Seismic Irregularity Evaluation of Reinforced Concrete Buildings Based on BNBC 2020

Abstract

The seismic performance of reinforced concrete buildings is strongly influenced by structural irregularities, which may amplify demand and lead to undesirable damage patterns during earthquakes. This study presents a code-based evaluation of seismic irregularities in five multi-storey reinforced concrete residential buildings designed according to the Bangladesh National Building Code (BNBC) 2020. Numerical models of the selected buildings were developed using ETABS, and seismic irregularities were assessed in terms of torsional irregularity, soft storey irregularity, mass irregularity, vertical geometric irregularity, and vertical discontinuity, following BNBC-defined criteria. The results indicate that torsional irregularity is the most critical and frequently observed form of irregularity among the studied configurations. Out of five buildings, four exhibited torsional irregularity, with two cases showing extreme torsional behaviour where the torsional irregularity ratio exceeded the BNBC limit by a significant margin, reaching values close to 1.9 at lower storeys. Soft storey irregularity was identified in one building, primarily at the ground floor, due to a substantial reduction in lateral stiffness compared to the upper storeys. No mass irregularity, vertical geometric irregularity, or vertical discontinuity was detected in any of the analysed cases. The findings highlight the sensitivity of torsional response to plan configuration and stiffness distribution, even when buildings comply with basic code requirements. This study emphasises the necessity of early-stage irregularity checks in seismic design to ensure safer and more resilient reinforced concrete buildings in seismic regions.