Criteria for Earthquake Resistant Design
of Structures — General Provisions and Buildings
The primary Indian Standard for seismic design of buildings. Defines seismic zones, design spectrum, base shear calculation, equivalent static and dynamic analysis methods, and structural irregularity provisions.
📋 2016 Revision: This edition introduced significant changes — revised zone map, new soil classification, updated response spectra, revised importance factors, and mandatory dynamic analysis criteria. Always use the 2016 edition for new designs.
IS 1893 (Part 1):2016 covers the earthquake resistant design of buildings and general structures. It is Part 1 of a six-part series, each addressing a specific structure type.
| Part | Scope |
|---|---|
| Part 1 | General provisions and buildings (this document) |
| Part 2 | Liquid-retaining tanks — elevated and ground-supported |
| Part 3 | Bridges and retaining walls |
| Part 4 | Industrial structures including stack-like structures |
| Part 5 | Dams and embankments |
| Part 6 | Tunnels and underground structures |
Scope Limitation IS 1893 Part 1 applies to buildings with regular and irregular configurations. For structures with special features (base isolation, dampers, etc.), specialist analysis beyond the scope of this code is required.
India is divided into four seismic zones (II to V) based on the expected intensity of ground shaking. The zone factor Z represents the peak ground acceleration (PGA) as a fraction of g for the design basis earthquake (DBE = 50% of MCE).
| Zone | Zone Factor Z | Seismic Intensity | Representative Cities |
|---|---|---|---|
| Zone II | 0.10 | Low | Hyderabad, Bengaluru, Chennai, Pune, Bhopal |
| Zone III | 0.16 | Moderate | Mumbai, Kolkata, Ahmedabad, Jaipur, Nagpur |
| Zone IV | 0.24 | Severe | Delhi, Jammu, Haridwar, Dehradun, Patna |
| Zone V | 0.36 | Very Severe | Srinagar, Shimla, Guwahati, Imphal, Andaman |
Zone I Removed in 2002 The earlier Zone I (very low seismicity) was merged into Zone II in the 2002 edition. The 2016 edition retains this four-zone classification. Always verify the zone of a specific site from the official IS 1893 seismic zone map, not from general city listings.
The design acceleration spectrum Sa/g is defined for 5% damping and three soil types. The spectral shape depends on the fundamental natural period T of the structure.
| Period Range | Hard Soil (Type I) | Medium Soil (Type II) | Soft Soil (Type III) |
|---|---|---|---|
| T = 0 to 0.10 s | 1 + 15T | 1 + 15T | 1 + 15T |
| T = 0.10 to 0.40 s | 2.50 | 2.50 | 2.50 |
| T = 0.40 to 0.55 s | 1.00/T | 2.50 | 2.50 |
| T = 0.55 to 0.67 s | 1.00/T | 1.36/T | 2.50 |
| T = 0.67 to 4.00 s | 1.00/T | 1.36/T | 1.67/T |
| T > 4.00 s | 0.25 | 0.34 | 0.42 |
Damping Correction The above values are for 5% damping. For other damping ratios, multiply Sa/g by the factor: (10/(5+ξ))^0.5, where ξ is the damping ratio in percent. For RCC structures, 5% is standard; for steel, 2% is often used.
The seismic weight W at each floor is the dead load plus a fraction of the imposed load. The fraction depends on the magnitude of the imposed load.
The total design lateral force (base shear) Vb is calculated using the design horizontal seismic coefficient Ah and the total seismic weight W of the building.
| Structure / Occupancy | Importance Factor I |
|---|---|
| Important service structures (hospitals, fire stations, power plants) | 1.5 |
| Residential, commercial, industrial buildings | 1.0 |
| Monuments, heritage structures | 1.0 |
| Structural System | Response Reduction Factor R |
|---|---|
| Ordinary Moment Resisting Frame (OMRF) — RC | 3.0 |
| Special Moment Resisting Frame (SMRF) — RC | 5.0 |
| RC Shear Wall with OMRF | 3.0 |
| RC Shear Wall with SMRF | 4.0 |
| Steel OMRF | 3.0 |
| Steel SMRF | 5.0 |
| Unreinforced masonry | 1.5 |
Ah Minimum Value The value of Ah shall not be less than Z/2 regardless of the I/R ratio. This ensures a minimum base shear even for highly ductile systems (Cl. 6.4.2).
The base shear Vb is distributed over the height of the building as lateral forces Qi at each floor. The distribution is parabolic — proportional to the product of floor weight and the square of its height above the base.
Applicability of ESA Equivalent Static Analysis (ESA) is permitted for regular buildings up to 15 m height in Zones II and III, and up to 10 m in Zones IV and V. For taller or irregular buildings, dynamic analysis is mandatory (Cl. 7.7).
Empirical Period Formula For RC moment-resisting frames: Ta = 0.075 × h^0.75. For steel frames: Ta = 0.085 × h^0.75. For RC shear wall buildings: Ta = 0.075 × h^0.75 / √Aw. (Cl. 7.6.2)
Dynamic analysis using the Response Spectrum Method is mandatory for buildings exceeding the ESA height limits and for all irregular buildings in Zones III, IV, and V.
| Requirement | Value / Rule | Clause |
|---|---|---|
| Minimum modes to consider | Modes contributing ≥ 90% of total mass | Cl. 7.7.5.2 |
| Minimum base shear (dynamic) | Not less than Vb from ESA; scale up if less | Cl. 7.7.2 |
| Accidental torsion | ±5% of floor dimension perpendicular to force direction | Cl. 7.9.2 |
Scaling of Dynamic Results If the base shear from dynamic analysis is less than that from the equivalent static method, all response quantities (forces, moments, drifts) must be scaled up proportionally (Cl. 7.7.2).
IS 1893:2016 defines plan irregularities and vertical irregularities that trigger mandatory dynamic analysis and additional design requirements.
| Plan Irregularity Type | Definition |
|---|---|
| Torsional | Maximum storey drift > 1.2 times average drift at that storey |
| Re-entrant Corner | Plan projection > 15% of plan dimension in that direction |
| Diaphragm Discontinuity | Abrupt variation in stiffness; cut-outs > 50% of gross area |
| Out-of-plane Offset | Lateral force-resisting elements offset from their position above |
| Non-parallel Systems | Lateral force-resisting elements not parallel to major axes |
| Vertical Irregularity Type | Definition |
|---|---|
| Soft Storey | Lateral stiffness < 70% of adjacent storey or < 80% of average of 3 storeys above |
| Extreme Soft Storey | Lateral stiffness < 60% of adjacent storey or < 70% of average of 3 storeys above |
| Mass Irregularity | Seismic weight of any storey > 200% of adjacent storey weight |
| Vertical Geometric | Horizontal dimension of LFRS > 130% of that in adjacent storey |
| In-plane Discontinuity | In-plane offset of LFRS greater than its length |
| Weak Storey | Lateral strength < 80% of adjacent storey above |
Soft Storey Prohibition Buildings with extreme soft storey (open ground floor for parking) are highly vulnerable. IS 1893:2016 requires that such buildings be designed with the soft storey stiffness increased to meet the code limits, or the columns/walls of the soft storey be designed for 2.5 times the seismic forces (Cl. 7.1).
| Topic | IS 1893 : 2002 | IS 1893 : 2016 |
|---|---|---|
| Seismic zone map | Based on 1984 data | Updated with new seismological data; some cities reclassified |
| Soil classification | 3 types (I, II, III) | Retained but with clearer N-value and Vs30 criteria |
| Response spectrum | Single plateau at 2.5 | Extended plateau for soft soils; revised long-period branch |
| Importance factor I | 1.0 / 1.5 | Revised table with more occupancy categories |
| Dynamic analysis trigger | Height > 40 m (RC), > 90 m (steel) | Stricter: height > 15 m (Zones II–III), > 10 m (Zones IV–V) for irregular buildings |
| Torsion provisions | Basic provisions | Detailed accidental eccentricity and torsional amplification |
| Diaphragm design | Not explicitly covered | New provisions for diaphragm forces and connections |
Ductile Detailing IS 1893 specifies the forces; ductile detailing of RC members to resist these forces is covered in IS 13920:2016. Both codes must be used together for seismic design of RC buildings in Zones III, IV, and V.