India is divided into 4 seismic zones (II to V). Zone factor (Z) is used to calculate design seismic base shear. IS 1893 was revised in 2016 — Zone I was merged into Zone II.
Seismic Zone Factors (IS 1893 Table 3)
Z = Zone Factor = Effective Peak Ground Acceleration (as fraction of g)
| Zone / City | Z Factor | Intensity | Risk / State | Examples |
|---|
| Zone II (Low) | 0.10 | VI (or less) | Low damage risk | South & Central Peninsula |
| Zone III (Moderate) | 0.16 | VII | Moderate damage risk | Mumbai, Kolkata, Chennai, Jaipur |
| Zone IV (Severe) | 0.24 | VIII | High damage risk | Delhi, Patna, Lucknow, most of NE |
| Zone V (Very Severe) | 0.36 | IX (or more) | Very high damage risk | Kashmir, NE India, Kutch, Uttarakhand |
Importance Factor (I) — IS 1893 Table 8
Multiplied with base shear — higher I = more conservative design
| Zone / City | Z Factor | Intensity | Risk / State | Examples |
|---|
| Residential, Commercial, Office | 1.0 | Normal | | Standard buildings |
| Schools, Assembly halls, Cinemas | 1.2 | Important | | Large occupancy |
| Hospitals, Fire stations, Emergency | 1.5 | Critical | | Post-earthquake functionality |
| Nuclear, Dams, Large bridges | 1.5–2.0 | Lifeline | | Special structures — as per specific IS codes |
Notes
• Base Shear: VB = Z/2 × I/R × Sa/g × W. This is the fundamental seismic design equation.
• Sa/g = spectral acceleration coefficient — depends on time period (T) and soil type (I, II, III)
• Soil Type I = Rock/Hard, Type II = Medium, Type III = Soft. Soft soil amplifies seismic waves.
• IS 1893:2016 removed Zone I — all of India is now Zone II or higher
• In Zone IV & V: ductile detailing per IS 13920 is MANDATORY for all RC frames
• For buildings < 15m in Zone II: seismic analysis may not be critical — but IS 1893 compliance is still required
• Seismic weight includes: full DL + 25% LL (if LL ≤ 3 kN/m²) or 50% LL (if LL > 3 kN/m²)
• Time period for RC frame: T = 0.075 × h^0.75 (h = height in metres)
• Time period for RC shear wall: T = 0.075 × h^0.75 × √(Aw/Ab) — modified by wall area