Modification of Park-Ang Damage Index to Accommodate Effect of Aftershocks on RC Structures

Seismic design codes do not consider the effects of aftershocks on structures. Moreover, most damage estimation methods disregard the effects of consecutive earthquakes. Recent earthquakes have demonstrated that the aftershocks can cause major damage to mainshock-damaged structures. After a strong seismic event, it should be determined if a mainshock-damaged building is safe for reoccupation in the event of aftershocks. This study examined the effects of both the main shock and aftershocks on the damage index of reinforced concrete (RC) structures. Records from 19 mainshocks that occurred in the Japan seismic region with moment magnitudes of greater than 4 were examined. More than 100 acceleration time series from these events (mainshock + aftershock events) were applied to evaluate the damage index. Seismic damage analysis was performed on a series of RC columns and RC frames. The natural period of these structures varied from 0.1 to 2.5 s. The damage index introduced by Park and Ang was employed to estimate the structural damage. Acceleration time series were applied to the structures in two steps. First, only mainshocks were applied to the structures and the damage index was obtained. Next, both the mainshock and aftershocks were applied to the structures and structures damaged by a mainshock were analyzed under periodic aftershock events. The results showed that the increase in structural period and the PGA of the aftershocks amplified the damage index under the effects of the aftershocks. A modification to Park-Ang damage index is proposed using a dimensionless term to accommodate the effect of aftershocks.