Proposed Refinements to Design Snow Load Derivation

In contrast to other naturally occurring loads such as wind-induced or earthquake loads, design snow loads are generally the result of a series of events that occur over an entire winter. As a result, a number of key variables are inherently generalized when developing standardized guidelines. While the ASCE 7 Standard is employed by engineers to define snow loading for structural design, a strict application of the standard is not necessarily synonymous with an optimized structural design due to these generalizations. The variability is increased further as building designs push the envelope in terms of geometry and energy performance. With improvements in design practice comes the need for refinement to the basis for loading derivation. Factors such as roof size, exposure, thermal capacity, and aerodynamics need to be considered when deriving loads. Design snow loads for roofs are typically considered as a fraction of the snow loading on the ground to account for the potential for snow that is drifted off of the roof surface. However, the potential for this loading relief decreases as the roof increases in size. The consideration given to area averaging effects when considering wind loading, as structural and cladding wind load components, is not realized when designing for snow loads. Wind directionality effects on the potential snow load distributions are also not accounted for. All step regions are treated as though there is an equal probability of occurrence of the loading magnitude. Further, with the exception of overheated structures such as greenhouses or unheated structures, the effects of thermal variations over a roof surface are not considered. However, variations to the internal operating temperatures and roof insulation values may lead to similar building performance characteristics. The use of alternative analysis tools including wind tunnel and finite area element modeling to determine the potential variability in design snow loads resulting from these factors is discussed and refinements to the current ASCE 7 snow load provisions that account for these factors are proposed.