Code of practice for the design of wood poles for overhead power and telecommunication lines

IS 5978: Scope & Key Specifications for Wood Poles

Scope Summary:

• Applies to wood poles used in overhead power and telecommunication lines.
• Covers dimensions, strength, and design parameters.
• Focus on poles' diameter, height, and setting depth for stability under wind and load.

Key Symbols (Clause 3.1):

Pole Dimensions (Clause 8.6 - Table 2):

Choose stoutest, straightest poles for unstable ground.

Design Considerations:

• Use diameter at 1.8 m from butt for strength checks.
• Account for overturning moments due to wind on pole and conductors.
• Ensure pole setting depth (Lg) provides stability.
• Refer to modulus of rupture (R) for bending strength.

flowchart TD
    A[Wood Pole Design] --> B[Determine Height (H) & Setting Depth (Lg)]
    B --> C[Select Diameter at Ground (Do) & Top (DT)]
    C --> D[Calculate Effective Diameter (de)]
    D --> E[Calculate

IS 5978 Key References & Specifications Summary

1. Pole Dimensions (Clause 8.6, Table 2)

• Minimum circumference at ground level and top for wood poles varies by class and height.
• Heights range from 6 m to 14 m.
• Circumference values (cm) differ by class groups (A, B, C).

(Refer to Table 2 in IS 5978 for detailed class-wise values)

2. Diameter Reference (Clause 1.8)

• Diameter measurements at 1.8 m from butt and at the top are critical.
• Use stoutest and straightest poles for stability, especially on unstable grounds.

3. Important Notes

• Use values with caution on abnormally unstable grounds.
• Select poles carefully for stability and durability.

Practical Tip:

• For design, always verify pole class and ground conditions.
• Refer to IS 5978 Table 2 for exact dimensions per pole class.

flowchart TD
    A[Select Pole Class] --> B{Measure Height}
    B -->|6m to 14m| C[Check Circumference at Ground Level]
    B -->|6m to 14m| D[Check Circumference at Top]
    C --> E[Verify against IS 5978 Table 2]
    D --> E
    E --> F[Confirm Pole Suitability]

For detailed dimensions and contact info, see IS 5978 Manak Bhavan, New Delhi.

IS 5978 - Definitions & Symbols (Clause 3.1)

Key Formula:

[ L_e = L_y - L_g ]

• Le is effective pole length for bending moment calculations.
• Overturning moment ( M = M_{c1} + M_{c2} + M_p ).

Notes:

• Use diameters at 1.8 m from butt and at top for design.
• Select stoutest, straightest poles for stability.
• Wind pressures (P) and (Pc) depend on regional data (refer IS 875 for wind loads).

flowchart TD
    A[Full Length (Ly)] -->|Subtract| B[Depth of Setting (Lg)]
    B -->|Result| C[Effective Length (Le)]
    C --> D[Calculate Overturning Moment (M)]
    D --> E[Design Checks]

This summary aids in understanding pole geometry and loadings per IS 5978.

IS 5978 - Key Specifications for Species of Timber (Clause 5.1.2 & Table 1)

Example from Table 1:

Important Symbols (Clause 3.1):

• de: Effective diameter of pole (cm)
• Do: Diameter at ground level (cm)
• H: Height above ground (m)
• M: Overturning moment at ground line (kg·m)
• R: Ultimate modulus of rupture (static bending) (kg/cm²)

Notes:

• Poles are designed using the modulus of rupture in green condition (wet timber).
• Species are grouped based on strength properties for design.
• Dimensions vary with species strength and defects.

flowchart TD
    A[Species of Timber] --> B[Average Weight @ 12% Moisture]
    A --> C[Static Bending Strength]
    A --> D[Modulus of

IS 5978: Classification & Strength of Wood Poles

1. Classification of Wood Poles (Clause 5.1)

2. Key Dimensions (Table 2, Clauses 5.1.3, 8.6)

• Dimensions given as minimum circumference (cm) at ground level and top for different classes and timber groups.
• Example for Class 1, Group A at 6 m height above ground:
  • Ground circumference = 60 cm
  • Top circumference = 50 cm

3. Symbols & Parameters (Clause 3.1)

4. Strength Considerations

• Ultimate breaking load depends on:
  • Modulus of rupture (R) of species (Table 1, not shown here)
  • Uniformity of taper
  • Defects and their location

5. Formula for Overturning Moment (Example)

[ M = M_{c1} + M_{c2} + M_p ]

Where:

• (M_{c1}, M_{c2}) = moments due to wind on conductors
• (M

IS 5978: Mechanical Properties of Timber (Key Points)

1. Timber Groups by Modulus of Rupture (Clause 4.1)

• Group A (Very Strong): Modulus of rupture ≥ 850 kg/cm²
• Group B (Strong): 630 ≤ Modulus of rupture < 850 kg/cm²
• Group C (Moderately Strong): 450 ≤ Modulus of rupture < 630 kg/cm²

2. Key Mechanical Properties (From Table 1, Clause 5.1.2)

3. Example Values for Teak (Terminalia grandis)

• Moisture Content: ~12%
• Static Bending Strength: 685 kg/cm²
• Modulus of Elasticity: 90,000 kg/cm²
• Maximum Crushing Stress: 340 kg/cm²
• Average Weight: 625 kg/m³

4. Important Symbols (Clause 3.1)

• de: Effective diameter (cm)
• Do: Diameter at ground level (cm)
• H: Height of pole above ground (m)
• M: Total overturning moment (kg·m)
• R: Ultimate modulus of rupture (kg/cm²)

Summary Diagram: Timber Strength Groups

graph LR
A[Group A: ≥ 850 kg/cm²] --> Strongest
B[Group B: 630-850 kg/cm²] --> Medium Strength
C[Group C: 450-630 kg/cm²] --> Moderate Strength

Note: Strength values are for small clear specimens in green condition; actual pole strength may vary due to defects and treatment effects.

IS 5978: Loads on Wood Poles – Key Formulas and Specifications

1. Key Symbols (Clause 3.1)

• de = Effective diameter of pole (cm)
• Do = Diameter at ground level (cm)
• DT = Diameter at the top (cm)
• H = Height above ground (m)
• Lg = Depth of setting (m)
• Le = Effective length = L - Lg (m)
• M = Total overturning moment at ground line (kg·m)
• Mc1, Mc2 = Overturning moments due to wind on conductors (kg·m)
• Mp = Overturning moment due to wind on pole (kg·m)
• P = Design wind pressure (kg/m²)
• Pc = Wind pressure on conductors (kg/m)
• R = Ultimate modulus of rupture in bending (kg/cm²)

2. Design Approach (Clause 6.1)

• Poles are designed as simple cantilevers.
• Use modulus of rupture (R) in green condition (moisture > 25%).
• Avoid defects near points of maximum bending stress (critical section).

3. Overturning Moment Calculation

[ M = M_{c1} + M_{c2} + M_p ]

Where:

• ( M_{c1} = P_c \times H_1 ) (wind on top conductor)
• ( M_{c2} = P_c \times H_2 ) (wind on next conductor)
• ( M_p = P \times d_e \times L_e^2 / 2 ) (wind on pole)

4. Modulus of Rupture (R) and Pole Dimensions

• Refer Table 1 (not shown here) for R values by timber species.
• Table 2 (Clause 8.6) provides minimum circumference at ground and top for different classes and species groups.

5. Excerpt from Table 2 (Minimum Circumference at Ground Level)

| Max Height (m) | Class 1 (cm) | Class 2 (cm) | Class 3 (cm) |

IS 5978: Design of Wood Poles - Key Points

1. Design Approach (Clause 6.1)

• Wood poles are designed as simple cantilevers (except when used as struts, cross arms, or braces).
• Use modulus of rupture in bending for green wood (moisture > 25%) from Table 1 for preliminary design.
• Position visible defects away from critical sections (max bending stress points).

2. Key Dimensions (Clause 8.6, Table 2)

(Refer to Table 2 for full classification and group details)

3. Important Symbols (Clause 3.1)

4. Basic Bending Stress Formula

[ \sigma = \frac{M \times c}{I} \leq R ]

Where:

• (M) = bending moment (kg·cm)
• (c) =

Selection of Pole Size (IS 5978: Clause 8.6 & 8.7)

Key Points:

• Pole size is selected based on standard dimensions given in Table 2 (Clause 8.6).
• Table 2 provides minimum circumferences at ground level and top for various pole heights and classes.
• Pole height is measured above ground level (H), and circumferences are given in cm.
• The safe working load = breaking load / factor of safety (Clause 8.7).
• The total overturning moment (M) at ground line due to wind loads on wires and pole is converted to an equivalent load at 60 cm from the top.
• Initial design can ignore pole wind load; calculate for wire wind load first, then iterate including pole wind load.

Important Symbols (Clause 3.1)

Pole Selection Procedure Summary

1. Calculate overturning moment (M):

[ M = M_{c1} + M_{c2} + M_p ]

• Start by ignoring (M_p) (wind on pole).
• Calculate (M_{c1}), (M_{c2}) from wind load on conductors.

2. Convert M to equivalent load at 60 cm from top for design.

3. Select pole size from Table 2 based on height and class with minimum circumference satisfying the moment.

4. Iterate including (M_p)

Use of Stays and Stability Considerations (IS 5978 - Clause 8.8)

Key Formula for Crippling Load due to Vertical Components in Stays:

[ P_c = \frac{P_u}{K_t \times K_g \times K_s} ]

Where:

• (P_c) = Crippling load on pole due to vertical stay forces
• (P_u) = Ultimate crippling load for the pole section
• (K_t) = Factor for tapered sections
• (K_g) = Factor for imperfect ground rigidity
• (K_s) = Factor for stability due to stays and line wires

Important Specifications:

• Pole Selection: Use the straightest and stoutest poles for stays, especially on unstable grounds (Clause 1.8).
• Pole Dimensions: Diameter at 1.8 m from butt and at top are critical for design (see Clause 1.8 table).
• Pole Design: Poles act as simple cantilevers (Clause 6.1).
• Modulus of Rupture (R): Use green condition values (moisture > 25%) from Table 1.
• Ultimate Resisting Moment at Ground Line:

[ M_u = \frac{32 \times 100 \times R}{X} ]

Where (X) is the diameter at ground line (cm).

Stability Factors Summary:

flowchart LR
    A[Vertical Forces in Stays] --> B[Calculate Vertical Component]
    B --> C[Apply Factors: K_t, K_g, K_s]
    C --> D[Determine Crippling Load on Pole]
    D --> E[Check Against Ultimate Load Capacity]
    E --> F{Safe or Unsafe?}
    F -->|Safe| G[Design Approved]
    F -->|Unsafe| H[Revise Design or Pole Selection]

Note: Always verify pole stability with respect to local soil conditions and stay arrangement per IS 5978 guidelines.

Safety Factors in IS 5978

• Minimum Factor of Safety (Clause 2.5):
  [ \text{Factor of Safety} \geq \max(2.5, \text{Statutory Rules}) ]

• Ultimate Resisting Moment at Ground Line (Clause 8.5):
  Let:

  • ( R ) = Ultimate modulus of rupture (kg/cm²)
  • ( X ) = Section modulus at ground line (cm³)
  • ( M ) = Resulting moment (kg·m)

  Then,
  [ \text{Ultimate resisting moment} = 32 \times X \times 100 \times R \quad \text{(kg·m)} ]

  Factor of Safety:
  [ \text{FOS} = \frac{32 \times 100 \times X \times R}{M} ]

• Safe Working Load (Clause 8.7):
  [ \text{Safe Working Load} = \frac{\text{Breaking Load}}{\text{Factor of Safety}} ]

  • Overturning moment due to wind on wires and pole converted to equivalent load at 60 cm from top.
  • Pole size selected from Table 2 based on duty.
  • Iterative process to include wind load on pole.

• Crippling Load with Stays (Clause 8.8):
  Takes into account:

  • Tapered sections
  • Imperfect ground rigidity
  • Pole-top stability from stays and wires

Quick Reference Table (Conceptual)

flowchart TD
    A[Calculate Section Modulus (X)] --> B[Determine Ultimate Rupture Modulus (

IS 5978: Testing Methods for Wooden Poles

Reference Standard:

• Testing methods as per IS: 1900-1961 (Methods of testing small clear timber specimens and wood poles).

Key Specifications & Testing Methods:

• Specimen Preparation:
  Small clear specimens are cut from poles avoiding defects to test mechanical properties.

• Load Application:
  Loads assumed on full-size poles per IS 1900-1961 method.

• Diameter Measurement:

  • Diameter at 1.8 m from butt (H) and at top are critical for design and testing.
  • Use these diameters cautiously on unstable grounds.

Important Table (Diameter for Pole Classes):

Typical Testing Procedures:

• Bending Test:
  Apply lateral load at specified height; measure deflection and failure load.

• Compression Test:
  Axial load applied to determine crushing strength.

• Shear Test:
  Evaluate resistance to shear forces.

Summary Formula for Bending Stress:

[ \sigma_b = \frac{M}{Z} = \frac{P \times L}{\frac{\pi}{32} \times d^3} ]

Where:

• (\sigma_b) = bending stress
• (M) = bending moment = (P \times L)
• (P) = applied load
• (L) = load application height
• (Z) = section modulus for circular cross-section
• (d) = diameter at test section

flowchart TD
    A[Specimen Preparation] --> B[Load Application as per IS 1900]
    B --> C[Measure Diameter at 1.8 m and Top]
    C --> D{Testing Types}
    D --> E[Bending Test]
    D --> F[Compression Test]
    D --> G[Shear Test]
    E --> H[Calculate Bending Stress]

Note: Always select straight, stout poles for testing to ensure reliability. Use IS

IS 5978 Annexures and Tables Key Points

1. Pole Dimensions (Clause 1.8)

• Diameter at 1.8 m from butt (H) and diameter at top (cm) are critical for design.
• Use straightest, stoutest poles for stability, especially on unstable grounds.

2. Strength of Timber Species (Clause 5.1.2, Table 1)

• Provides mechanical properties for various timber species in green condition at 12% moisture.
• Key properties include:
  • Density (kg/m³)
  • Static Modulus of Rupture (kg/cm²)
  • Modulus of Elasticity (×10³ kg/cm²)
  • Maximum Crushing Stress (kg/cm²)
  • Shearing Stress (kg/cm²)

Usage Notes:

• Use these values for design and selection of poles.
• Adjust for specific site conditions and species availability.

flowchart TD
    A[Select Timber Species] --> B{Check Mechanical Properties}
    B --> C[Density]
    B --> D[Modulus of Rupture]
    B --> E[Modulus of Elasticity]
    B --> F[Max Crushing Stress]
    B --> G[Shearing Stress]
    C --> H[Design Pole Dimensions]
    D --> H
    E --> H
    F --> H
    G --> H
    H --> I[Final Pole Selection]

For full tables and detailed annexures, refer to IS 5978 official document.