[ Page 7 of 8 ]

Long-spans amplify the collaborative relationship between architects and engineers

By Sara Hart

The membrane stretched between the arches consists of four layers of PVC-polyester mesh and is translucent to bring in additional daylight, which is supplemented by bright spotlights whose beams are diffused by large mirrors. The mesh layers form air pockets that provide thermal insulation. PVC membrane was chosen for three reasons: It’s lightweight, has a life expectancy of at least 20 years, and has enjoyed 30 years of being successfully engineered.

External horizontal props between the top chords of the arches (above) restrain any torsion that might be caused by eccentrically connected membranes. The sliding doors (shown partially opened below) are supported by a 3-foot-thick concrete slab. The doors can be completely opened in only 15 minutes. Photography: © Cargolifter

They’re called doors, but bear little resemblance to any other kind. Semicircular in plan and a quarter of a sphere in three dimensions, they have an arch length of 551 feet (by comparison, the height of the Statue of Liberty is 153 feet), and the designers had to minimize the tonnage of steel to keep the costs from being exorbitant. The solution illustrates the shell principle. The inner part of each of eight door panels is made of identical horizontal, vertical, and diagonal steel members and clad with corrugated-metal sheets. The panels slide along motorized rails in two directions—three slide inside of each other and nest into one fixed panel in each direction. When closed, the panels form smooth, tightly fitting spheres.

Although David Geiger died suddenly in 1989 at the age of 54, his firm continues his work and still focuses primarily on long-span construction, adapting and improving on the Cabledome in each new project (see Tech Brief, page 286). Could these engineers design a span across an entire city? The answer is, probably. In 1950, Bucky Fuller proposed to cover a large portion of midtown Manhattan with a transparent, climate-controlled dome 2 miles in diameter. Although it may sound far-fetched, the concept is technically and economically attainable. One day soon there might be a compelling reason to do just that.