Fragility analysis of woodframe buildings considering combined snow and earthquake loading

This paper describes a study to evaluate appropriate percentages of design snow load (factors for use with nominal values provided by the current design load standard for structural design in the United States, ASCE 7-02) when snow load is being taken into account in a seismic fragility analysis. The procedure is illustrated through the development of seismic fragility curves for one and two-story woodframe structures in three locations (Memphis, TN; Carbondale, IL; and Boston, MA) having both moderate snow and seismic hazards. The fragilities are cast in terms of displacement criteria (maximum shearwall drift) with the snow load serving to add seismic weight to the structure. The structures are analyzed using a nonlinear dynamic time-history analysis procedure. The seismic hazard is defined using USGS seismic hazard maps and uncertainty in the seismic hazard at each location is characterized by a suite of ordinary ground motion records. The ground snow hazard is defined through an analysis of data from first-order weather stations at the sites considered. Through a series of multi-hazard convolutions, parametric studies, and the construction of fragility curves, percentages of design snow load are determined for use in constructing displacement-based seismic fragilities and calculating failure probabilities (by convolving with appropriate seismic hazard functions). Practical implications for fragility analysis considering multiple hazards and performance-based design of woodframe structures also are discussed.