Main Shock-Aftershock Sequential Analysis on Reinforced Concrete Columns

Abstract eng:
This paper focuses on developing a main shock-aftershock (MSAS) sequential seismic analysis framework on reinforced concrete (RC) columns. Historical seismic events demonstrated the vulnerability of existing RC columns when they were subjected to a main shock followed by series of aftershocks. Especially, aftershocks during the 2011 Christchurch earthquake in New Zealand aggravated damages to Christchurch and the central city area economically and structurally. A series of aftershocks identified after a main shock caused severe damage on structures already weakened by the main shock. As there is growing attention to importance of aftershocks, a few main shock-aftershock sequential analyses have been suggested and developed. One of the common approaches to obtaining aftershock ground motion time series is to repeat main shock ground motion time series using frequency-invariant scaling factors. Another common approach is to randomly select time series from a set of main shock records and scaling them down to achieve desired amplitudes for aftershock motions. However, these conventional approaches are not capable of obtaining ground motion time series properly representing characteristics of aftershock ground motions because the frequency contents of aftershock ground motions usually differ from those of main shock ground motions. This research demonstrates the importance of properly representing aftershock ground motions in estimating the seismic responses of an RC column, and presents the differences in frequency contents between aftershock ground motions and the corresponding main shock ground motions. Time history seismic analyses are conducted using a finite element analysis program, OpenSees. The main shock motions recorded during the 1994 Northridge, California, and the 1997 Umbria Marche, Italy earthquakes are used. The aftershock motions are selected or obtained: (1) from recordings during the seismic events, (2) by scaling main shock motions to match the peak ground acceleration (PGA) values of aftershock motions, or (3) by spectrally matching main shock motions to the aftershock motions. The peak displacements and residual displacements of the column using the spectrally matched motions are closer to those results using real aftershock motion records, as compared to using the scaled motions. This demonstrates that the frequency contents of ground motions have significant impacts on the seismic responses of the RC column. Finally, in order to accommodate the forward prediction condition where aftershock ground motions are not available, an empirical relationship for the ratio of aftershock to main shock horizontal pseudo spectral accelerations (PSAs) at various periods is developed.

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