As per IS 1893 (Part 1 & Part 5) : 2025 | Seventh Revision
| Cl. 6.2.2, 6.2.3, 8.2 | Table 1, 3, 4
AHD = ANH(T) × Z × I / R
AVD = (2/3) × AHD
VBD,min = 0.625 × Z × I / R
Key changes from IS 1893:2016 →Zone VI added (Himalayas/NE India, Z=0.50–0.75) |
Z now depends on Return Period (not fixed per zone) |
Importance Factor I=1.0 uniform; hazard via TR selection |
ANH replaces Sa/g with updated long-period shape (1/T0.9) |
OMRF banned in Zones IV, V, VI |
Site classes A–D (Vs-based) replace old Type I–III |
Spectrum extended to 10 s
1. Structure Category & Return Period (Table 1)
Structure Category
Design Type
Design Return Period (TR)
Importance Factor (I)
Uniform for all categories (TR governs hazard)
2. Seismic Zone & Zone Factor (Table 3)
Earthquake Zone (Cl. 6.2.1)
Z = —
⚠️ OMRF is prohibited in Zones IV, V and VI as per IS 1893 (Part 5):2025. Only SMRF and ductile systems are permitted.
Zone Factor (Z) for TR
from Table 3, IS 1893:2025
Elastic Force Reduction Factor R (Table, Part 5)
R = 5.0
Damping Ratio (Table 7)
η=1.00
Site Class (Table 4 — Vs30 based)
Site C
Site Class E requires site-specific hazard assessment per IS 1893:2025 and cannot be calculated with this general formula.
3. Fundamental Time Period (T) — Cl. 7.6, Part 5
Structure Type for Ta
Building Height (h)
m
Base Dimension along EQ direction (d)
m
Codal Time Period Ta
sec
Max T from analysis must not exceed 1.25 × Ta for regular buildings (Cl. 7.6.2, Part 5).
Time Period T (from 3D analysis / software)
sec
For normal structures: T limited to 0.6 s; important: 2 s; critical: 4 s (conservative per Nepalese/draft guidance). Check compliance with 1.25×Ta cap.
4. Results
Earthquake Zone
Design Return Period (TR)
years
Zone Factor (Z)
Importance Factor (I)
Elastic Force Reduction Factor (R)
Damping Multiplier (η)
Design Time Period (T)
sec
Normalised Spectral Acceleration ANH(T) — selected site + damping
Design Horizontal Seismic Coefficient
—
AHD = ANH × Z × I / R
Design Vertical Seismic Coefficient
—
AVD = (2/3) × AHD
Design Horizontal Coeff. (AHD)
Design Vertical Coeff. (AVD)
Minimum AHD (Cl. — min base shear):
AHD,min = 0.625 × Z × I / R = —
| Governing AHD = —
AHD at same T — All Site Classes:
AHD — Site A/B (Hard Rock/Rock)
AHD — Site C (Dense/Medium Soil)
AHD — Site D (Stiff Soil)
5. Response Spectrum — ANH vs T (IS 1893:2025)
Curves shown at selected damping. Vertical marker = design T. Note the modified long-period shape (1/T0.9) compared to 2016 code (1/T).
6. AHD vs T — All Site Classes
AHD for selected Zone, TR, I, R and damping. Vertical marker = design T.
7. Zone Factor Table (Table 3, IS 1893:2025) — for reference
Zone
TR=73 yr
TR=225 yr
TR=475 yr
TR=975 yr
TR=2475 yr
TR=4975 yr
TR=9975 yr
II
0.037
0.050
0.060
0.075
0.100
0.113
0.150
III
0.063
0.085
0.100
0.125
0.167
0.188
0.250
IV
0.140
0.175
0.210
0.233
0.280
0.292
0.350
V
0.200
0.250
0.300
0.333
0.400
0.417
0.500
VI NEW
0.300
0.375
0.500
0.600
0.750
0.938
1.125
For Strength Design use: Normal=475yr, Important=975yr, Critical=2475yr, Special=4975yr. For Serviceability Check use: Normal=73yr, Important=225yr, Critical=475yr, Special=975yr.
Reference — IS 1893 : 2025 Key Clauses
Part 1, Cl. 6.2.1: Earthquake zones II–VI |
Cl. 6.2.2.2: Zone factor Z from Table 3 per return period |
Cl. 6.2.3.2: ANH spectral acceleration |
Table 4: Site classes A–E (Vs30 based) |
Part 5, Cl. 8.2: AHD = ANH × Z × I / R |
Cl. 8.2.3: Elastic Force Reduction Factor R (renamed from 2016) |
Part 5 Table: OMRF prohibited in Zones IV, V, VI |
Min base shear: VBD,min = 0.625 × Z × I / R |
Spectrum: Extended to T = 10 s
