Seismic Fragility Analysis of Base-Isolated Building Frames Excited by Near- and Far-Field Earthquakes

The present study aims at the probabilistic seismic risk assessment of a base-isolated building frame under near- and far-field earthquakes by conducting a fragility analysis. For this purpose, a base-isolated 10-story reinforced concrete frame was considered having lead rubber bearing as the base isolation system. Fragility curves were developed for an ensemble of far-field and near-field (including directivity and fling-step effect) earthquakes, and for a number of damage measures, namely, maximum interstory drift ratio (MIDR), maximum base shear (MBS), maximum roof drift ratio (MRDR), maximum top floor acceleration (MTFA), and maximum isolator displacement (MID). Moreover, to investigate the effect of frequency contents of near-field earthquakes, the peak ground velocity (PGV) to peak ground acceleration (PGA) ratio of near-field directivity earthquakes was considered as a variable. Two sets of near-field directivity earthquakes were considered, one having a low $\mathrm{PGV}:\mathrm{PGA}$ ratio, i.e., less than 150 ($\mathrm{cm}/\mathrm{s}/g$), and the other having a high $\mathrm{PGV}:\mathrm{PGA}$ ratio, which is greater than 150 ($\mathrm{cm}/\mathrm{s}/g$). The incremental dynamic analysis was conducted to create the fragility curves by assuming different threshold values of damage states, namely, slight, moderate, extensive, and collapse. The results of the study indicate that even for the low PGA level, the near-field earthquakes cause a high probability of exceedance for the base-isolated frame. The $\mathrm{PGV}:\mathrm{PGA}$ ratio has a significant effect on the damage probability for the near-field earthquakes because the high $\mathrm{PGV}:\mathrm{PGA}$ ratio had greater damage probability compared to that of low $\mathrm{PGV}:\mathrm{PGA}$ ratio.