SAN FRANCISCO—Structural engineers from Arup, a global design and business consulting firm, with design architects Renzo Piano, local architects Chong Partners, and other design and construction team members, created the visually striking and environmentally friendly roof assembly recently constructed atop the new California Academy of Sciences building in Golden Gate Park. This undulating 2.5-acre "living roof" blanketed with 1.7 million native California plants is one of the key contributors to the building’s overall sustainability, helping it to potentially earn a top rating of LEED Platinum from the U.S. Green Building Council.

With seven "hills" to mimic the seven hills of San Francisco, the roof structure consists of a grillage of curved steel beam sections—some spanning up to 96 feet—that support an undulating concrete slab. The curved steel beams form a structural "skeleton" whose concrete "skin" was applied from above with the aid of temporary timber formwork to achieve a carefully contoured finished surface.

The combination of complex geometry, architecturally exposed steel, and seismic detailing made the steel connection design a challenge. Three-dimensional computer modeling techniques were used to create connection details that were both structurally and aesthetically acceptable. The structural engineers then provided these computer models to the steel fabricators who prepared their own computer models to define the shape of the steel members and construct the intricate connections.

Above the concrete structure, the living roof build-up consists of a waterproof membrane, root barrier, insulation, filter fabric, drain mat, and 6 inches of lightweight soils. Slope stability is provided by a 24-foot by 24-foot grillage of rock-filled metal cages (gabions) anchored to the concrete slab. The total imposed load from this build-up, including the weight of water-saturated soil, was 35 pounds per square foot. This roof load, while notably higher than a traditionally designed roofing system, resulted in a relatively nominal increase in the total cost of the structure.