The buildings with the presence of setback irregularity are now being increasingly used in the urban areas. The present work proposes an irregularity index for quantifying the setback irregularity based on the dynamic characteristics of the buildings. This paper also proposes a modified equation for the fundamental period of vibration, for building frames with setback irregularity. Furthermore, the equations for estimating the maximum inter storey drift ratio (Ir) and maximum displacement ductility (μmax) are also proposed. These equations are proposed on basis of the regression analysis conducted on the seismic response databank of 305 building models with different types of setback irregularity for each height category. The proposed equations are represented as a function of the irregularity index, and are validated for 2D and 3D building models with setback irregularity
