Joint Earthquake-Snow Hazard Characterization and Fragility Analysis of Wood-frame Structures

This paper presents a study to statistically characterize the joint earthquake-snow hazard and subsequently develop maximum inter-story drift fragility curves for a series of archetype woodframe structures. Of particular focus are structures built in heavy-snow regions where seismic design may govern (and hence the roof snow load contributes additional seismic mass). While load standards such as ASCE 7 provide guidance on combining design loads for life safety design, for example, when considering base shear, guidance is not yet available for other performance levels (or limit states with specified non-exceedance probabilities), other structural responses (e.g., maximum inter-story drift), and hazard levels other than that implied in the life safety design (e.g., 2%/50 years). All of these are expected to become more significant as performance based-design procedures continue to evolve and gain acceptance in the design community. Using Stampede Pass, WA as the study site, snow loads and earthquake loads were modeled as stochastic pulse processes and the joint snow-earthquake hazard contours were constructed (using simulation) to characterize the joint snow-earthquake hazard at different hazard levels. The uses of the joint hazard contours in performance-based engineering framework applications also are discussed and the suitability of current constant companion load coincidence factors, developed for use in strength-based design, is examined. The peak inter-story drift distribution and the seismic fragility curves were then developed for a set of archetype wood-frame structures at different joint hazard levels. The results show that the current strength-based design procedures are not risk-consistent for these types of wood-frame structures, affirming that recently developed displacement-based design procedures may provide a more risk-consistent design methodology.