Reliability-Based Design Snow Loads. II: Reliability Assessment and Mapping Procedures

Abstract

This paper describes the development of reliability-targeted ground snow load maps for use in building (roof) design. The proposed procedures aim to ensure that structures designed achieve a target safety index, taken to be 3.0 as defined in ASCE 7 (ASCE 2010). When applied to the U.S. state of Colorado, the reliability-targeted mapping procedure shows that to achieve this target reliability index, design ground snow loads may need to be larger or smaller than the 50-year return period load that ASCE 7 currently stipulates for design, depending on the site and winter climate conditions at that site. Sites with larger coefficients of variation in the annual maximum snow load (in Colorado, lower-altitude sites) generally need design loads larger than the 50-year values. The paper also advances snow reliability assessment procedures through the proposal of a new model for the ground-to-roof conversion factor needed for quantifying roof snow loads as a function of ground snow loads. Spatial smoothing and altitude-dependent mapping procedures are described.