The Tennessee-built doors made from 30,000 square feet of glass from Owatonna, Minnesota, rotate on hydraulic pistons. For those cold winters, though, much smaller regular-sized entrance doors embedded in the glass can open instead, keeping the frigid temperatures where they belong: outside. Located at the west entrance, where the Vikings expect roughly 70 percent of fans to enter, the five large doors can remain open during contests, bringing natural air into the stadium. With all these translucent and transparent materials, even on a cloudy day in Minneapolis enough natural light streams into the building that the new stadium looks like it opens to the outside. The west end of the building features the five largest pivoting glass doors in the world, 55 feet wide and ranging from 95 feet to 75 feet tall. The Vikings wanted to welcome the warm air, not just shun the cold, and that's where the idea for huge glass doors started. Minneapolis has its long, cold winters, but that doesn't mean that football in September is frigid. This gutter has a melting system within to take the cascading snow and turn it into full liquid before the weight becomes a Metrodome-type issue. The steep pitch of the roof (1:12) fits the Northern European design motif and serves a practical purpose of helping snow slide into a snow gutter that rings the top of the building. The roof is so minimal that the Vikings can still do a flyover before the game. The roof that sits over the 1.75 million square foot stadium is one of the lightest stadium roofs in the world, despite its snow load requirements. It's set asymmetrically to allow for the sunnier south side of the building to use more ETFE than the north side. The lightweight but strong material allowed designers to run a superspan that includes a single 989-foot-long single ridge truss the entire length of the field. More than just bringing the light in, ETFE also ensures the snow will stay out. "We arrived on the ideal solution for this market, that indoor-outdoor experience," says Lester Bagley, executive vice president of stadium development for the Vikings. About 60 percent of the roof is clad in these translucent ETFE pillows, amounting to a total of 240,000 square feet, while the remainder is a traditional steel deck and membrane roof. The outer layer of ETFE foil is printed with a silver ink to create a "frit" that helps reduce the heat of the sun on the roof surface and deflect the sun, while the inner layer of air helps maintain the desired temperature, altogether controlling the thermal needs of the 66,200-seat building. The air units do not run constantly or create airflow, but simply maintain a minimum pressure within each panel. Each panel has a series of air supply units that, while not structural, helps the "pillows" stabilize the foil, according to engineering firm Thornton Tomasetti. It's also translucent-so much so that a stadium in New Zealand with a roof made of this stuff can grow natural grass indoors.Įach of the ETFE panels in the Vikings' roof is made of three layers of ETFE foil, welded into panels with edges captured into aluminum frames, which are themselves connected to the steel brackets of the roof structure. ETFE is lighter and stronger than traditional roofing materials. The key is ETFE, a space-age, plastic-like material used in construction for roughly 30 years. Instead, Trubey helped design a stadium that, he says, "felt like outdoors all the time." "The only reason to have a retractable roof was because others had one," says Bryan Trubey, HKS architect and designer of the building. But the Vikings are trying something else.
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