The $1.07 billion stadium under construction for the Minnesota Vikings will sport a number of plastics innovations, such as a clear fluoropolymer roof to offer a climate-controlled outdoor feel and a snow-melt system of cross-linked polyethylene (PEX) pipe to prevent it from collapsing like the football team's old Metrodome home in 2010.
The Minnesota Vikings new home, which will be called U.S. Bank Stadium, is about a year away from completion.
Uponor Inc. of Apple Valley, Minn., and its partners, designed a 70,000 linear-foot PEX system that is unique to North American stadiums to solve the problem of snow sliding off the pitched roof onto the ground below.
The multi-purpose stadium taking shape in Minneapolis also will have 2,000 flat-screen monitors for fans, polymer bathroom partitions instead of baked enamel and stainless steel, and 54 rows of telescopic polypropylene seats that can be retracted to allow for baseball games.
And, there's one plastic development still in the test phase that a lot of people are rooting for: bird-friendly window film that won't take away from spectacular views of the downtown skyline.
The 1.75 million-square-foot stadium will put almost 200,000 square feet of glass less than a mile from the migratory passageway of the Mississippi River. Bird collisions will happen beyond the Vikings taking on Falcons, Cardinals, Ravens and Seahawks during games.
Between 365 million and 988 million birds are killed every year in the United States alone when they crash into man-made structures, particularly glass windows, according to 2014 research by the Smithsonian Migratory Bird Center and U.S. Fish and Wildlife Service. Many experts predict the Vikings' stadium will add to those fatalities and one Minnesota company — St. Paul-based 3M — is trying to come up with a new product to deter birds from flying into it.
“3M is working with the Vikings, Audubon Society, and the Minnesota Sports Facilities Authority (MSFA) to look for a possible bird-safe film solution. However, the testing [is in] initial laboratory tests, so commenting on them would be inappropriate as it is still too early,” 3M spokeswoman Fanna Haile-Selassie said in an email to Plastics News.
In the wings
Christine Sheppard, the bird collisions campaign manager for the American Bird Conservancy (ABC), which is based in The Plains, Va., said 3M is looking at a tall order from bird lovers, city council members in the Twin Cities and some of the public.
“They're hoping 3M will come up with an ultraviolet solution,” Sheppard said in a telephone interview. “This is something that's not really innovative and new because it has been explored in window film for quite a while. There's actually an existing glass with a UV signal but we haven't seen a commercial window film yet.”
An ornithologist, Sheppard tests different kinds of coatings in and on windows to determine what is visible to birds but not people. She works in earnest as glass becomes a bigger part of the built world, placing a fine mesh net in front of the windows in her research tunnel to prevent collisions.
“There was a prototype that we tested and somebody else tested that looked relatively effective but there are issues with these UV patterns,” Sheppard said. “Think about it. When can you go out without sunblock? It's early morning and that's when these birds are actively flying around and smacking into stuff. So a UV pattern is never going to be as effective as one that is visible in other wave lengths, unfortunately.”
In Minnesota, bird-safe design requirements didn't go into effect until after the Vikings stadium's building permit was approved, Sheppard said, drawing the ire of bird activists.
“It's very distressing,” she added. “Unless it's the letter of the law, they don't have to do it but the spirit of the law and the spirit of the state is that they are trying to lead on this.”
Jennifer Hathaway, MSFA communications director, said in an email, “The bird-safe glass issue was not brought to our attention until after the building design had been completed. Bird-safe glass was only one of many requests that the Minnesota Audubon Society had for the MSFA. We continue to work [with them] on a potential retrofit solution, but have no information to share at this time. We were able to agree to all of the other operational things [they] wanted for the new building, such as turning off lights during migration periods and designing the lighting to be down lit, not up lit.”
Weather considerations, however, were at the forefront of the structure's design.
In December 2010, 17 inches of snow fell on the Vikings' facility, tearing the huge fabric roof panels the day before a game against the New York Giants. The stadium was out of commission for months.
This time, the south half of the stadium will be covered with ethylene-tetraflorouethylene (ETFE), a durable transparent polymer that resists low and high temperatures, abrasion, corrosion and weathering.
With the new roof to rise some 205 feet in the east and 270 feet in the west, the need to prevent snow and ice from sliding onto the ground — and people — below brought about a collaboration of experts in the design and construction fields, including the staff at Uponor.
The PEX pipe maker has created snow-melt systems to keep snow from accumulating on stadium fields but for this project they were asked to take it up a notch, Joe Grubesic, Uponor director of sales for the Midwest, said in a telephone interview.
“This is definitely unique,” Grubesic said. “To our knowledge it's the only roof snow-melting system on any stadium in North America. We've done many turf-conditioning projects to keep snow and ice from accumulating on the field and to keep temperatures high enough for the grass to grow when it would otherwise be dormant but nothing like this.”
Uponor's systems call for filling PEX pipes with glycol and water — the glycol keeps the water from freezing — and heating the mix to melt snow.
For the Vikings system, Uponor staff worked with a mechanical engineer from a construction partner to go over numbers about roof pitches, wind speeds, ambient temperatures and other factors to design a system that could handle the harsh winters of Minnesota. Their solution: Let the snow slide off the roof into catch basins that will be attached like gutters 100 to 125 feet above the ground. The 58,000 square feet of catch basins will hold 70,000 linear feet of PEX pipe that is 3/4 of an inch in diameter.
The system will be activated prior to any projected winter weather events so the snow-melt mix has time to heat up and turn falling flakes into water that flows into a drain system. The system has six zones for efficiency.
“Because of wind speed or the direction of the falling snow, you could have more snow accumulate or drift in one area than another,” Grubesic said. “So the six zones operate separately from one another in case there's a higher need on one end of the stadium vs. another.”
Uponor manufactures pipes for North American customers at its U.S. headquarters in Apple Valley. The Vikings system was made there, preassembled and rolled onto plastic mesh mates.
“All the contractor needs to do is unroll these mats and make the connections,” Grubesic said. “That will really speed installation.”
He isn't sure Uponor has a new market segment it can attack but he doesn't rule out getting more requests for roof systems as architects and builders push design limits.
“The complexity of the project definitely challenged us as did the construction time frame but when the opportunity is there to do something unique you make the time. We made it a priority,” Grubesic said. “It's a design and technological feat in my eyes and it was a true team effort.”
Up on the roof
Sixty percent of the roof that will cover the Vikings' stadium will be made of ETFE. The clear material will offer vista views and let in natural light while its pitch and non-stick surface will have snow sliding into the catch basins for collection and melting.
Of all the reasons to select ETFE, transparency topped the list, Kevin Taylor, senior vice president of the Dallas-based architectural firm HKS Inc., said in an email.
“It will enhance the connection to nature and the city,” he said. “Its transparency will also allow spectators to have an experience that is more in line with an outdoor stadium without the rain or snow.”
ETFE is extruded into sheets that are welded together into roof panels that will get an added ink-frit pattern to reduce solar heat from building up. The sheets are so thin they measure about 200 microns each, which is 1/64 of an inch.
The result is a very lightweight but strong material.
“Glass façade or roof systems typically weigh 10 to 20 pounds per square foot,” Taylor said. “ETFE systems weigh less than 1 pound per square foot. The systems are engineered for the local codes but the weight-strength ratio is extreme.”
The roof will be strong enough to hold 400 times it weight and it won't need large truss supports, which should cut down on shadow issues for players and fans.
Because of Minnesota's northern climate, a retractable roof was ruled out because of the limited number of times it would be used and the high cost it would add — millions of dollars — for the complicated mechanical system.
Stadium designers are on to something new anyway with ETFE.
“Clear is the new retractable,” Taylor said.
The material provides other benefits as well. ETFE doesn't degrade from exposure to UV light. It is expected to have a 30- to 50-year life expectancy. And, minimal maintenance will be required. The non-stick surface makes the exterior side almost self-cleaning.
“ETFE is very similar to Teflon, which is very slippery and non-porous like your typical frying pan,” Taylor said. “Thus, under the action of rain, any debris or dirt will wash off naturally.
Installation of the ETFE panels began in June and should be wrapped up in the late fall. The rest of the roof will be metal deck and membrane roofing.