With a roof made of fabric similar to that used in trampolines, it's not hard to envision why 43 centimeters of snow tore through Minneapolis's Hubert H. Humphrey Metrodome last month. What is perhaps harder to imagine is why anyone would consider keeping in place an inflatable domed stadium that most engineers agree is antiquated. Forensic investigators and engineers are still studying the December 12 accident, although it is unclear whether any federal agencies have been summoned to investigate.

Under pressure, few inflatable stadiums
Most stadiums either have a truss-supported roof or lack one altogether. Inflatable roofs are usually dome-shaped, because that provides the optimum amount of volume for the pressurized air to maintain shape.

The roof fabric is pliable, usually made of tensile fiberglass or polyester, hemispherical, and attached at the foundation with heavy weights. Inflation fans, located just under the roof, keep the dome inflated and blow into a common duct that circumnavigates the building. The air is then funneled into the interior arena from that duct. Inflatable dome-roofed structures remain stable as long as the stadium's internal air pressure equals or exceeds outside forces such as those exerted by wind, snow or even earthquakes. All such systems rely on two types of exit doors on the ground level and a revolving door that can be used to help moderate pressure.

Pete Sala, managing director of the 2.7-hectare  Carrier Dome at Syracuse University in New York State, says theirs is one of only three permanent, inflatable football stadiums in the country—the third is the Silverdome in Pontiac, Mich.

And the Metrodome is currently the only active air-pressurized stadium in the National Football League. The former residents of the Silverdome, the Detroit Lions, have played at Ford Field since that facility opened in 2002. The Silverdome is now used for music and various sports events.

The University of Northern Iowa (U.N.I.) in Cedar Falls replaced its air-pressurized system in 1994 after its roof collapsed numerous times during snowstorms. The pressurized RCA Dome (formerly called the Hoosier Dome and home to the NFL's Indianapolis Colts), no longer exists either. Another air dome, Vancouver's BC Place, where the opening ceremonies of the 2010 Winter Olympics were held, is being replaced with a retractable fabric roof. Sala says the stadium was built about the same time as the Carrier, which opened in September 1980.

U.N.I. also got rid of the air-supported part of the roof—and [although it] still has a fabric center, it is not held up with air anymore," Sala says. "U.N.I. Dome was part of our generation—it's part hybrid now," he notes.

But the Carrier Dome is here to stay. It's always been properly inspected and maintained, Sala says, pointing out that Syracuse gets more snow than Minneapolis. Further, the Carrier roof has held up even with as much as 1.2 meters of snow falling in a day and half, and the two meters that have already dropped on the area this winter. "Our typical routine when snow is predicted is we go through a series of checks and balances and protocol…to keep snow off the roof," he says.

What worked in 1970
The technology behind air-pressurized domes was popularized by the late architect and engineer David Geiger, who designed the U.S. Pavilion for Expo 70 in Osaka, Japan; but inflatable domes predate Geiger. The U.S. Patent and Trademark Office shows images for a filing in April of 1958 by Woldemar A. Bary for an inflatable structure.

It's a technology, though, that even Geiger may have foreseen as obsolete. His own designs graduated to an unpressurized cable-dome structure, as evidenced by his work on the roof of the Florida Suncoast Dome—now called Tropicana Field—in Saint Petersburg, for example.

More recently, some stadiums with air-inflated supported "tensegrity" domes—a structural system that is still commonly used for other sports facilities such as tennis courts—are being shuttered. Tensegrity is a term coined by the futurist inventor Buckminster Fuller to describe structures that balance compression with tension, but need not specifically apply to inflatables.

What happened in Minneapolis
At the Metrodome last month, the main problem was the weight of the snow, says civil engineer Dario Gasparini of Case Western Reserve University. He adds: "There's also wind, but the weight of the snow itself required action, meaning…if you had a one-foot blanket of snow on the entire Metrodome, that weight is five to seven times the weight of the roof itself."

Engineer Steve Maki, Metrodome facilities manager, explains that they have 20 "blowers," or fans, on the stadium's "mechanical" level, located just above the upper concourse and below the roof; and each blower pumps at 100 horsepower and 2,800 cubic meters per minute. Eighteen of them have heating coils drawing 65 degrees Celsius to heat the interior "bowl" or seating area, he says.

"So to keep that weight inflated or up, you had to increase the air pressure inside the building roughly five- to seven-fold," he says, adding that either the blowers do that job or not. If not, the snow must be removed either mechanically or thermally. Gasparini warns that one must be very careful with equipment up on the dome so as not to cause a puncture.

And "ponding" is also a risk with this type of roof design, which can occur when snow is heated to melt off but runoff backs up rather than flows and eventually causes elastic fabric to stretch and tear.

Risky walk in the snow
Blowing away the snow and meltwater without causing a fabric tear is exactly what engineer Maki tried to do on December 11 at the Metrodome. He and six other workers were on the roof, melting snow with water from fire hoses.

The Metrodome's roof—which is Teflon-coated fiberglass and weighs approximately 265 metric tons—is inspected every three years by tensile structure contractor Birdair, Inc., in Amherst, N.Y. The fabric of the $52.5-million dome stretches about four hectares.

About 5 A.M. on December 12, one of the roof's triangular fabric panels located on the west side of the building ruptured and "then it appears that then the roof started to come down—and then sequentially two adjacent (diamond-shaped) panels failed," Maki says. "We had an additional diamond panel rupture suddenly on the Wednesday night [December 15] after initial deflation."

"Finally," he says, "we had a diamond panel that was extremely loaded that we knew could fail any time. The inner liner has drain holes which were not lining up with the outer fabric so the panel was not draining as we heated the building. We attempted to drain it by shooting the lowest point with a shotgun slug; however, that action caused the panel to rupture anyway."

Although investigations are ongoing, Maki says the first panel tear was "obviously" snow-related, and then a shift of strain to the center of the dome caused the next two ruptures. "I can tell you the conditions were the worst I'd ever seen in terms of a combination of snow accumulation, wind and decreasing temperatures," he says. "Certainly we followed all the procedures and processes we've devised and expanded on over time, [but] the situation was too unsafe, so I pulled our people off the roof."

"There are better technologies"
Larry Griffis of the American Society of Civil Engineers and principal engineer with Walter P. Moore, an engineering and consultancy firm in Houston, has been retained to investigate the Metrodome incident, but is not allowed to comment. He says, however, that Geiger—whom Griffis called a "brilliant" engineer—would have agreed that air-pressurized dome technology is outdated.

Even though such structures have advantages—particularly, being lightweight, of high strength and low cost—maintaining such facilities is not nearly as attractive as it was a couple decades ago. The risk of deflation alone has proved problematic, and has occurred several times at inflatable dome facilities, particularly due to snow loads.

Griffis also points out that maintenance costs tend to be higher than many owners want to pay; for example, maintaining air pressure can be both laborious and expensive. Maki, for example, says his facility's monthly electricity bill runs about $60,000, and special doors are required to balance air pressure inside the dome.

Maki says the Minnesota Vikings, who didn't make the NFL play-offs this season, are in serious discussions for a new stadium; whether that means moving to a new location or not is uncertain, but the Metrodome's days as a home for a professional football team could be numbered. A primary motivator for the decision will, arguably, be the stadium’s outdated roof design—its inflatability rather than the tensegrity aspect. (Tensegrity structures are supported primarily by high-strength cables whose tension forces are balanced either by air or other support structures, Griffis says.)

Griffis adds that air-supported structures such as the Metrodome are a particular class of tensegrity structures supported by differential air pressure. Tensegrity structures, however, can be self-supporting without the use of air pressure by employing cables, interior vertical pipe struts and/or a perimeter concrete compression ring. Examples of such structures include the Atlanta–Fulton County Stadium and Tropicana Field.

"There are better technologies out there," Griffis says. "We've probably seen the last of air-inflated tensegrity domes."

Editor's Note: Scientific American contacted Birdair, the company that manufactures the fabric dome. After calls were twice referred to their legal department, messages ultimately went unanswered.