Beyond Prescriptive Codes: A Validated Linear-Static Methodology for Seismic Design of Soft-Storey RC Structures

Reinforced concrete (RC) buildings with masonry-induced soft-storey irregularities exhibit extreme seismic vulnerability, a critical risk often underestimated by conventional code-based design. Standard equivalent static methods typically fail to capture the intense concentration of seismic demand at the flexible ground level, leading to unconservative designs that do not meet performance objectives. This research proposes a corrective linear-static methodology to address this deficiency. A new Equivalent Lateral Force profile (ELF1) was developed, derived from modal analyses of 235 representative soft-storey archetypes. This profile was integrated with a more realistic response modification coefficient (Rᵢ₁ = 5.04), which was determined to be 37% lower than the normative R-factor (R=8) prescribed by code. Nonlinear static (pushover) analyses confirmed that the conventional design resulted in "irreparable" damage (mean GDI = 0.82). In contrast, redesigning the structure using the proposed ELF1 and Rᵢ₁ methodology successfully mitigated damage concentration, upgrading the structural performance to a "repairable" state (mean GDI = 0.52). Finally, Incremental Dynamic Analysis validated the approach; the redesigned structure satisfied FEMA P695 collapse prevention criteria, achieving an Adjusted Collapse Margin Ratio (ACMR) of 2.10. This study confirms the proposed ELF1 method is a robust and practical design alternative that accurately accounts for soft-storey mechanisms within a simplified linear framework.