Seismic Zone VI, in IS 1893 (Part 1): 2025 — A Critical Review and Design Implications

The publication of IS 1893 (Part 1): 2025 represents a significant update to Indian seismic design practices, formally recognizing Seismic Zone VI as the highest hazard category. With a zone factor of Z = 0.75, this new zone substantially increases the reference design seismic demand beyond the previous maximum of Zone V. This change requires designers to consider not just higher force levels, but also more fundamental aspects of safety, such as system integrity, redundancy, ductile response, and reliable load transfer mechanisms. This paper critically reviews the code evolution leading to Zone VI and examines its design rationale and implications. It demonstrates the practical impact through a numerical comparison of a typical mid-rise reinforced-concrete building designed for both Zone V and Zone VI conditions. The study synthesizes the main structural consequences for configuration control, torsional behaviour, soft-storey vulnerability, diaphragm and collector design, and foundation-soil interaction. It consolidates these findings into a practical checklist for senior designers. The paper concludes by highlighting a shift from implicit life-safety goals to explicit collapse-prevention objectives and outlines directions for future research and practice.