SBJ/June 29-July 5, 2015/In Depth

Innovative roof design has Vikings seeing potential

The green discussion at U.S. Bank Stadium starts with the innovative roof design at the Minnesota Vikings’ new facility.

When it opens in July 2016, the $1 billion stadium will feature the largest translucent roof in North America. To achieve the clear look, the roof incorporates ETFE, a plastic-like material originally developed by DuPont 40 years ago. Over the past decade, ETFE has been used for roofs at the Cube aquatics center at the Beijing Olympics and the Allianz Arena soccer stadium in Munich.

ETFE is lightweight, durable and resists corrosion. For going green, it’s energy efficient, cost effective and recyclable, according to Birdair, a supplier of roof covers for stadiums.

The translucent roof at U.S. Bank Stadium will give fans the experience of being outdoors.
Photo by: HKS; Minnesota Vikings
Architect HKS and the Minnesota Sports Facilities Authority are targeting LEED certification for the stadium, and the roof will help it gain points in the energy category.

Using ETFE will create the experience of being outdoors without feeling the full effect of the elements, so there is also potential for the roof to earn points for innovation in design, said John Hutchings, HKS’s sports principal-in-charge of the Vikings’ project.

“The idea behind it was to use a material that allows you — as opposed to the hermetically sealed Metrodome where you couldn’t see whether it was a blue sky or a cloudy day — to see exactly what the weather is like,” Hutchings said.

In the NFL’s harshest winter climate, the sun can still shine brightly on a clear day, so heat buildup inside the stadium is an issue, said Brian Wolfe, an associate with HKS and the firm’s sustainable design coordinator.

The key to regulating the elements indoors is something called thermal fritting, an etching design element layered over ETFE that reflects the sun’s rays away from the roof. The fritting helps with both heat prevention and heat retention, depending on the time of the year. HKS studied the sun’s movement over the course of the day to determine the maximum location of the fritting patterns, Wolfe said.

What’s just as important as ETFE is the roof’s sloping design to minimize snow buildup. The Metrodome, the
Vikings’ old home, featured a roof supported by an air-compression system, and it suffered multiple collapses from heavy snows.

For the new stadium, the roof’s sharp angles, combined with the ETFE technology, should “help us shed snow very efficiently,” said Michele Kelm-Helgen, chair for the Minnesota Sports Facilities Authority, the Vikings’ landlord and stadium owner.

In terms of cost, on a project where the Vikings have picked up the tab on several cost overruns since the budget was approved, having a fixed roof saves about $100 million compared with retractable roofs like those in use at AT&T Stadium and Lucas Oil Stadium. HKS designed those stadiums as well.

The lightweight ETFE technology results in a roof weighing 2,000 to 3,000 tons less compared with the roof at the Indianapolis Colts’ facility, and with a reduction in lighting requirements over the hard roof at Lucas Oil Stadium, Hutchings said.

In the end, the ETFE roof presents a sophisticated yet efficient design, resulting in one of the most cost-effective structures in the NFL, said Mark Williams, director of HKS’s sports and entertainment practice.

“On the exterior side, it’s a very recognizable, branded component of design,” Williams said. “There will be no doubt when people see that image that that’s Minnesota’s stadium. It will be very identifiable.”

Apart from the roof, stadium developers focused on stormwater management, energy efficiency and public transportation to form additional green elements.

In Minnesota, where flooding can be a major issue during spring thaws, the new stadium is connected to a new stormwater filtration system with thousands of feet of underground pipe outside the facility. It’s designed to collect and treat up to 1.1 million gallons of water (based on the worst-case flood scenario) before diverting it back into the ground, compared with piping it directly into the city’s sewer system, Kelm-Helgen said.

“That’s a big part of the story for Minnesota,” Wolfe said. “I know it’s not going to be one [green element] that people are going to see, but I think the benefits of it are huge to the local water system and the Mississippi River. Storm water and melted snow will be cleaned before it hits the river.”

On the energy side, project officials are using LED sports lighting in the design. The Vikings are covering the cost of the $1.2 million upgrade. The annual savings from using a higher quality light that burns brighter and uses less energy is expected to run in the range of $400,000 a year, authority officials said.

“When the design of the stadium was initiated, LED sports lighting wasn’t up to the level needed for pro facilities,” Wolfe said. “It’s come a long way, and we have been able to incorporate it. The authority and the Vikings felt it was worth the effort.”

Two new light-rail lines will pass by the stadium, and the authority expects more than 30 percent of attendees for all events will arrive by train, Kelm-Helgen said. The stadium also will connect to the city’s pedestrian skyway system linking hotels, parking ramps and residences.

“We have over eight miles of skyways … and 40,000 residents that live in downtown Minneapolis, so all those people will be able to walk any time of year [indoors] to the stadium,” she said.

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