The Effect of Directivity Pulse of Near Fault Earthquakes on Seismic Response of a Tall Building

A special class of ground motions near the fault regions has distinct characteristics that are different from far-field ground motions. One of the critical features of these ground motions is the existence of a velocity pulse in the ground motion record in the direction perpendicular to the fault rupture. These pulse-like ground motions generally occur when the fault rupture propagates towards a site located near the fault. The accumulation of energy in the seismic wave front results in a velocity pulse with a relatively long period in the direction perpendicular to the fault line. This phenomenon could adversely affect the seismic performance of tall buildings with relatively long fundamental periods.In this paper the effect of velocity pulse on seismic response of a 42-story reinforced concrete building with a central core wall structural system is evaluated. The building has already been studied in the TBI project of the Pacific Earthquake Engineering Research Center (PEER). The structural model of the building is developed based on the information contained in the PEER report. The model is first verified by conducting a nonlinear response analysis using one of the ground motion records in the PEER report and comparing the results with that report. After verification, the model is subjected to three earthquake records each containing a velocity pulse (i.e., Northridge 1994, Cape Mendocino 1992, and Chuetsu-Oki, Japan 2007). Subsequently, the velocity pulse of each record is removed using a recently proposed wavelet-based signal processing approach and the building is analyzed again to determine the impact of the velocity pulse. The results of the analyses show that the velocity pulse significantly affects the seismic response of the building. By removing the velocity pulse, the lateral drift and rotation of the coupling beams decrease by about 50% and 60%, respectively. Also, the forces in the structure (shear and bending moment) are reduced by about 40%. The dominant effect of the velocity pulse on the seismic response of the building is due to the high energy content in the frequency range of the pulse velocity. However, due to the different frequency content of each record, the effect of the pulse period cannot be accurately assessed. Therefore, the study of buildings with the same lateral bearing system and different natural periods (i.e.,