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

Tamanna Akter, Fatema Yasmin

Abstract


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.

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References


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