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Air-supported domes, such as the Minneapolis Metrodome, appear to be giving way to tensioned structures.
By Mason Riddle and Barb Ernster
In an almost prophetic statement, David Campbell, principal and CEO of Geiger Engineers, Suffern N.Y., told Fabric Architecture in the September 2010 issue that an “air-supported dome has never collapsed, but it can deflate…due to climatic conditions such as snow and ice.” (“Air domes: last of a dying breed”). Just three months later, snow and ice did just that to the Mall of America Metrodome in Minneapolis, Minn., forcing the NFL Vikings football team to find a new home field for the remainder of the season and dislocating several other scheduled shows and events. An unprecedented heavy snow load caused the fabric to rupture at 5:00 a.m. on December 12th, and the air-supported roof deflated. At least nine of the 106 Teflon® roof panels need to be replaced, according to estimates by the Metropolitan Sports Facilities Commission (MSFC). Repairs are expected to be completed this spring.
Steve Maki, director of facilities and engineering for MSFC, said the snow load was unusual. “In my 25 plus years here, this was extraordinary. We had a lot of snow and it was surprising how much it kept coming and coming.”
Comparing it to the Halloween blizzard of 1991 that dropped 28 inches of snow on the Twin Cities, Maki said this snow had high moisture content and came much faster. Heating fans directed towards the roof’s surface to melt the snow couldn’t keep up, so work crews went up on the roof to remove snow with hot water from fire hoses. Wind conditions, however, made it unsafe for them to continue beyond the afternoon hours on December 11.
“We were going to start again at eight the next morning, but we didn’t get to that process,” Maki said. “This was a situation where even with all of our process and procedures we could not get ahead of it. I can’t stress [enough] how the wind affected things.”
History lesson
Birdair Inc. in Buffalo, N.Y., manufactured and installed the fabric roof in 1982 and is contracted to make the new panels. Birdair, owned by ChemFab when the Metrodome was built, is now a member of the Taiyo Kogyo group. Warranties for the Metrodome have long expired, but roof inspections and fabric testing done just last summer indicated that it was still above specifications in terms of its original characteristics, according to Maki.
Maki notes that there are not many air-supported structures like the Metrodome any more. Tension-supported structures are more common now. The move away from air-supported dome facilities is particularly true for professional sports because teams want to distinguish themselves and want to be outside, not under a dome, says Campbell. Baseball fields are uniquely sized and vary stadium by stadium, he says, which adds to the team’s identity and the game’s drama. In fact, the Minnesota Twins baseball team, which occupied the Metrodome since it opened in 1982, left last fall for its new state-of-the-art open-air stadium, Target Field. Most sports franchises realize that it is next to impossible for an air-supported dome to provide the amenities now expected by the public, such as sophisticated digital technology, restaurants and bars, and adequate private facilities.
Engineer David Geiger ignited the air-supported dome “craze” with his design for the U.S. Pavilion built at Expo ’70 in Osaka, Japan. It spawned many other air-supported dome facilities, including the 1975 Pontiac Silverdome, the 1980 Syracuse University Carrier Dome, and the 1982 Hubert H. Humphrey Metrodome, now named for the Mall of America.
Dome or no?
The advantages of these domes, Campbell asserts, is that they are more cost-effective and can be constructed with relative expediency compared to a fixed or retractable roof stadium. They also offer an unobstructed open interior space, since there is no need for support columns, and allow for multipurpose activities year round. The disadvantages are that they cannot attain the insulation values of hard-walled structures, so heating and cooling costs are higher. And they rely on people to actively manage or monitor them “24/7”; someone must activate the system before a snow or ice storm. The processes to manage them, however, are now being done extremely well and a deflation incident is rare, says Campbell.
In the Metrodome’s case, the building was preheated in anticipation of the storm, and heat was directed to the roof’s surface when the snow started to come. The larger question is, can they prepare for future storms such as the one on December 11th?
“If we knew what the perfect answer is, I’d say yes, we should do it,” says Maki. “I’m sure that we would only have an air-supported structure again. As with all things, you look to see what you can do to make changes and make it better.”