Don't under-design roofs for area's snow load

Designing farm buildings to withstand snow and ice

Heavy snowfalls and ice storms can lead to roof collapses or damage in poorly designed buildings according to Bob Schultheis, natural resource engineering specialist, University of Missouri Extension.

It often comes down to weight. Remember, a foot of snow is about equal in weight to an inch of ice, and an inch of ice weighs about 5 pounds per square foot.

"Most farm building failures occur because the roof structure was under-designed for the area's snow load, the bracing or fasteners were short-changed during construction, or because wind-drifted snow distributes unevenly over roofs covered with ice," Schultheis said.

To help prevent future such building failures, Schultheis says a few basic design rules for agricultural buildings should be followed in southwest Missouri.

First, roofs of farm buildings should be designed to support a minimum total load of 25 pounds per square foot. This figure includes the snow load and the dead load (weight of building materials). If cages, feeders, hoists or ceilings are hung from roof trusses, figure this extra weight into the design.

The second item to remember is that the height of the eave greatly affects how big the poles, posts or metal frames must be to prevent wind-racking, how many anchor bolts are needed or how deep poles must be embedded in the ground, and how much extra bracing is needed between trusses, poles or frames.

Usually a 4- to 5-foot embedment depth is recommended for wood poles and posts.

"Where rock prevents getting this depth, the posts should be encased in concrete. Failure to do so invites wind uplift or sidewall shift from excessive roof loads," Schultheis said.

Third, keep in mind that buildings over 40 feet wide are generally less expensive to build with steel truss frames than with wood trusses. Wood, however, is more forgiving for short periods of time under excessive loads, and may withstand a 200 to 400 percent overload for two to three days without failure.

"A wood roof will give you audible advance notice of failure by screeching of nails and sounds of wood splintering. Steel roofs, on the other hand, usually fail when the design load is reached and without any advance notice," Schultheis said.

And finally, weak designs often show themselves with a dip or sag in a roofline or beam, or a leaning sidewall. Adding 2 x 6 knee braces to posts and trusses, tying opposing walls together with cables and turnbuckles, sistering larger joists to existing ones, or adding extra support posts under long beam spans are just a few ways of beefing up some of these weaker buildings.

For anyone considering construction of a post-frame building, Schultheis recommends NRAES-1 Post-Frame Building Handbook, which is available from Extension Publications by calling 1-800-292-0969.

For more information, Schultheis can be reached at the Webster County Extension Center in Marshfield, Mo., at 417-859-2044.

1/14/08
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Date: 1/4/08

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