Code Provisions for Soft-Storey Irregularity in Multi-Storey Buildings: A Comparative Review of International Seismic Design Standards

Open ground and intermediate storeys, introduced to accommodate parking, retail, or lobby space, create an abrupt reduction in lateral stiffness that classifies a building as vertically irregular with a “soft storey.” This irregularity has been repeatedly implicated in catastrophic structural collapses during the 1923 Great Kanto, 1978 Miyagi-ken- oki, 1995 Hyogoken-Nanbu, 1999 Izmit, 2005 Kashmir, and 2011 Van earthquakes, among others. In response, national seismic codes have adopted markedly different quantitative criteria — based variously on inter-storey drift, storey stiffness, or storey rigidity ratios — to mitigate this irregularity. This paper synthesizes the soft-storey provisions of six major seismic design frameworks (Turkish Earthquake Code TSC 2007, Indian Standard IS 1893 Part 1:2002, New Zealand Standard NZS 4203:1992, the Japanese Building Standard Law, Eurocode 8, and ASCE 7, the last two informing the Bangladesh National Building Code) and re-examines two independent parametric case studies that applied these criteria to identical reinforced- concrete frame models under both equivalent-static and time-history analysis. The comparative synthesis shows that the Japanese lateral-stiffness-ratio method (Rs < 0.6) consistently identifies soft-storey irregularity at markedly smaller first-storey heights than the Turkish, Indian, and American criteria, indicating that it is the most conservative of the reviewed provisions, while Eurocode 8 offers only a qualitative amplification rule with no explicit numerical threshold. The case-study re-analysis further reveals that dynamic (time-history) analysis can detect soft-storey irregularity that static analysis misses, particularly under the Japanese criterion, underscoring the limitation of relying solely on equivalent-static procedures. These findings offer a consolidated reference for code-drafting bodies and practicing engineers, particularly in seismically active, rapidly urbanizing regions such as Bangladesh, where current provisions closely mirror the comparatively permissive ASCE 7/IS 1893 approach.