Site-Specific Snow Load Models and Hazard Curves for Probabilistic Design

Abstract

This paper describes a study to develop site-specific probabilistic snow load models for use in reliability analyses of structures in the United States. Annual maximum snow water-equivalent depth data measured at 93 first-order weather stations was obtained from the National Climatic Data Center. These were then converted to annual maximum ground snow loads and statistics at each site were obtained using graphical fitting techniques. The 50year maximum roof snow load values (in ratio to code-specified nominal values) were then evaluated site by site using the statistics for annual maximum ground snow, extreme value theory, Monte Carlo simulation, statistics for ground to roof conversion factors, and the nominal ground snow load values in ASCE 7. Finally, the 84 sites in the continental United States (excluding nine sites in Alaska) were grouped on the basis of the 5% exclusion values of actual to nominal 50year maximum roof snow load. Aggregated statistics for 50year roof snow load in three geographic regions (designated Northeast, Midwest/Mid-Atlantic, and Northern Midwest/Mountain West) were determined. The roof snow load models developed in this study can be used in code calibration studies and development of next-generation partial safety factors. Snow load models such as those developed herein are developed using data from weather stations typically located at airport or military base locations. Therefore, they are not applicable to special locations such as mountainous regions or gorges. ASCE 7 suggests that design snow loads in such cases should be based on local snow information with consideration given to the orientation, elevation, and records available at each location. Using the state of Oregon as an example, this study also develops county-scale design ground snow load curves and then postprocesses this information into snow hazard curves. Both the design ground snow load curves and the snow hazard curves are presented as functions of elevation for each county area (defined as areas having similar ground snow load versus elevation relationships). To evaluate structural performance of a building subjected to snow load in a performance-based framework, snow hazard curves can be convolved with fragility curves for the building. This study demonstrates one approach for developing snow hazard curves for special regions (mountain areas and gorges) for use in such performance-based design applications.

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