Innovative new materials have been creeping into sports stadium construction around the world, providing both practical and aesthetic benefit. Now, those innovations are finally coming to US soil, giving stateside sports fans a fresh take on a stadium roof.

When the Minnesota Vikings host the Green Bay Packers at the new US Bank Stadium in Minneapolis on September 18, it will be the first regular-season NFL game played under a roof made of a material called ETFE. The translucent plastic lets enough daylight in to bring an outdoor feel to the 66,200-seat building, but it also gives Vikings fans a fully indoor stadium.

“We arrived on the ideal solution for this market, that indoor-outdoor experience,” says Vikings executive vice president of stadium development Lester Bagley. “A stadium for all seasons.”

Commonly used in everything from greenhouses and skylights to solar cells and groundbreaking architecture, Ethylene tetrafluoroethylene film is durable, translucent, and much lighter than glass, so engineers can use it to form roofs, windows, and facades with much less structural support. Its elasticity and strength allows designers to stretch the film across large expanses, like the Estadio Cuauhtémoc soccer stadium in Puebla, Mexico and the Allianz Arena soccer venue in Munich, Germany. Those whimsical bubbles on the facade of the “Water Cube” at the 2008 Beijing Olympics? Also ETFE.

Vikings Stadium FootballJim Mone/AP

Looking Up

Sixty percent of the roof—240,000 square feet in total—is clad in translucent ETFE pillows, bathing the playing surface in natural light even on overcast days. Air supply units within the three-layer ETFE foil pillows maintain a minimum pressure. Held in place by an aluminum frame connected to the steel brackets of the roof, the triple layering helps control the thermal needs of the stadium.

If you’ve ever been to Minnesota, you’ll know that snow is a very big deal. The Vikings’ old home, the Metrodome, collapsed under the weight of accumulated snow from a storm in 2010. The ETFE roof was specifically designed to dispatch snow efficiently. Its outer ETFE foil is printed with a silver ink to create a glaze that deflects sunlight and melts snow. The roof is also built with a 1:12 pitch (one inch of rise for every foot of run), matching the Northern European strategy of encouraging snow to slide off the roof. Architect Bryan Trubey of HKS also added a snow gutter, which lines the top of the stadium with a snow-melting system to further limit any concern of the snow’s weight collapsing the roof.

Snow weight wasn’t the only concern in building such a large indoor venue. A large truss span sticks out on one side and gives the US Bank Stadium the appearance of a ship’s prow. Construction costs on a stadium with a 989-foot-long single-ridge truss would have ballooned if it had been made with steel and membrane. Trubey says shifting to ETFE not only cut costs and gave Minneapolis a unique motif, but it took the stadium’s roof from what was projected to be one of the heaviest in the world for its size to one of the lightest.

Los Angeles Rams v Minnesota VikingsThe exterior of the U.S. Bank Stadium on September 1, 2016 in Minneapolis, MN.Hannah Foslien/Getty Images

Other Roofs

Minnesota isn’t the only city with a polymer-swathed stadium on the way. For the Atlanta Falcons’ new Mercedes-Benz Stadium, set to open in June 2017, architects at HOK designed an eight-panel retractable roof that mimics a pinwheel. Also, a single layer of ETFE drapes the outside of Mercedes-Benz Stadium, making it the first single-layer use of the material at a US sports facility.

With the intricate angular geometry of the architecture, the 160,000-square-foot sheet of material is supported by thin steel cables and provides a view of Atlanta’s skyline—something that would have required bulky steel beams and vastly inflated engineering costs if attempted with glass.

Other lightweight polymers are in favor as well. Architecture firm Rossetti went with Polytetrafluoroethylene or PTFE, commonly known as Teflon, to provide a solution for the the world’s largest tennis stadium, the USTA’s Arthur Ashe Stadium in New York. Built on a former dump site in 1997, the ground under the nearly 24,000-seat venue couldn’t support any more weight, so designers created a tent-like structure out of roughly 6,500 pounds of steel. Workers drilled eight column supports 175 feet through suspect soil straight to bedrock. The structure planted atop the columns holds two giant panels of PTFE fabric that open and close over the stadium.

“It is more of an umbrella that is open and the fabric is all stretched out,” says Rossetti principal Jon Disbrow. The lightweight solution not only helps with foundational concerns, but it reduced cost on the project, he says.

Follow Tim Newcomb on Twitter at @tdnewcomb.

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The NFL’s First Translucent Roof Is a Super-Tough Monster