Metrodome Roof Collapse

The Hubert H. Humphrey Metrodome, commonly called the Metrodome, is a domed sports stadium in downtown Minneapolis, Minnesota, United States. The football playing field has been known as Mall of America Field at Hubert H. Humphrey Metrodome since October 2009. Opened in 1982, it replaced Metropolitan Stadium, which was on the current site of the Mall of America in Bloomington and Memorial Stadium on the University of Minnesota campus. The Metrodome is home to the National Football League’s Minnesota Vikings, and is occasionally used by the Big Ten’s University of Minnesota Golden Gophers baseball team. The stadium was also the home of the Minnesota Twins from 1982 to 2009 and theGolden Gophers football team from 1982 to 2008.

The stadium is 28 years old, making it the ninth oldest stadium in the National Football League. Locally, its common nickname is simply The Dome. Over time it acquired other nicknames connected to its uniqueness.

The stadium is well known for its fiberglass fabric roof that is self-supported by air-pressure. The Metrodome was also the second major sports facility to have a domed roof supported completely by air, the first being the Pontiac Silverdome. The Metrodome is similar in design to BC Place Stadium and the RCA Dome.

The Metrodome’s air-supported roof was designed by the inventor of air-supported structures, David H. Geiger, through his New York-based Geiger Berger Associates and manufactured and installed by Birdair Structures. An air-supported structure supported by positive air pressure, it requires 250,000 ft³/min (120 m³/s) of air to keep it inflated. The air pressure is supplied by twenty 90-horsepower fans. The roof is made of two layers: the outer layers is Teflon coated fiberglass and the inner is a proprietary acoustical fabric. By design, the dead air space between the layers insulates the roof; in winter, warm air is blown into the space between layers to help melt snow that has accumulated on top. At the time it was built, the 10 acres of fabric made the roof the largest expanse ever done in that manner. The outside Teflon membrane is 1/32nd of an inch thick and the inner liner of woven fiberglass is 1/64th of an inch thick. The entire roof weighs roughly 580,000 pounds. It reaches 195 feet, or about 16 stories, at its highest point.

To prevent roof tears like those that occurred in its first years of service, the Metropolitan Sports Facilities Commission adopted a twofold strategy to prevent future occurrences: When snow accumulation was expected, hot air was pumped into the space between the roof’s two layers. Workers also climbed on the roof and used steam and high-powered hot-water hoses to melt snow. In addition, before the storm that caused the December 2010 collapse, the inside of the stadium was heated to nearly 80 °F (26.7 °C).

To maintain the differential air pressure, spectators usually enter and leave the seating and concourse areas through revolving doors, since the use of regular doors without an airlock would cause significant loss of air pressure. The double-walled construction allows warmed air to circulate beneath the top of the dome, melting accumulated snow. A sophisticated environmental control center in the lower part of the stadium is manned to monitor weather and make adjustments in air distribution to maintain the roof.

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