Seismic Response Evaluation of Irregular Reinforced Concrete Buildings Using an Integrated IAF–NRI–ISS Framework

Structural irregularities can alter seismic deformation and internal force requirements, but comparisons of different types of irregularities using a single set of response measures remain limited. This research introduces a response-index framework for comparing seismic performance of irregular reinforced concrete (RC) buildings. Nine ten-storey RC models, one regular reference and eight with irregularities such as mass, re-entrant corners, vertical stiffness, and geometric irregularities, were evaluated through Response Spectrum Analysis. Six response parameters—roof displacement, maximum storey drift, drift ratio, base shear, column axial force, and column bending moment—were assessed using the Irregularity Amplification Factor (IAF), Normalized Response Index (NRI), Deformation–Force indices (DI–FI), and Irregularity Severity Score (ISS). The framework integrates response amplification, normalization, behavioral classification, and overall ranking within a unified reference-based process. The vertical stiffness model S-2 achieved the highest ISS (1.461), primarily due to a bending moment of 404.57 kip-ft, followed by S-1 (ISS = 1.285). Sensitivity analysis showed that S-2 consistently remained the top-ranked option, except when the bending moment was excluded. The framework provides a concise basis for identifying dominant response mechanisms and comparing the rankings of irregular configurations, thereby aiding seismic assessment and informing design decisions for RC buildings.