The Overstrength Paradox: A Case Study on Code-Induced Performance Inversion in Torsionally Irregular RC Frames

Abstract

Background: Seismic codes prescribe member oversizing to mitigate torsional irregularity. This case study documents the “Overstrength Paradox”—observed within a controlled boundary (six-story RC SMRF, unidirectional pushover)—where code-mandated stiffness enhancements altered the failure hierarchy for the specific configuration examined. Methods: A parametric numerical investigation on a six-story RC SMRF (IS 1893:2016, IS 13920) evaluated three variants—non-compliant, analytically amplified (design eccentricity), and geometrically upsized—via nonlinear static pushover analysis (ASCE 41-13). Seismic demand was scaled using Importance Factor (I = 1.0, 1.5, 2.0). Results: At I=1.0, the torsion-compliant structure exhibited high overstrength (Ω = 6.56–7.59) yet showed shear-critical collapse (D/C ≈ 1.19) despite codecompliant shear reinforcement. At I=2.0, it sustained moderate overstrength (Ω = 3.07–3.49) but achieved Immediate Occupancy through ductile flexural action. This inversion confirms linear force-scaling fails to yield proportional safety margins in overstrength-stiffened asymmetric systems. Conclusions: For the specific configuration examined, seismic safety depended on demand-activated ductile pathways rather than nominal reserve strength. These findings serve as a documented diagnostic indicator of hierarchy vulnerability. The mechanism may generalize to other plan-irregular systems but requires systematic validation.