structural timber in the building - Specification
1986 Edition

Per IS 3629 (1986), permanent structural timber species include untreated heartwood of high durability with sapwood content not exceeding 15%, treated heartwood of moderate and low durability species especially those classified as treatability classes 'a' and 'b', heartwood of moderate durability with class 'c' treatability subjected to pressure impregnation, and sapwood of any durability class which must be thoroughly treated. Locally available species are preferred, and for species not listed, strength testing per Clause 4.2 and IS 883 is required. The standard references IS 399-1963 and IS 401-1982 for detailed durability and treatment classifications.

Allowed defects in structural timber under this code include wane provided it is not combined with knots and the resulting strength reduction does not exceed that caused by maximum permitted knots; worm holes are permitted except those attributed to powder post beetles and are treated similarly to knots for strength considerations. Other minor defects not impairing mechanical properties are acceptable. Prohibited defects include loose grain, splits, compression wood in conifers, heartwood and sap rot, crookedness, worm holes from powder post beetles, and pitch pockets. IS 3364 (Part 2)-1976 details measurement and classification of these defects.

The standard categorizes timber durability into three classes: Class I (high durability) with a service life of 120 months or more, Class II (moderate durability) with 60 to 120 months, and Class III (low durability) below 60 months. Timber requiring preservative treatment includes heartwood of moderate and low durability species, heartwood of high durability species if sapwood exceeds 15%, and all sapwood regardless of durability. Untreated high durability heartwood with sapwood content of 15% or less may be used as is. Preservative treatments conform to IS 401-1982, ensuring protection against fungi, insects, and marine borers.

Permissible stresses in timber are derived by dividing basic stresses from clear specimens (as per IS 1708-1969) by safety factors defined for different conditions (inside, outside, wet) and timber groups. For Grade I timber, factors of safety range from 4 to 10 depending on stress type and exposure. Adjustment factors are applied to account for timber grade: 1.16 multiplier for Select Grade and 0.84 for Grade II. Timber is classified into Groups A, B, and C based on modulus of elasticity and extreme fibre stress, influencing design stresses. This approach ensures adequate safety and durability across environmental conditions.

To maintain timber integrity, the standard advises storing timber elevated off the ground in a dry, ventilated environment protected from direct sunlight, rain, and hot winds. Stacks should be arranged with spacers to allow air circulation, and heavy weights applied atop to prevent warping. Ends must be coated with protective materials like coal tar or aluminum lead paint to reduce end cracking as per IS 1141-1973. Handling during erection should minimize unintended stresses by avoiding impacts, bending, or twisting. Proper storage and careful handling ensure timber retains its strength and durability for structural use.