Criteria for Earthquake
Resistant Design of Structures

Overview & Philosophy

IS 1893 (Part 1):2016 provides guidelines for designing structures to resist seismic forces. The code does not aim to prevent any damage during an earthquake but ensures that the structure:

The design approach uses the Force Reduction Factor (R) to account for this ductility. Instead of designing for the full inertial force, we design for a reduced force, assuming the structure will yield safely. Cl. 6.1

Seismic Zone Map

India is divided into four seismic zones based on the Modified Mercalli Intensity (MMI) scale and historical seismicity. The Basic Seismic Coefficient (A_b) varies by zone. Fig. 1 / Table 2

Key Terminology

• Fundamental Natural Period (T_a): The time taken for a building to sway back and forth once. Taller buildings have longer periods.
• Response Reduction Factor (R): Represents ductility. Higher R means more ductility allowed, reducing design force. (e.g., Ordinary Moment Frame R≈3, Special Moment Frame R≈5).
• Importance Factor (I): Increases design force for critical buildings like Hospitals, Fire Stations, Power Plants (I = 1.5) vs. Residential (I = 1.0).
• Damping Ratio (ξ): Usually assumed as 5% for RCC structures. Lower for steel (2%).

Design Parameters

The lateral seismic force depends on four main parameters defined in the code. Cl. 6.2

1. Zone Factor (Z): Depends on the location (Zone II to V).
2. Importance Factor (I): Depends on building function (Table 6).
   • Residential, Educational, Office: I = 1.0
   • Hospitals, Fire Stations, Police Stations: I = 1.25 (Note: 2016 update changed some categories)
   • Essential facilities (Power, Telecom), Hazardous Chemical plants: I = 1.5
3. Response Reduction Factor (R): Depends on the frame system (Table 7).
   • Ordinary RC Moment Frame: R = 3.0
   • Special RC Moment Frame: R = 5.0
   • RC Shear Wall: R = 5.0
   • Braced Frames (Steel): R = 3.0–6.0
4. Average Response Acceleration Coefficient (S_a/g): Based on the natural period T and soil type (Site Class).
   • Soft Soil: Longer duration shaking, higher Sa for long periods.
   • Rock/Hard Soil: Short duration, high frequency.

Equivalent Static Analysis Method

For regular, low-to-medium rise buildings, the complex dynamic motion is simplified into a static lateral force applied at the base. Cl. 7.8

Where:

• V_b: Base Shear (Total lateral force at the bottom).
• W: Seismic weight of the building (Dead Load + Appropriate portion of Live Load).
• A_b: Design Horizontal Seismic Coefficient.

The coefficient A_b is calculated as:

Natural Period Approximation (for Regular Buildings):

Where h is the height of the building in meters. d is the base dimension of the building parallel to the force direction.

Once V_b is found, it is distributed vertically along the height:

Response Spectrum Method

Mandatory for irregular buildings, tall buildings (>40m in Zones IV & V), or structures on soft soil. This considers multiple vibration modes. Cl. 7.6

Steps involve:

1. Calculate natural periods (T_n) and mode shapes for the first several modes.
2. Find spectral acceleration (S_a/g) for each T_n.
3. Calculate modal mass participation factor.
4. Combine results using SRSS (Square Root of Sum of Squares) or CQC (Complete Quadratic Combination) method.
5. Minimum Requirement: Include enough modes so that the modal mass participation is at least 90% of the total mass.

P-Delta & Drift Checks

Inter-story drift (relative displacement between floors) must be limited to prevent damage to non-structural elements and avoid P-Delta instability. Cl. 7.11.1

P-Delta Effect: Must be checked if the story drift exceeds 0.02 times the story height or if the stability coefficient exceeds 0.1.

Related Codes

IS 1893 must be read alongside these codes for a complete design solution: