Criteria for Earthquake Resistant Design of Structures, Part 1: General Provisions and Buildings
2002 Edition

India is divided into four seismic zones based on expected maximum earthquake intensity according to the MSK64 scale: Zone II (low risk, intensity VI or below), Zone III (moderate risk, intensity VII), Zone IV (high risk, intensity VIII), and Zone V (very high risk, intensity IX and above). Each zone is assigned a Zone Factor (Z) that reflects the peak ground acceleration used for design, with values approximately 0.10 for Zone II, 0.16 for Zone III, 0.24 for Zone IV, and 0.36 for Zone V. These factors help define the seismic forces considered in structural design. For critical projects, site-specific seismic hazard assessments are recommended.

IS 1893 Part 1 recommends two principal dynamic analysis approaches: the Time History Method, which involves using actual or simulated earthquake ground motion records with dynamic principles; and the Response Spectrum Method, which employs design response spectra to estimate peak structural responses. After dynamic analysis, the base shear obtained must be compared with that from simplified fundamental period calculations, and if the dynamic analysis yields a lower base shear, response quantities should be scaled accordingly to ensure safety and consistency.

The design base shear (VB) represents the total lateral seismic force at the building’s base and is computed using the formula VB = Ah × W, where Ah is the design horizontal seismic coefficient and W is the seismic weight of the structure. The base shear is then apportioned along the building’s height based on the weight and elevation of each floor, typically with forces at floor i computed as Qi = VB × (Wi × hi^2) / Σ(Wi × hi^2). When dynamic seismic analysis is performed, the base shear from such analysis must be at least equal to that calculated from the fundamental period method; otherwise, the dynamic responses are scaled.

In seismic zones III, IV, and V, foundation designs must avoid differential settlement-prone footing types and ensure interconnection between individual spread footings or pile caps with tie beams, especially in Zones IV and V unless founded directly on rock. The code specifies permissible increases in allowable bearing pressures based on soil and foundation types. Minimum standard penetration test (SPT) N-values are prescribed for various depths and zones, emphasizing avoidance or improvement of liquefiable soils. Isolated footings without ties are prohibited on soft soils with low N-values to prevent seismic-induced failures.

Buildings exhibiting irregular geometry or soft storeys must be designed to enhance lateral strength and stiffness to mitigate seismic vulnerabilities. Soft storeys are identified where lateral stiffness is less than 70% of the storey above or 80% of the average stiffness of three storeys above. Extreme soft storeys, such as stilt levels, have even lower stiffness thresholds. Design strategies include adding shear walls, bracing, and using ductile moment-resisting frames to ensure uniform stiffness distribution and prevent soft storey failure mechanisms. Attention to torsional irregularities and vertical continuity in lateral load-resisting systems is critical for seismic resilience.