What is the required factor of safety for designing wood poles?

A factor of safety of 2.5 on the ultimate stress of the timber species used (Clause 7.1).

Which types of wood are recommended for poles?

The code groups timbers into A, B, and C. Group A (e.g., Sal), having the highest strength, is most preferred, followed by Group B (e.g., Kail, Teak) (Table 1).

What is the permissible bending stress for Sal wood poles?

For Sal (Group A), the permissible stress in bending for 'Inside Location' is 125 kgf/cm² (Table 2).

How deep should a wood pole be planted?

This code does not specify planting depth but refers to IS 1039 for guidance. A common rule is 1/6th of the pole's total length.

IS 5978

: 1970

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

CurrentSpecializedCode of PracticeBIMStructural Engineering · Timber and Timber Stores

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This code provides guidelines for the structural design of single solid wood poles used for supporting overhead power and telecommunication lines. It covers material selection, preservative treatments, calculation of design loads like wind, and determination of permissible stresses for various timber species.

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

Quick Reference Values

Factor of Safety on ultimate stress2.5 (Clause 7.1)

Permissible Bending Stress (Group A Timber, Inside Location)125 kgf/cm² (Table 2)

Permissible Bending Stress (Group B Timber, Inside Location)100 kgf/cm² (Table 2)

Permissible Bending Stress (Group C Timber, Inside Location)75 kgf/cm² (Table 2)

Basic Wind Pressure (High Intensity, up to 15m height)150 kgf/m² (Table 3)

Key Formulas

fb = M / Z ≤ Fb — Bending stress check at ground line, where M is max bending moment and Z is section modulus

M = P1*h1 + P2*h2 + ... — Calculation of Bending Moment from various transverse loads (wind on pole, wind on conductor, etc.)

Practical Notes

The wind pressure values in Table 3 are outdated. It is crucial to use the latest version of IS 875 (Part 3) for calculating wind loads.

The critical design section for a cantilever pole is at the ground line, where bending moment is typically maximum.

Effective preservative treatment of timber as per IS 401 is essential for the durability and service life of the pole.