Damping Modification Factors for Response Spectra of Pulse-like Near-fault Ground Motions

The damping modification factor quantifies the effect of damping ratio on seismic response of a structure. It is related to the spectral characteristics of ground motion, and is used to obtain the high damping spectra from the low damping spectra. This work aims to explore the damping effect on structural responses induced by near-fault ground motions, and to propose estimation formulas for damping modification factor. A total of 50 ground motions were selected, and the wavelet-based equivalent pulses were extracted. The response spectra were calculated based on the selected ground motions and the equivalent pulses, with the assumed 13 damping ratios ranging from 0.05 to 0.50. The damping modification factors were obtained based on the reference spectra with a damping ratio of 0.05. The results show that increasing damping ratio reduces the spectral values the most significantly if the period ratio T/Tp (structural period to pulse period) is between 0.5 and 1.2, and in this range the same level of damping modification factors are obtained from the equivalent pulses. Based on the numerical results, the empirical functions were suggested for estimating the damping modification factor for acceleration, velocity and displacement, respectively. The proposed formulas take into account the near-fault effect, and are applicable within the period range of 0.5 ＜ T/Tp ＜ 1.2.