When the Houston Astrodome opened in 1965, it instantly became an architectural icon, launching an era of domed stadiums for professional football and baseball teams in cities such as New Orleans, Indianapolis, and Seattle. By the late '80s, Major League Baseball had moved onto retractable-roof stadiums, beginning with Toronto’s SkyDome (now the Rogers Centre), home of the Blue Jays, built in 1989. In 2002, Houston’s Reliant Stadium (now NRG Stadium) became the first National Football League (NFL) stadium to be topped with a retractable roof. It was quickly succeeded by other sun-roofed facilities, including the Lucas Oil Stadium in Indianapolis and the AT&T Stadium outside of Dallas.
This July, the $1.087 billion U.S. Bank Stadium in Minneapolis may signal another era in stadium roof design. The roof of the new home for the NFL’s Minnesota Vikings will be fixed, but 60 percent of it will be essentially transparent, thanks to 240,000 square feet of ETFE (ethylene tetrafluoroethylene) plastic. Couple that with a full-height glass curtainwall at the stadium’s northwest end zone, and fans will enjoy nearly as much natural light inside as if they were in an open-air venue. “We wanted to have a true indoor–outdoor experience,” says Vikings executive vice president of public affairs and stadium development, Lester Bagley. “A stadium for all seasons.”
The 66,200-seat stadium occupies the site of the Vikings’ former home, the Hubert H. Humphrey Metrodome, built in 1982. It too had a distinctive roof, although not in a positive way. The glass-fiber fabric dome notoriously and dramatically collapsed on Dec. 12, 2010, under about 17 inches of accumulated snow. The roof was repaired in time for the Vikings to play on inside the aging dome for the following season, but for past two seasons, during U.S. Bank Stadium’s construction on the Metrodome site, the team has played across town at the University of Minnesota’s open-air TCF Bank Stadium. In January, a playoff game against the Seattle Seahawks had a game-time temperature of -6 F.
With the Metrodome’s roof collapse still fresh in everyone’s minds, Dallas-based HKS Architects had to be meticulous when designing U.S. Bank Stadium’s novel roof to withstand heavy snow loads. The roof pitches 14 degrees, starting from 205 feet tall at its east end to a peak height of 270 feet to the west. Snow slides into a gutter ringing the stadium roof perimeter that’s equipped with a heated melting system.
HKS principal John Hutchings, AIA, says that ETFE's high strength-to-weight ratio allows for one of the lightest stadium roofs in the world. He adds that the ETFE portion of the roof actually melts the snow more quickly than its opaque, zinc-paneled counterpart. “Our design anticipated a certain amount of radiant heat [from the sun], because we use that in the winter to heat the building itself,” he explains.
In cold months, air warmed by the greenhouse effect will be harnessed to help heat the stadium. In warm months, the peaked roof provides room for the balmy air to rise due to stack ventilation. “It can be comfortable [in the stadium] on a 95-degree day in August,” Hutchings says. He also believes U.S. Bank Stadium will enjoy higher energy efficiency than a retractable-roof stadium because the latter, even when closed, is subject to air leakage. “This is a major concern in extremely cold climates, where any breaks in the roof envelope lead to drastic stack effect in the building,” he says. “The ETFE roof allows for better control of the interior stadium environment.”
When members of the Minnesota Sports Facilities Authority (MSFA), the state agency developing the U.S. Bank Stadium, toured retractable-roof facilities before embarking on the project, they often heard from stadium operators that attendees preferred the roof be closed for optimal comfort. “It’s that 70-degree temperature that people shoot for,” says MSFA chair Michele Kelm-Helgen. A retractable roof can add upward of $100 million to construction costs, she says, but most open up fewer than five times per year. “HKS convinced us that the equivalent of a retractable roof was ETFE,” she says. “It’s the largest such span in North America, and in combination with that [nearly 300-foot] glass wall, you’re going to feel like you’re outside.”
Although U.S. Bank Stadium’s expansive, transparent roof is unprecedented for an American football venue, it does allude to one of its host city’s most ubiquitous architectural features: the Minneapolis Skyway System, an interlinked collection of elevated, glass-enclosed pedestrian footbridges that connect 69 city blocks over 11 square miles downtown. The stadium, at a larger scale, also aims to create a climate-controlled indoor space that offers the benefits of daylight and outdoor views.
During a recent press tour hosted by the Minnesota Vikings, the new stadium was filled with natural light despite overcast skies. When the sun emerged from behind the clouds, the shift in light on the playing field and the concourses was palpable. Hutchings says that no one liked the hermetically-sealed domes of the past: “You didn’t know if it was blue sky or raining outside.” Now the experience is just the opposite.