How is structural timber classified?

It is classified into three groups (Group A, B, and C) based on its Modulus of Elasticity and extreme fiber stress in bending.

What code governs the actual sizing and structural design of timber members?

IS 883 covers the structural design and permissible stresses, whereas IS 3629 governs material classification and grading.

Are dead knots allowed in structural timber?

Loose or dead knots are generally not permitted, but tight knots are allowed up to specified limits depending on the timber grade (Select, Grade I, or Grade II).

IS 3629

: 1986

Structural Timber - Use in Buildings - Code of Practice

CurrentSpecializedCode of PracticeStructural Engineering · Timber and Wood

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Specifies the requirements for structural timber used in buildings, including its classification, grading, permissible defects, seasoning, and preservation. It acts as the primary material specification standard before conducting structural design.

Provides recommendations for the selection, grading, and use of structural timber members in different parts of buildings.

Quick Reference Values

Modulus of Elasticity (Group A Timber)> 12.5 kN/mm²

Modulus of Elasticity (Group B Timber)9.8 to 12.5 kN/mm²

Modulus of Elasticity (Group C Timber)5.6 to 9.8 kN/mm²

Maximum Slope of Grain (Select Grade)1 in 20

Maximum Slope of Grain (Grade I)1 in 15

Maximum Slope of Grain (Grade II)1 in 10

Practical Notes

Always classify timber into Group A, B, or C based on strength properties before proceeding to design.

Ensure proper seasoning and preservative treatment according to IS 401 and IS 1141 to prevent decay and insect attack.

Strictly monitor the maximum size of knots and slope of grain at the site, as these natural defects severely impact the structural strength.