Effects of frequency contents of aftershock ground motions on reinforced concrete (RC) bridge columns

Abstract

This study focuses on exploring effects of frequency contents of aftershock ground motions on seismic responses of reinforced concrete (RC) bridge columns. It has been well recognized that RC columns damaged by a sizeable main shock event become more vulnerable to following aftershocks. Therefore, it is essential to use proper main shock-aftershock sequential ground motions in seismic analyses to ensure the safety and integrity of infrastructure. Using frequency –invariant scaling factors, conventional methods were developed in the past. These methods combine sequential ground motions when performing a time history nonlinear analysis. However, these conventional methods neglect frequency contents of aftershock ground motions, which are usually different from those of main shock ground motions.This research demonstrates the importance of properly representing aftershock ground motions in estimating seismic responses of RC columns, and presents the differences in frequency contents between aftershock ground motions and the corresponding main shock ground motions. Time history seismic analyses using a finite element analysis program OpenSees are carried out. First, main shock motions recorded during the 1994 Northridge, California, the United States of America, the 1997 Umbria-Mache, Italy and the 1999 Chi-Chi, Taiwan earthquakes are used in this study. Then, the corresponding aftershock motions are selected or obtained: (1) from recordings during the seismic events, (2) by scaling main shock (which represents a traditional method), or (3) by spectrally matching main shock motions to the aftershock motions. The peak displacements and residual displacements of the RC columns using the spectrally matched motions are closer to those results using real aftershock motion records, as opposed to using the scaled motions. This demonstrates that the frequency contents of aftershock ground motions have significant impacts on the seismic responses of RC columns.