IS Standards Series · Part 6 of 9

Seismic Design per IS 1893:2016

Earthquake-resistant design per IS 1893:2016 Part 1 — seismic zones, design base shear V_B = A_h·W, design spectrum, response reduction factor R, lateral force distribution, and IS 13920 ductile detailing requirements.

Seismic Zones and Zone Factors
Design Spectrum and A_h
Response Reduction Factor R
Design Base Shear
Lateral Force Distribution
IS 13920 Ductile Detailing
Comparison with Other Codes

1. Seismic Zones and Zone Factors

IS 1893:2016 divides India into four seismic zones (II, III, IV, V) based on peak ground acceleration. Zone V is the highest hazard zone, covering the northeast states, Jammu & Kashmir, Himachal Pradesh, and Uttarakhand.

Zone	Zone Factor Z	PGA (g)	Representative Cities
Zone II (Low)	0.10	0.05g	Hyderabad, Bengaluru, Chennai, Pune
Zone III (Moderate)	0.16	0.08g	Mumbai, Kolkata, Jaipur, Bhopal
Zone IV (High)	0.24	0.12g	Delhi, Lucknow, Chandigarh, Srinagar
Zone V (Very High)	0.36	0.18g	Guwahati, Shimla, Shillong, Imphal

2016 revision: IS 1893:2016 replaced the 2002 edition. Zone I was removed (all previously Zone I areas moved to Zone II). The zone factor Z is now expressed as a fraction of PGA for 2% probability of exceedance in 50 years (return period 2475 years), down from the earlier 10% in 50 years (475 years).

2. Design Spectrum and Design Seismic Coefficient A_h

IS 1893:2016 Cl.6.4.2 — Design Horizontal Seismic Coefficient

IS 1893A_h = (Z/2) · (S_a/g) · (I/R)

Z = zone factor (Table 3)
S_a/g = spectral acceleration coefficient (design spectrum)
I = importance factor
R = response reduction factor

Design Spectrum S_a/g — IS 1893 Cl.6.4.2

Period T (s)	Rock / Hard soil (Type I)	Medium soil (Type II)	Soft soil (Type III)
0 ≤ T ≤ 0.10	1 + 15T	1 + 15T	1 + 15T
T = 0.10	2.50	2.50	2.50 (plateau)
0.10 < T ≤ 0.40	2.50	2.50	2.50
0.40 < T ≤ 0.55	2.50	2.50	2.50
0.55 < T ≤ 0.67	—	—	2.50
T > 0.40 (Type I)	1.0/T	—	—
T > 0.55 (Type II)	—	1.36/T	—
T > 0.67 (Type III)	—	—	1.67/T
T > 4.0 (all types)	0.25/T²	0.36/T²	0.42/T²

Soil Types

Type I (Rock/Hard Soil): N > 30 (SPT), V_s30 > 360 m/s
Type II (Medium Soil): 10 ≤ N ≤ 30 (SPT), 180 ≤ V_s30 ≤ 360 m/s
Type III (Soft Soil): N < 10 (SPT) or liquefiable soil, V_s30 < 180 m/s

Fundamental Period T — IS 1893 Cl.7.6

IS 1893 Cl.7.6 — Empirical Period

RC frameT = 0.075·H^0.75s

Steel frameT = 0.085·H^0.75s

Shear wall / braced frameT = 0.09·H / √ds

H = total height above base (m); d = base dimension in direction of shaking (m)

3. Response Reduction Factor R and Importance Factor I

The response reduction factor R accounts for ductility and over-strength of the structural system. Higher R implies better ductile performance; IS 1893 Table 9 specifies R for each system type.

Structural System	R
RC ordinary moment resisting frame (OMRF)	3.0
RC special moment resisting frame (SMRF) — IS 13920	5.0
RC shear wall with SMRF	5.0
RC shear wall without moment frame	3.0
Steel ordinary moment frame	3.0
Steel special moment frame	5.0
Unreinforced masonry	1.5

Importance Factor I

Occupancy Category	I
Normal residential / commercial / industrial buildings	1.0
Schools, colleges, assembly halls	1.2
Hospitals, emergency services, power stations, dams	1.5
Nuclear power plants (IS 1893 Part 4)	Special provisions

4. Design Base Shear — IS 1893 Cl.7.5

IS 1893 Cl.7.5.3 — Total Design Lateral Force

IS 1893V_B = A_h · WkN

W = seismic weight of the structure (DL + fraction of LL per Cl.7.3)

Worked Example

5-storey RC SMRF, Delhi (Zone IV, Z = 0.24), Type II soil, I = 1.0, R = 5.0, H = 15 m, W = 10,000 kN:

T = 0.075 × 15^0.75 = 0.075 × 7.62 = 0.57 s
S_a/g for Type II soil at T = 0.57 s: 1.36/0.57 = 2.39
A_h = (0.24/2) × 2.39 × (1.0/5.0) = 0.12 × 2.39 × 0.20 = 0.057
V_B = 0.057 × 10,000 = 575 kN

5. Lateral Force Distribution — IS 1893 Cl.7.6.3

IS 1893 Cl.7.6.3a — Floor-Level Force Distribution

IS 1893Q_i = V_B · (W_i·h_i²) / Σ(W_j·h_j²)kN

W_i = seismic weight at floor i; h_i = height of floor i above base

This parabolic distribution concentrates more force at upper floors compared to a linear (triangular) distribution, which is conservative for the first mode response of regular buildings. IS 1893 Cl.7.6.3b adds an additional force of 0.004·V_B at the roof for torsion effects.

6. IS 13920:2016 Ductile Detailing

IS 13920 is mandatory for RC structures in seismic zones III, IV, and V. It prescribes ductility requirements for beams, columns, and shear walls.

Beam Detailing (IS 13920 Cl.6)

Width ≥ 200 mm; depth/width ratio ≤ 4
Minimum 2 bars top and bottom throughout (A_st,min = 0.24√f_ck/f_y each face)
Confinement hoops in plastic hinge zones: spacing ≤ min(d/4, 8φ_bar, 100 mm) over 2d from face of column
Positive moment capacity at face ≥ 50% of negative moment capacity

Column Detailing (IS 13920 Cl.7)

Minimum dimension: 300 mm (Zone III–V SMRF)
Reinforcement: 1%–4% A_g
Special confining reinforcement over L_o = max(D, l_c/6, 450 mm) at each end
Confining hoop spacing: min(B/4, 75 mm) within L_o; max 100 mm outside

Shear Wall Detailing (IS 13920 Cl.9)

Minimum reinforcement: 0.25% in both directions (each face)
Two curtains of reinforcement required if thickness > 200 mm or if shear stress > 0.25√f_ck
Boundary elements required when extreme fibre stress > 0.2·f_ck

Shear Wall Calculator → Seismic Design Basics → Next: Wind Loads →

7. Comparison with Other Codes

Feature	IS 1893:2016	ASCE 7-22 (US)	Eurocode 8 (EU)
Hazard basis	Z factor (zoned map)	S_S, S₁ (mapped spectral)	a_g (peak ground acc.)
Return period	2475 yr (2016 ed.)	2475 yr (MCER)	475 yr (default)
Ductility classes	OMRF, SMRF	IMF, SMF, OMF	DCL, DCM, DCH
Base shear	A_h·W	C_s·W (seismic)	S_d(T₁)·m·λ
Response reduction	R (1.5–5.0)	R (3–8+)	q-factor (1.5–6.5)

Preliminary design only. Seismic design requires site-specific hazard assessment and project-specific review by a licensed structural engineer familiar with IS 1893:2016 and IS 13920:2016.

← Previous Steel Design per IS 800 Next → Wind Loads per IS 875 Part 3

Seismic Design per IS 1893:2016

Contents

1. Seismic Zones and Zone Factors

2. Design Spectrum and Design Seismic Coefficient Ah

Design Spectrum Sa/g — IS 1893 Cl.6.4.2

Soil Types

Fundamental Period T — IS 1893 Cl.7.6

3. Response Reduction Factor R and Importance Factor I

Importance Factor I

4. Design Base Shear — IS 1893 Cl.7.5

Worked Example

5. Lateral Force Distribution — IS 1893 Cl.7.6.3

6. IS 13920:2016 Ductile Detailing

Beam Detailing (IS 13920 Cl.6)

Column Detailing (IS 13920 Cl.7)

Shear Wall Detailing (IS 13920 Cl.9)

7. Comparison with Other Codes

Related Tools & Guides

2. Design Spectrum and Design Seismic Coefficient A_h

Design Spectrum S_a/g — IS 1893 Cl.6.4.2