Snow Loads Reach "Design" Levels - What's Next?

Rain on snow in the past 24 hours has produced snow loads in the area equal to the 'design' snow loads for structures. In other words, the amount of snow present is the maximum anticipated amount the structures were designed to carry. But so far no structural failures. Why?

By Monday, the depth of undisturbed snow on the ground had increased to 27 inches. Measured weight of the snow was found to be 26 pounds per square foot. Warmer weather since then has delivered about 1/2 inch of rain to the area which was largely absorbed into the snow profile, bringing the total weight of the snow to 30 pounds per square foot. This is the amount of load for which many of the area structures were designed for, with manufactured housing being designed for as low as 25 pounds for square foot. The City of Moscow in updated press releases is advising owners of structures with flat or mild sloping roofs to remove roof snow and for all citizens to be aware of the amounts on roofs and potential failure hazard.

So, why no failures, yet? This year owners and managers of buildings have been vigilant to get the snow off of flat roofs, and in some cases, even sloped roofs. Last year heavy snowfall closed one major business temporarily and caused one agricultural building failure. So people are aware and taking precautions this year. Also, accumulations on sloped and poorly insulated roofs tend to be less than on the ground or flat, well-insulated roofs. The other reason is that building codes incorporate safety factors. Building codes take into account uncertainties in (future) loading conditions, and legitimate uncertainties in the materials and construction. So, in reality, properly designed structures should be able to withstand the design loads, and then some. Just how much "and then some" is depends on the individual structure, materials, and any "over design" (additional safety factor). The structures that are at immediate risk would be those not build to building code standards.

Now what? For the sake of the roofs in the area the best scenario would be dry weather above freezing to take the snow off the roofs. But another possibility would be cold weather and more snow, in which case roofs that are still carrying the already fallen snow will crowd any factors of safety and at some point become in peril of collapse. Still another weather possibility is warm, windy, wet weather, which will be potentially bad for roofs as the rain will be absorbed by any standing snow, increasing the weight. Then the snows will melt from ground and roofs, causing excessive runoff. Wind accelerates snow melt by convective heat transfer (the opposite of the chill factor effect). Indeed, area weather forecasts contain all three advisories (warmer temperatures and rain, wind, and flooding). The next 48 hours will be interesting.

Sources:

Construction Codes, City of Pullman, 325 S.E. Paradise Street, Pullman, Washington 99163.

Pullman / Moscow Regional Airport, Observed Weather for Past 48 Hours, weatherforyou.com.

City of Moscow Press Release, 221 E. 2^nd Street, Moscow, Idaho 83843.

Snow Loads on Roofs and Decks, Jeff Filler, Associated Content.

National Weather Service Weather Warnings for Moscow, Idaho, weather.msn.com.