December 19, 2007
Perhaps the most famous cantilever in America is one of the shortest: Frank Lloyd Wright’s 1935 design for the exterior concrete terraces at Fallingwater, the longest of which extends a mere 15 feet to hover over the rush of Pennsylvania’s Bear Run stream. Much has been made of the ongoing structural repairs the cantilevers have needed since they were built, but there has never been a question about preserving them.
A good cantilever can make or break a project. Writing in Space, Time and Architecture, Sigfried Giedion saw in Wright’s extensive use of the cantilever a revolution in design, where space becomes an “essential part of the structure, molded as an inseparable part of it.” Cantilevers found widespread favor in the development of Modern architecture in the last century. Mies van der Rohe used them to extend his “universal space” and to further dematerialize the field of vision in his projects. Architects who practiced certain strains of Modernism—think of the hanging corners at Hugh Stubbins’s 1977 Citicorp Center in New York—preferred to disrupt, sometimes unintentionally, architectural stability with the cantilever.
Mega-projects with mega-budgets can more readily reach for these spectacular feats of engineering. But smaller projects, when done well, can still retain the power of the cantilever to draw our eye across its gravity-defying expanse. Wright used cantilevers two ways—for overhangs and habitable space—which is how we can think of three new projects completed in 2007 in Michigan, Japan, and Germany.
For the corporate offices of the Lamar Construction Company, located outside of Grand Rapids, Integrated Architecture designed a 52-foot-wide and 100-foot-long cantilever for the building’s second floor. Michael Corby, AIA, a principal with Grand Rapids–based Integrated Architecture, says his office had never attempted a cantilevered design such as this. “We used a temporary scaffolding to build the trusses in the air,” Corby says. “We then pulled the scaffolding away and the steel just hovered there,” he says. The building has two volumes separated by a 16-foot gap—a lower, pre-engineered, orange (the company’s signature color) steel box for storage and offices, and the cantilevered glass box for offices and a dining area. The interior of the primary cantilever is defined by the two 16-foot-deep trusses, each of which arrived on-site in three pieces. The trusses, spaced 26 feet apart, were also painted orange. A secondary cantilever extends outward to the edge of the box, perpendicular to the trusses.
Paul Dannels, AIA, a principal with Ann Arbor–based Structural Design Incorporated, designed the building’s structural system and considers the cantilever a low-tech solution. “You really just have to buy into the size and scale of the trusses,” he says. The trusses tie into a secondary steel structure contained within a poured-in-place concrete shaft that includes an elevator and a staircase. That steel structure is anchored in a 6-foot-deep subgrade concrete footing, 62 feet wide and 90 feet long. The length of the footing, which extends underground along the horizontal axis of the cantilever, counteracts the tendency of the cantilever to want to pull the building over. All cantilevers, from simple to complex, have to be designed to resist the moment of this force.
Additionally, a 4-inch camber was designed into the unsupported end of the volume. Corby says once they pulled away scaffolding, the structure dropped 3¼4 inch. The remainder, 31¼ inches, takes into account other loads. Since the building is located near Lake Michigan, it experiences a high wind load. “It’s unusual to have a building that is so influenced by twist, or torsion of the concrete shaft,” Dannels says. “As the wind hits the building, it wants to turn like a weather vane.” The steel frame in the concrete shaft resists that movement.