The Roof That Isn’t There

On December 12, 2010, the much-maligned Metrodome finally gave out. After nearly a foot and a half of snow accumulated on top of the home of the NFL’s Minnesota Vikings, the roof caved in spectacular fashion, dumping truckloads of powder upon the football field.

The collapse left the Vikings out in the cold. The team eventually would get its wish as the city of Minneapolis approved a new stadium to be built in the same spot. But while construction crews razed the defunct Metrodome and built the Vikes’ new billion-dollar home, the team played outdoors at nearby TCF Bank Stadium on the campus of the University of Minnesota, where gametime temperatures dropped to -4 degrees Fahrenheit for this month’s playoff matchup against Seattle.

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The roof, then, has a sort of history when it comes to Minneapolis stadiums. The new home of the Vikings (and host of Super Bowl LII), U.S. Bank Stadium, does something altogether new in North America. Its designers brought the best of the outside in, all while keeping U.S. Bank Stadium a fully indoor venue.

The Roof

The Metrodome was a marvel when it opened in 1982. Air once supported the fiberglass fabric roof—though that air couldn’t stand up to the power of snow in Minnesota. In the decades since then, the plethora of new, crazy-expensive stadiums built in America has included many with retractable rooves: those that can slide open when the sun shines and close the lid when the rain comes.

But the Vikings are trying something else. “The only reason to have a retractable roof was because others had one,” says Bryan Trubey, HKS architect and designer of the building. Instead, Trubey helped design a stadium that, he says, “felt like outdoors all the time.”

The key is ETFE, a space-age, plastic-like material used in construction for roughly 30 years. ETFE is lighter and stronger than traditional roofing materials. 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.

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Each 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. 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. The air units do not run constantly or create airflow, but simply maintain a minimum pressure within each panel.

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. 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.

“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. “A stadium for all seasons.”

More than just bringing the light in, ETFE also ensures the snow will stay out. 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. It’s set asymmetrically to allow for the sunnier south side of the building to use more ETFE than the north side. 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. The roof is so minimal that the Vikings can still do a flyover before the game. In an indoor venue.

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. 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 Doors

Minneapolis has its long, cold winters, but that doesn’t mean that football in September is frigid. The Vikings wanted to welcome the warm air, not just shun the cold, and that’s where the idea for huge glass doors started. 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. 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.

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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. 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.

The Tennessee-built doors made from 30,000 square feet of glass from Owatonna, Minnesota, rotate on hydraulic pistons. The curtain of glass also breaks up the exterior black zinc metal panels that will eventually wear into a weathered gray.

The Build

Eric Grenz, construction executive for Mortenson Construction, says 100,000 cubic yards of concrete were used in the construction of the building. The highest point of concrete rose 289 feet on the ring beam, taller than the Statue of Liberty. To keep the project on track, Mortenson needed to conduct year-round concrete pours. And year-round means workers pour concrete even in -30 degree temperatures, requiring heated cement trucks and heated forms that can properly cure the concrete.

Incredibly intricate logistics at the downtown site that sometimes had Mortenson managing a dozen cranes inside the bowl. “We had a 4D schedule, an animation of the entire sequencing of the job as a whole,” Grenz says. “It really helped to understand the flow.”

Besides Viking football games, Minneapolis’ newest megastructure is already slated to host the Super Bowl in 2018 and the NCAA men’s basketball Final Four in 2019. So plenty of fans will get to gaze up at the new vision for a colossal roof, one that feels like it’s hardly even there.

Follow Tim Newcomb on Twitter at @tdnewcomb.

Source: https://gardencourte.com
Categories: Outdoor