Evaluation of Reduction Factors for Vertically Irregular RC Frames: Conventional vs. Adaptive Pushover Analysis

The significant computational demand of Nonlinear Time-History Analysis (NTHA) often leads seismic design practitioners to opt for the more efficient Nonlinear Static Pushover Analysis. However, conventional pushover methods, typically reliant on the first-mode pattern, fail to capture the significant influence of higher modes. This limitation becomes particularly critical in vertically irregular structures, where geometric discontinuities can amplify higher-mode effects, leading to an inaccurate assessment of seismic performance. This study addresses this gap by employing Displacement-Based Adaptive Pushover Analysis (DAP) to account for these higher-mode effects. We investigated five distinct configurations of vertically irregular reinforced concrete (RC) moment-resisting frames to evaluate and compare the seismic reduction factors derived from both conventional Static pushover analysis first mode pattern CSPA and DAP, benchmarking these results against the prescribed limits in Eurocode 8. The findings are compelling: geometric irregularities located near the base of the structure lead to a substantial discrepancy between the reduction factors calculated by DAP and CSPA. Critically, the adaptive analysis consistently yielded significantly lower and more conservative reduction factors, which frequently fell below the code's minimum values. This result suggests that current code provisions may dangerously overestimate the seismic capacity and ductility of such structures, masking their true vulnerability. The study concludes that for vertically irregular frames, particularly those with significant discontinuities in lower stories, adaptive pushover analysis is a necessity for achieving a safe and realistic seismic performance assessment.