Fragility Curves and Methodology for Estimating Postearthquake Occupancy of Wood-Frame Single-Family Houses on a Regional Scale

Abstract

With the exception of older dwellings built on cripple walls, wood-frame single-family houses in California have shown adequate levels of safety against collapse in previous earthquakes. However, damage to chimneys and structural walls includes widespread types of damage that have resulted in houses being posted with yellow or red tags, restricting their postearthquake occupancy. By using analytical models, we develop fragility curves, sometimes also referred to as fragility functions, for estimating the probability of damage to masonry chimneys and severe damage to shear walls in wood-frame single-family houses as a function of the number of stories of the house, the year the chimney was built, and the ground motion intensity. Fragility curves are provided as a function of two different ground motion intensity measures: the 5%-damped average spectral pseudoacceleration at a period of 0.16 s, and the peak ground acceleration. The proposed fragility curves are used within a fully probabilistic methodology for estimating the number of wood-frame single-family houses with chimney and wall damage in Napa, California, due to the 2014 South Napa earthquake. Results are compared to estimates computed using three previously proposed methodologies and with empirical data collected after the earthquake. The proposed fragility curves and methodology lead to estimations of the number of houses with chimney and wall damage that are between 2 and 10 times less biased than those computed using previous damage estimation procedures.