Seismic fragility analysis: Application to simple linear and nonlinear systems

Seismic fragility of a structural, nonstructural and/or geotechnical system is the probability that a system response exceeds a critical value under seismic ground motions of specified intensities. Fragility curves are plots of system fragilities versus a scalar measure of seismic intensity. Traditionally, peak ground acceleration (PGA) has been used as an intensity measure. Recent studies show that pseudo‐spectral acceleration provides a superior measure of seismic intensity than PGA. It is shown that pseudo‐spectral acceleration may not be a satisfactory measure of seismic intensity for the fragility analysis of nonlinear systems, so that fragility curves based on pseudo‐spectral acceleration are not recommended for these systems. An alternative intensity measure based on two parameters, the earthquake moment magnitude m and the distance r from the seismic source to system site, is proposed for the fragility analysis of nonlinear systems. The relationship between a system probability of failure and (m, r) is called fragility surface. Findings are for single degree of freedom systems and synthetic ground motion records.