Criteria for Earthquake Resistant Design of Structures, Part 1: General Provisions and Buildings

IS 1893 (Part 1) - Scope: Key Specifications & Tables

Scope:
IS 1893 (Part 1) covers criteria for seismic design of buildings and structures considering seismic zones, soil types, and structural importance.

1. Design Spectrum (Clause 6.4)

• Defines response spectrum curves for rock/soil sites based on natural periods and damping.
• Used to calculate spectral acceleration ( S_a/g ) for seismic forces.

2. Zone Factor, ( Z ) (Clause 6.4.2)

3. Allowable Bearing Pressure Increase (Clause 6.3.5.2, Table 1)

• N = Standard Penetration Test (SPT) value at foundation level.
• Values from IS 6403 or IS 1888 for allowable bearing pressure.

4. Rounding Off (General Rule)

• Follow IS 2:1960 for rounding numerical values.
• Retain significant places as per specified values.

Summary Diagram of Soil Type vs Foundation Permissible Increase

graph TD
  A[Foundation Type] --> B[Soil Type I (N >

IS 1893 Part 1: Seismic Zoning & Zone Factors

1. Zone Factor (Z) Definition (Clause 3.33)

• Represents the peak ground acceleration (PGA) as a fraction of gravity (g).
• Used to derive the design response spectrum.
• Based on maximum considered earthquake (MCE) risk in the seismic zone.

2. Seismic Zones & Zone Factors (Table 2, Clause 6.4.2)

• Zones correspond to earthquake intensity (MSK scale):
  • Zone II: Intensity VI or less
  • Zone III: Intensity VII
  • Zone IV: Intensity VIII
  • Zone V: Intensity IX and above

3. Application Notes

• Zone factors are approximate estimates for design PGA.
• Important projects require detailed seismic hazard analysis.
• Importance factor (I) and response reduction factor (R) are used alongside Z in design.
• Refer to IS 1893 for natural period calculations and damping values.

4. Seismic Zoning Map

• Classifies India into zones II to V based on seismic risk.
• Helps assign Zone Factor (Z) for design.

Summary Formula for Design Base Shear:

[ V_b = Z \times I \times R^{-1} \times W ]

Where:

• ( V_b ) = design base shear
• ( Z ) = zone factor (from table above)
• ( I ) = importance factor
• ( R ) = response reduction factor
• ( W ) = seismic weight of the structure

flowchart LR
    A[Seismic Zone] --> B[Zone Factor (Z)]
    B --> C[Design Base Shear Calculation]
    C --> D[Structural Design]
    A --> E[Seismic Intensity (MSK)]
    E --> B

References: IS 1893 (Part 1): 2002

IS 1893 Part 1: Basic Assumptions & Definitions - Key Points

1. Basic Assumptions (Clause 6.2)

• Earthquake ground motions are impulsive, irregular, and short-duration; steady-state resonance is rare.
• Earthquake unlikely to coincide with maximum wind, flood, or sea waves.
• Use static elastic modulus values for materials unless dynamic values are available (refer IS 456, IS 1343, IS 800).

2. Key Definitions (Clause 3.1 & 4.25)

• Weak Storey: Lateral strength < 80% of storey above.
• Seismic Weight (W): Total weight considered for seismic analysis.
• Design Horizontal Seismic Coefficient (Ah): Base for lateral force calculation.
• Fundamental Natural Period (T or T1): Time period of the first mode of vibration.
• Base Shear (B or Vb): Total lateral force at the base due to earthquake.

3. Important Symbols

4. Fundamental Formula for Base Shear (Clause 7.6)

[ B = A_h \times W ]

Where:

• ( A_h ) = design horizontal acceleration coefficient depending on ( T ) (see Clause 6.4.2)
• ( W ) = seismic weight of the building

5. Approximate Fundamental Period (Ta) for RC Frame Buildings

[ T_a = 0.075 \times h^{0.75} ]

• ( h ) = height of building in meters

6. Seismic Base Shear Calculation Summary

- Calculate \( T_a \) using building height.
- Determine \( A

IS 1893 (Part 1): Classification of Structures & Importance Factors

Importance Factor (I) — Clause 6.4.2, Table 6

Notes:

• Design engineers may adopt I > 1.5 based on economy, strategy, or special considerations.
• Temporary structures (excavations, scaffolding) are exempt.
• Buildings not covered explicitly may be assigned higher I values.

Definition (Clause 3.14):

Importance Factor (I) adjusts design seismic forces based on the structure's function, hazard consequences, post-earthquake use, historic or economic importance.

Use in Design:

Design seismic force, ( F ), is calculated as:

[ F = I \times S_a \times W ]

Where:

• ( I ) = Importance factor
• ( S_a ) = Spectral acceleration coefficient
• ( W ) = Seismic weight of the structure

flowchart LR
    A[Structure Classification] --> B{Important Service/Community?}
    B -- Yes --> C[Importance Factor = 1.5]
    B -- No --> D[Importance Factor = 1.0]
    C & D --> E[Calculate Design Seismic Force F = I × Sa × W]

This classification ensures safety and functional continuity of critical structures during earthquakes.

IS 1893 (Part 1) – General Design Considerations: Key Formulas & Specs

1. Design Lateral Force (Clause 7.5)

The base shear, ( V_b ), is calculated as:

[ V_b = A_h \times W ]

Where:

• ( A_h ) = Design horizontal seismic coefficient
• ( W ) = Seismic weight of the structure

2. Distribution of Design Force (Clause 7.7)

Lateral forces at floor level ( i ), ( F_i ), are distributed as:

[ F_i = \frac{W_i h_i}{\sum W_i h_i} \times V_b ]

Where:

• ( W_i ) = Weight at floor ( i )
• ( h_i ) = Height of floor ( i ) from base

3. Design Spectrum (Clause 6.4)

The design response spectrum ( S_a/g ) is defined by:

Where ( T_1, T_2, T_3 ) depend on soil and zone factors.

4. Design Imposed Loads for Earthquake Force (Clause 7.3)

• Use full dead load plus appropriate portion of imposed load (usually 25-50%) as seismic weight ( W ).
• Include self-weight of structural and non-structural components attached to the structure.

flowchart TD
    A[Calculate Seismic Weight (W)] --> B[Determine Design Horizontal Coefficient (A_h)]
    B --> C[Compute Base Shear V_b = A_h * W]
   

IS 1893 (Part 1) — Seismic Loads & Load Combinations

Key Load Combinations (Clause 6.3.1 & 6.3.3)

When considering earthquake forces:

• Basic Load Combination:

  [ 1.5(DL + IL) \quad \text{or} \quad 1.2DL + 1.2EL + 0.5IL ]

• For simultaneous horizontal and vertical seismic forces (Clause 6.3.3):

  [ 1.2DL \pm 1.0EL + IL ]

  where:

  • DL = Dead Load
  • IL = Imposed Load
  • EL = Earthquake Load (Response Quantity)

Load Factors for Plastic Design of Steel Structures (6.3.1.1)

• Plastic design load combinations include:

  Combination	Formula
  1	(1.5(DL + IL))
  2	(1.2DL + 1.2EL + 0.5IL)
  3	(1.2DL + 1.2EL - 0.5IL) (if applicable)

Design Imposed Loads (Clause 7.3)

• Use imposed loads as per IS 875 (Part 2) for calculating earthquake forces.
• Imposed loads affect the seismic weight and thus the seismic force.

Summary Table of Load Combinations:

graph TD
    A[Dead Load (DL)]
    B[Imposed Load (IL)]
    C[Earthquake Load (EL)]
    D[Load Combinations]
    A --> D
    B --> D
    C

IS 1893 (Part 1) - Key Formulas and Specifications for Analysis & Design

1. Design Spectrum (Clause 6.4)

• Defines the elastic response spectrum for seismic design.
• Spectral acceleration, ( S_a ), is given as a function of time period ( T ) and soil type.
• Typical formula for spectral acceleration:

[ S_a = \begin{cases} 0.36 \times a_g & T \leq T_1 \ \frac{0.36 \times a_g \times T_1}{T} & T_1 \leq T \leq T_2 \ 0.12 \times a_g & T \geq T_2 \end{cases} ]

Where:

• ( a_g ) = design horizontal seismic coefficient
• ( T_1 ), ( T_2 ) = characteristic periods depending on soil type

2. Distribution of Design Force (Clause 7.7)

• Lateral seismic force ( F_x ) at floor level ( x ):

[ F_x = \frac{W_x h_x^k}{\sum W_i h_i^k} V ]

Where:

• ( W_x ) = seismic weight at floor ( x )
• ( h_x ) = height of floor ( x ) from base
• ( k ) = exponent (usually 1 to 2, commonly 1.5)
• ( V ) = total design lateral force

3. Analysis of Building Subjected to Design Forces (Clause 7.8.4.3)

• Use static equivalent lateral force method.
• Consider forces as static horizontal forces applied at floor levels.
• Analyze using accepted mechanics principles (e.g., lateral load analysis, shear, bending moments).

4. Simplified Formulae for Dams (Concrete/Masonry)

• Modified clauses provide dynamic force formulas based on seismic coefficient and dam geometry.
• Forces are often expressed as a percentage of dam weight, adjusted for hydrodynamic effects.

Summary Table: Typical Parameters for Design Spectrum

IS 1893 (Part 1) - Dynamic Analysis Methods Key Points

1. Dynamic Analysis Types (Clause 7.8.2)

• Time History Method (Clause 7.8.3):
  • Uses actual ground motion records.
  • Requires accepted dynamic principles.
• Response Spectrum Method:
  • Uses response spectra derived from ground motion.

2. Base Shear Comparison (Clause 7.8.2)

• Calculate base shear (V_B) from dynamic analysis.
• Calculate base shear (V_{B}) using fundamental period (T_a) (Clause 7.6).
• If (V_B < V_{B}), multiply all response quantities by (\frac{V_{B}}{V_B}).

3. Free Vibration Analysis (Clause 7.8.4.1)

• Perform undamped free vibration analysis to find:
  • Natural periods (T).
  • Mode shapes ({\phi}).
• Use appropriate masses and elastic stiffness.

Key Formula for Base Shear Scaling:

[ \text{Scaled Response} = \text{Response from Dynamic Analysis} \times \frac{V_{B}}{V_B} ]

Summary Table:

flowchart TD
    A[Start Dynamic Analysis] --> B{Choose Method}
    B -->|Time History| C[Use Ground Motion Records]
    B -->|Response Spectrum| D[Use Response Spectrum]
    C --> E[Calculate Base Shear \(V_B\)]
    D --> E
    E --> F[Calculate \(V_B\) using \(T_a\)]
    F --> G{Is \(V_B < V_B\)?}
    G -->|Yes| H[Scale Responses by \(\frac{V_B}{V_B}\)]
   

IS 1893 (Part 1) - Design of Foundations & Soil Interaction: Key Points

1. Increase in Allowable Bearing Pressure (Clause 6.3.5.2, Table 1)

• N-value: Standard penetration test value at foundation level.
• Allowable bearing pressure per IS 6403 or IS 1888.
• Total increase in bearing pressure including seismic forces must not exceed above limits.

2. Minimum Recommended N-values by Seismic Zone & Depth

3. Important Specifications

• Avoid sites with loose sands (SP) having N < 15 (Zones III-V) or N < 10 (Zone II) due to liquefaction risk.
• Use deep piles to reach non-liquefiable strata or improve soil by compaction/stabilization.
• Isolated footings without tie beams not permitted on soft soils (N < 10).
• Piles should be designed for lateral loads ignoring soil lateral resistance in liquefiable layers.

4. Design Spectrum & Zone Factor (For seismic force calculation)

| Seismic Zone | II | III | IV

IS 1893 (Part 1) – Design for Torsion and Eccentricity

Key Concepts & Formulas:

• Design Eccentricity (e_di) at floor i accounts for torsion due to offset between Centre of Mass (C.M.) and Centre of Rigidity (C.R.).

• Clause 7.9.2 defines design eccentricity ( e_a ) as:

[ e_a = \max \left( e, , 0.05b \right) ]

where:

• ( e ) = static eccentricity at floor i (distance between C.M. and C.R.)

• ( b ) = floor plan dimension perpendicular to force direction

• ( 0.05b ) = minimum eccentricity to account for torsional effects (5% of floor dimension)

• Shear forces on lateral force resisting elements must be increased to include torsional moments caused by ( e_a ). Negative torsional shear can be neglected.

Design Steps:

1. Calculate static eccentricity ( e ).
2. Determine minimum eccentricity ( 0.05b ).
3. Use the larger of the two as design eccentricity ( e_a ).
4. Apply lateral forces at the displaced C.M. to create torsion.
5. Calculate torsional moments and corresponding shear forces on elements.

Summary Table:

flowchart LR
    A[Calculate Static Eccentricity (e)] --> B[Calculate 0.05b]
    B --> C[Select max(e, 0.05b) as Design Eccentricity (e_a)]
    C --> D[Apply

IS 1893 (Part 1) - Deformation and Drift Limits

1. Storey Drift Limit (Clause 7.11.1)

• Maximum storey drift due to design lateral force (partial factor = 1.0):

  [ \delta_{max} \leq 0.004 \times h ]

  where,
  (\delta_{max}) = maximum storey drift,
  (h) = storey height.

• No drift limit for single-storey buildings designed to accommodate storey drift.

2. Deformation Compatibility (Clause 7.11.2)

• For seismic zones IV & V, non-seismic members must sustain vertical load under moments from storey deformation = (R \times) storey displacement.
• (R) = Response Reduction Factor (see Table 7 of IS 1893).

3. Use of Computed Fundamental Period (Clause 7.11.2)

• For displacement checks only, seismic forces may be computed using the actual fundamental period (T) without applying the lower bound force limit (Clause 7.8.2).

4. Key Table: Response Reduction Factor (R)

(Refer IS 1893 Table 7 for exact values)

Summary Diagram of Drift Limit and Deformation Compatibility

graph LR
A[Storey Drift \n δ ≤ 0.004h] --> B[Check for each storey]
B --> C{Single Storey?}
C -- Yes --> D[No drift limit]
C -- No --> E[Apply drift limit]
E --> F[Check deformation compatibility for non-seismic members]
F --> G[Ensure vertical load capacity under R × displacement]

References:

• Clause 7.11.1: Storey drift limit = 0.004 × storey height.
• Clause 7.11.2:

IS 1893 (Part 1) Special Provisions for Irregular & Soft Storey Buildings

1. Soft Storey Definition (Table 5)

2. Weak Storey

• Storey lateral strength < 80% of storey above.
• Lateral strength = sum of strengths of all seismic force resisting elements in that storey.

3. Irregularities to Consider

• Torsional Irregularity: Max storey drift at one end > 1.2 × average drift at two ends (with design eccentricity).
• Re-entrant Corners: Projections > 15% of plan dimension.
• Diaphragm Discontinuity: Abrupt stiffness change > 50% or openings > 50% of diaphragm area.
• Out-of-Plane Offsets: Discontinuities in lateral force path.
• Non-parallel Systems: Vertical elements not parallel or symmetric.

4. Seismic Weight Distribution (Clause 7.4.3)

• Weight between floors distributed inversely proportional to distance to floors above and below.

5. Importance Factor (Table 6)

6. Key Formula for Soft Storey Stiffness Check

[ k_i < 0.7 \times k_{i+1} \quad \text{or} \quad k_i < 0.8 \times \frac{k_{i+1} + k_{i+2} + k_{i+3}}{3} ]

where ( k_i ) = lateral stiffness of storey i.

7. **Multiplying Factors for Damping (Table

IS 1893 (Part 1): Use of Isolation and Energy Dissipation Devices

Key Points:

• Base Isolation & Energy Dissipation Devices: Allowed for earthquake resistance only if:
  • Standard devices with detailed experimental performance data are used.
  • Designer demonstrates sufficient protection via detailed analysis.
  • Locally assembled devices must be experimentally validated.
  • Design reviewed by competent authority.

Important Related Specifications:

• Important buildings (hospitals, schools, emergency facilities): 1.5
• Other buildings: 1.0 | | Damping Multiplying Factors (Table 3) | Adjust seismic forces based on damping %:
  | Damping (%) | 0 | 2 | 5 | 7 | 10 | 15 | 20 | 25 | 30 |
  | Factor | 3.20 | 1.40 | 1.00 | 0.90 | 0.80 | 0.70 | 0.60 | 0.55 | 0.50 |
  Use factor to multiply spectral acceleration for damping different from 5%. | | Seismic Weight Distribution (Clause 7.4.3) | Weights between storeys distributed inversely proportional to distance from adjacent floors. |

Design Considerations:

• Use standard isolation devices with proven performance.
• Perform dynamic analysis including isolation/dissipation device behavior.
• Consider reduced seismic forces due to increased damping using multiplying factors.
• Review building importance factor for critical facilities.
• Validate local devices experimentally before use.

Formula for Adjusted Spectral Acceleration (Sa):

[ S_{a,adj} = S_a \times \text{Damping Factor} ]

Where:

• ( S_a ) = spectral acceleration at 5% damping
• Damping Factor from Table 3 above

flowchart TD
    A[Start: Select Isolation/Energy Dissipation Device] --> B{Is device standard with experimental data?}
    B -- No --> C[Perform Experimental Validation]
    B -- Yes --> D[Perform Detailed Dynamic Analysis]
    D --> E{Does analysis show sufficient protection?}
    E -- No --> F[Redesign Device/System

Change in Occupancy (IS 1893 Part 1, Clause 6.1.8)

• When occupancy changes causing a higher Importance Factor (I), redesign the structure as per seismic requirements for the new higher I.
• Importance Factors (Table 6):

  Structure Type	Importance Factor (I)
  Hospitals, schools, emergency buildings, power stations, large halls	1.5
  All other buildings	1.0

Retrofitting Guidance

• Retrofitting must upgrade the structure to meet seismic design criteria of a new structure with the increased importance factor.
• Use ductile detailing as per IS 13920 for enhanced seismic performance.
• Consider changing lateral load resisting system or strengthening existing elements.

Key Tables for Seismic Design (IS 1893 Part 1, Clause 6.4.2)

Seismic Weight Calculation (Clause 7.3.1)

Damping Multiplying Factors (for Response Spectrum)

IS 1893 Part 1: References and Annexures - Key Points

1. Important Adjunct Indian Standards (Clause 2.1)

• IS 456:2000 - Plain & Reinforced Concrete
• IS 800:1984 - Steel Construction
• IS 875 (Parts 1-5):1987 - Design Loads (Dead, Imposed, Wind, Snow, Special)
• IS 1498:1970 - Soil Classification
• IS 4326:1993 - Earthquake Resistant Design of Buildings
• IS 13920:1993 - Ductile Detailing for RC Structures
• IS 1893 Part 4 - Industrial Structures

2. Dynamic Amplification Factor (Clause 1.5)

• Use 1.5 for dynamic amplification factor in general.
• Use 1.0 if 3D dynamic analysis is performed.

3. Connections Between Building Parts (Clause 7.12.4)

• Connections must transmit forces ≥ (2/3) × W or 0.05 × weight of smaller part.
• Frictional resistance alone is insufficient.

4. Seismic Zone Factors (Annex E, Page 35)

5. Earthquake Intensity Scale (Annex D)

• Intensity ranges from 1 (Not noticeable) to 12 (Landscape changes)
• Damage Grades 1 to 5 describe severity from fine cracks to total collapse.
• Building Types:
  • A: Mud, field-stone (weak)
  • B: Ordinary brick, prefabricated
  • C: Reinforced concrete, well-built wood (strong)

Summary Table: Dynamic Amplification Factor