UNSYMMETRICAL SNOW LOADING ON BUILDINGS



By

Charles C. Roberts, Jr., Ph.D., P.E.

Every year, roof structures fail after moderate snowfalls that would not seem to be excessive. A significant ingredient in this phenomenon is the condition of nonuniform snow load. The design of most buildings assumes that the loading or force on the roof is uniformly distributed. In moderate snowfalls with windy conditions, drifts develop on the down-wind side of a roof, which can be a significant load on the structure at a particular location.


Figure 1

Figure 1 shows a large snowdrift on the downwind side of a roof ridge. A snowfall as low as 3-4 inches can result in drifting on the order of 2 - 3 feet. The unsymmetrical loading from snowdrifts can cause different failure modes


Figure 2

when compared to uniform loading. Figure 2 is an example of how unsymmetrical loading severely damaged a conical roof. A conical roof is quite strong when the loading is uniformly distributed downward. This allows the roof rafters and sheet metal to be in plane and resist the load. When a snow load is applied on one side of the roof, the roof rafters deflect downward in a local area, causing a buckle in the structure. The buckle is inherently weak and can cause local rafter failure, which is what lead to the collapse of the conical grain bin roof.


Figure 3

Figure 3 shows a large drift that was over 2 feet deep on the down- wind side of the roof ridge. This caused a localized collapse of the roof in the area of the drift. It should be noted that the ground snow depth was approximately 12 inches, while the drift depth on the roof was over 2 feet. Snow density is a significant factor affecting the amount of unbalanced load on a structure. Most building designers and many building codes assume snow loading to be uniform for simplicity's sake. There is insufficient data available in the engineering literature on characteristics of nonuniform snow load development. Consequently, no consensus has been reached regarding how to handle building design with unbalanced snow loading. In certain cases, as shown above, the uniform snow load assumption may not be valid, and a collapse may occur as a result of a nominal snowfall.


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