Shovels, Cleats, and Fabrics: Just Another Snow Story

December 20, 2010 · 0 comments

The Minnesota Vikings are hosting football outdoors for the first time in 29 years in their hometown, thanks to the collapse last week of the fabric roof of their home, the Metrodome. Last Sunday the football team decamped to a dome in Detroit, but tonight they’re expecting six inches of snow in Minneapolis to greet the Chicago Bears (themselves no strangers to snow and cold, of course).

The weather story this time is not just the falling snow, but the valiant efforts of workers and volunteers who have prepared the University of Minnesota’s FieldTurf synthetic field for this Monday night game. Not only does the snow need to be cleared, but the frozen field needs to be warmed sufficiently to prevent a slew of injuries. One player called the surface “hard as concrete.” Unlike NFL stadiums, which deal with a season that stretches into December, the university’s field is normally shut down by now, and does not have heating coils underneath to blunt the effects of freezing air temperatures. In addition the stadium as a whole was “winterized,” or put in cold storage with pipes dissembled to withstand freezing, so reawakening the facility for the game was quite a process.

The conditions of the game make one appreciate the need for a dome for winter sports in Minnesota, but last week’s spectacular roof collapse raises the architectural question: how to design a large roof for Minnesota’s famously varied climate.

The keepers of the Metrodome have good reason to believe that, an occasional roof collapse aside, fabric is still the right answer. The extremes of temperature in Minneapolis stretch and contract any covering, so in fact a flexible roof is ideal. And the maintenance of a fixed structure also means significant snow removal costs which may outweigh the occasional rip and fix for a fabric roof. (Thanks to the forecasts for heavy snow, workers were on the Metrodome roof trying to clear and melt snow last Friday before retreating in a lost cause .) The main downside of fabrics these days is the energy cost of the air pressure between the two sheets of the dome to keep the roof inflated.

Here’s a radio press conference discussing the climatic considerations of the stadium after a previous collapse of the Metrodome roof, also due to snowfall, back in 1982.

It takes a particular kind of storm to damage the roof. The design calls for warm air forced between the outer and inner layers to help melt the snow, but particularly cold storms can overcome that defense, especially if coupled with sufficient water content for a heavy accumulation and winds to drift the snow and cause particularly devastating loads in particular spots on the dome. Apparently even in Minnesota this doesn’t happen often enough to make other roof solutions less expensive or more convenient.