|How loads are specified|
|Nothing in safety, resistance, or loads for structural design is cut and dry. Most of the variables shown in this article can vary, and each variable has a probability distribution. For example, roof snow loads S = Sr + Cw Cb Ca Cs Ss Cw Ca Cs are educated approximations while for Sr, Cb, Ss can be modelled using probability distributions.
Considering the period building codes have been used, snow loads have been specified in the National Building Code of Canada (NBCC) since 1953. Technology has advanced exponentially while more environmental data has been collected. The improvement in computing capability has allowed loads and safety to be addressed in a more sophisticated manner than in earlier years.
The following is a brief history on the development of loads, but first, here are some terms that will be used that need explanation:
Step 3: Limit States design approach allowed loads to be combined in a manner that provided a more consistent margin of safety than allowable strength design.1
Step 4: With the additional environmental data, the accidental (rare) load could be better defined. Use of the accidental load captures unique situations that were not caught by using a specified load with a load factor. For example, on the coastal areas of the U.S., there are significant spring snow falls. The effect of these storms was not accounted for previously but are now. The 2020 snow study increased the design snow loads two to three times in some coastal areas. This change reflected local knowledge of snow loads for those municipalities.
Step 5: Is an exciting development, it shows multiple probability distributions can be used to establish design loads, where in the past, only one probability distribution was used for each load type.
1 For more on Limit States Design in Canada, read the paper “Safety and Limit States Design for Reinforced Concrete” by J.G. MacGregor published in the Canadian Journal of Civil Engineering, V. 3, No. 4, Dec. 1976, pp. 484-513.
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