Seismic Performance of Vertical Irregular Steel Frame Structure under Mainshock-Aftershock

Abstract

Abstract: The majority of the structures are involved with architectural importance and it is highly impossible to achieve uniform structural properties in all directions. Hence earthquake resistant design codes considered it as irregular frames based on relative difference in the story properties. In many cases these irregularities are responsible for structural collapse of buildings under earthquake ground motions. The seismic response of buildings with irregular distribution of stiffness along the height may be differentfrom that ofregular building. Also, past earthquakes showed that structure may be subjected to sequence of ground motions but current codes do not have guidelines for such cases. It is considered that aftershock do not cause any more damage to damaged structure by the preceding mainshock ground motion. In this study steel moment resisting frame buildings are evaluated to understand the seismic response of vertical stiffness irregular frames subjected to mainshock-aftershock ground motions. The 9-story steel moment resisting frame building situated in Los Angeles is used in this study was originally developed as part of the SAC steel project. In this study soft and stiff storey case was considered at three different locations along the height, i.e., at bottom storey, mid-height storey and top storey. The single modification factor is used for irregularity. For comparison purpose dynamic properties of regular and irregular frames are kept same. Two sets of 6 records were selected representing a seismic hazard level of 2% and 10% probability of exceedance in 50 years respectively as mainshock. Two sets of 30 mainshockaftershock ground motion are considered for the study. These ground motions were developed using randomized approach. A non-linear time history analysis of regular and irregular building is carried out separately under mainshock and mainshockaftershock. The effect of building irregularity was studied for single storey modification at bottom storey, mid-storey and top storey with comparison to regular building. The comparison of a regular and irregular building is carried out in terms of maximum roof displacement and interstorey drift ratio. Also, comparison of frames under mainshock and mainshock-aftershock is done.