Seismic Design Methodology Based on Performance Objectives for Mass-Irregular RC Structures in Elevation

Abstract

Vertical irregularity in buildings often arises from architectural or functional requirements that lead to irregular mass distribution over different stories—such as open ground floors, setbacks, or changes in occupancy use. These irregularities can significantly affect seismic performance, making an accurate assessment essential for structural safety. This study presents a novel irregularity index that incorporates the interstory drift ratio into a performance-based framework for evaluating the seismic behaviour of ten-story reinforced concrete buildings with irregular mass distribution. Displacement-based adaptive pushover method is employed to analyse the response of vertically irregular structures under seismic loading. A detailed investigation is conducted focusing on key response parameters, including base shear versus roof displacement, interstory drift, and story-shear distribution, to identify the factors most critical to structural behaviour. The results highlight the influence of interstory drift on the seismic response of buildings with vertical mass irregularities. Based on these findings, a performance index is developed using the interstory drift ratio, which reflects sensitivity to variations in mass along the building height. This approach contributes to the broader understanding of how mass irregularity affects structural performance and offers an additional tool for evaluating seismic behaviour in irregular buildings.