Friday, August 15, 2008

Retractable roofs

The Houston Astrodome in Texas, opened in 1966, was the first stadium with a roof over the playing area. It set a trend for sports fields for the next twenty years. Its roof, designed to resist 135-mph (216-kph) winds, has a clear span of 642 feet (196 meters); it is 208 feet (64 meters) high at the apex. It was not, however, the first arena to have a roof. It was predated by almost 2,000 years by the Flavian Amphitheater in Rome, better known as the Colosseum. The Colosseum measured 620 by 510 feet (189 by 156 meters), and the perimeter of the fourth story had stone brackets supporting wooden masts from which an awning (velarium) was suspended across the interior to shield spectators from the sun. The velarium, was not fixed; teams of sailors handled the rope-and-pulley system that allowed it to be opened and closed depending on the weather.

The Toronto SkyDome, designed by architects Rod Robbie and Michael Allen and inaugurated in June 1989, was the first modern stadium with a fully retractable roof. SkyDome provides 2 million square feet (186,000 square meters) of usable floor space for up to 30,000 spectators. The 8-acre (3.24-hectare), 11,000-ton (10,000-tonne) roof rises 282 feet (86 meters) above the field level. It consists of a fixed panel and three movable panels, framed with steel trusses and covered with a polyvinyl chloride (PVC) membrane laminated to an insulated steel sheet, moving on a system of tracks and bogies. The roof can open in twenty minutes to uncover the entire field area and over 90 percent of the seating. Since SkyDome, many similar structures have developed the new technology that enables very large buildings, once considered static, to become (at least in part) flexible—an architectural feat.

Amsterdam Arena, the Netherlands, was opened in September 1996, the first retractable roof stadium in Europe. The stadium is 540 feet (165 meters) wide and 770 feet (235 meters) long; the roof, soaring 255 feet (78 meters) above the playing field, consists of two movable panels that retract across the short span. The designer was Rob Schuurman. Bank One Ballpark in Phoenix, Arizona, designed by Ellerbe Becket, was completed in early 1998. Two 200-horsepower motors open or close the retractable roof over the 48,000-seat stadium in under five minutes. Each half of the roof consists of three movable trusses that telescope over a fixed end truss. Either side can be opened to any position, independently of the other. The 52,000-seat Colonial Stadium in Melbourne, Australia, was opened in 2000. Its 540-foot-span (165-meter) retractable roof, employing a lightweight space-truss structure, opens or closes in less than eight minutes. Other arenas, such as the Sports Park Main Stadium of the Oita Prefecture, Japan, and Miller Park, Milwaukee, Wisconsin, were completed in 2001. The former, designed by Kurokawa Kisho Architectural Urban Design and the Takenaka Corporation, has a retractable 895-foot-diameter (274-meter) hemispherical steel-framed shell roof; Miller Park has a seven-panel roof.

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