Keywords: pulse-like motions, wavelet transform, inelastic time history analysis, steel moment frames, demand.

This paper investigates inelastic dynamic behaviour of steel moment frames subjected to a special class of ground motion which are characterized by strong pulses in the velocity time history and called 'pulse-like ground motions'. These large pulses are frequently clear in the fault-normal component of the ground motions obtained in near-fault area where the earthquake rupture has propagated toward the site. This type of ground motion is distinguished from typical far field records and may generate large energy demands that force the structures to dissipate such a high energy with few large displacements [1,2]. Therefore, the study of near-fault ground-motion characteristics and its effect on the seismic behaviour of structures subjected to these motions are very important active topics both to the seismological and to the engineering communities. Selection of such motions and the effect of their features on structural behaviour are fundamental questions raised here.

In the first phase of this work, for the characterization of these special types of ground motion, a wavelet based approach is performed to identify and extract the large pulse of the 1978 Tabas earthquake recorded at a near-fault area. In this procedure, the largest velocity pulse from the normal-fault component of the Tabas earthquake is extracted and compared with the remaining part of the motion. If the extracted pulse is large relative to remaining features in the ground motion, the ground motion is classified as pulse-like motion [3] and selected as an input motion in the nonlinear analysis of structures. The Daubechies wavelet of order four (db4) as a most widely-used mother wavelet which has been recognized as effective in identifying features in the seismic data, has been applied herein.

In the second phase, the inelastic dynamic behaviour of steel moment frames subjected to the original record of the Tabas earthquake, extracted pulse and residual ground motion is evaluated to gain a better insight into the effect of strong pulse and its contribution on structural demand. The interstory drift ratio defined as the relative displacement between two consecutive story levels normalized by the story height and selected in this paper as an important measure of the seismic demand along with the maximum displacement of the stories. These attempts will provide an appropriate platform for incorporating the special effects of near-fault ground motion in seismic design codes.

1

J.F. Hall, T.H. Heaton, M.W. Halling, D.J. Wald, "Near-source ground motion and its effects on flexible buildings", Earthquake Spectra, EERI, 11(4), 569-605, 1995. doi:10.1193/1.1585828

2

W. Liao, C. Loh, S. Wan, "Earthquake Responses of RC Moment frames Subjected to Near-fault Ground Motions", The Structural Design of Tall Buildings, 10, 219-229, 2001. doi:10.1002/tal.178

3

J.W. Baker, "Quantitative Classification of Near-Fault Ground Motions Using Wavelet Analysis", Bulletin of the Seismological Society of America, 97, 1486-1501, 2007. doi:10.1785/0120060255

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