Tensegrity domes, patented by Buckminster Fuller in 1962 from a concept by Kenneth Snelson, are membrane structures consisting of radial trusses made from steel cables under tension with vertical steel pipes spreading the cables into the truss form. They have been made circular, elliptical, and other shapes to cover stadiums from Korea to Florida. While the first permanent air supported membrane domes were the radar domes designed and built by Walter Bird after World War II, the temporary membrane structure designed by David Geiger to cover the United States pavilion at Expo '70 was a landmark construction. Geiger's solution to a 90% reduction in the budget for the pavilion project was a "low profile cable-restrained, air-supported roof employing a superelliptical perimeter compression ring". Its very low cost led to the development of permanent versions using teflon-coated fiberglass and within 15 years the majority of the domed stadiums around the world used this system, including the Silverdome in Pontiac, Michigan. The restraining cables of such domes are laid diagonally to avoid the sagging perimeter found to occur with a standard grid.