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 interstory drift fragility curves for a series of archetype engineered light-frame wood structures. Of particular focus are structures built in moderate-seismic, heavy-snow regions. For these light-frame structures, the additional seismic mass due to the presence of roof snow may be significant. Although load standards such as ASCE 7 provide guidance on combining design loads when considering life safety (e.g.,flexural and shear limit states), guidance is not yet available for other performance levels (limit states with specified nonexceedance probabilities), performance (rather than safety) based limit states or damage indicators (e.g.,maximum interstory drift), and hazard levels other than those implied in life safety design (e.g.,2%/50years). All of these are expected to become more relevant as performance-based design procedures continue to evolve and gain acceptance. Using Boston and Stampede Pass, Washington, as study sites, snow loads and earthquake loads were modeled as stochastic processes and simulation was used to construct the joint snow-earthquake hazard contours from which the joint snow-earthquake hazard at different hazard levels could be characterized. One approach is proposed for the selection of appropriate companion load coincidence factors considering multiple hazards for use in performance-based design. Finally, peak interstory drift distributions and the seismic fragility curves at different joint hazard levels were developed for a set of archetype wood-frame structures. The results show that the current strength-based design procedures are not risk-consistent for these types of structures. As an alternative, recently developed displacement-based design procedures may provide a more risk-consistent design methodology.