete, it would weigh about 70 lbs/sf. The FRP panels weigh on average only five lbs/sf. The weight savings makes SFMOMA a safer building, or a building that is less expensive to make safe.
In the past, one of the challenges to extensive use of FRP cladding has been fire safety regulations. Kreysler solved this issue by inventing a specially formulated composite integrating fire resistance throughout the panels. Bill Kresyler describes the patent-pending process as “a proprietary blend of synthetic resins and natural aggregate that provides an attractive but extremely durable and highly fire-resistant product.”
Keep watching for further details.
The newly expanded SFMOMA is expected to open in 2016.
The complex three-dimensional geometry of the rippled façade, which will be the signature architectural feature of the museum’s new building, relies on digital fabrication to produce 700 façade panels. Each façade panel has its own unique mold geometry with complex double curvature. Kreysler & Associates utilizes CNC machinery to mill EPS foam molds in the precise geometrical shape of the rippled façade panels.
The fabrication process begins with a CNC Hotwire machine to control a heated wire that melts a path through raw blocks of EPS foam. The Hotwire is a 4-axis CNC machine making rough cuts approximately one inch above the final mold surface. Grasshopper scripts for Rhino were developed to program Gcode for the Hotwire machine. Each panel requires its own code of several hundred or thousand coordinates. The Hotwire machine is limited to defining ruled surfaces.
The foam molds are then transferred to a large 5-axis CNC router which machines the foam molds to the final panel shape. The 5-axis machine defines critical indexing keys and other alignment geometry. Toolpaths are programmed using Delcam’s Powermill software.
The SFMOMA façade panels will be the largest architectural application of composites technology in the United States. Kreysler & Associates developed the first composites system (patent-pending) to successfully pass the NFPA 285 multi-story fire-resistance test, required for such extensive use of composites in façade applications. The composite panels will have a sand-blasted polymer concrete finish.
The rippled façade of the new SFMOMA building is an example of how cutting-edge digital fabrication and composites technologies are transforming the face of our built environment.
images courtesy of Courtesy of MIR and Snøhetta via archdaily.com