Structural Timbers

Timber is the wood of mature trees that has been felled, seasoned and cut into usable sizes. Structural timber must be strong, durable, straight-grained and reasonably free from defects.

Classification

1. By growth of the tree
   • Exogenous trees grow outward by adding annual rings under the bark. They give almost all structural timber. They are divided into softwoods (conifers such as pine, deodar, fir) and hardwoods (broad-leaved trees such as sal, teak, sissoo).
   • Endogenous trees (bamboo, palm) grow from inside and have no annual rings. They are used only for light work.
2. By use and strength: structural codes (e.g. IS 883) place species in strength groups A, B and C according to their bending strength and stiffness. Each species is then graded (Select, Grade I, Grade II) by visible defects.
3. By seasoning and treatment: green, air-seasoned, kiln-seasoned, and preservative-treated timber.

Characteristics required for structural use

• High strength-to-weight ratio and adequate stiffness (modulus of elasticity).
• Durability: resistance to decay, fungus and termites (heartwood is better than sapwood).
• Straight grain, few knots, small slope of grain, low shrinkage after seasoning.
• Low and stable moisture content (below about 12–18 percent for roof and floor members).
• Good workability, nail and bolt holding capacity, and ready availability.
• Timber is anisotropic: strength along the grain is much greater than across the grain, so members must be placed to load the wood along the grain.

(a) Factors affecting strength

1. Moisture content: below the fibre saturation point (about 25–30 percent) strength and stiffness increase as the wood dries. Above it, strength is nearly constant. Timber is tested and designed at a standard moisture content (about 12 percent).
2. Density / specific gravity: denser wood has more cell wall material and so is stronger and stiffer.
3. Defects: knots, slope of grain, shakes, checks, splits, wane, decay and insect attack reduce the effective section and cause stress concentration. Slope of grain and knots mainly reduce tensile and bending strength.
4. Direction of loading: strength parallel to grain is many times that perpendicular to grain; compression, tension and shear all depend on the angle to the grain (Hankinson's formula).
5. Duration of load: the longer the load acts, the lower the strength. Codes allow higher stresses for short-term loads (for example wind or earthquake).
6. Temperature, seasoning and treatment: high temperature and poor seasoning reduce strength; some preservatives and fire-retardants slightly reduce it.
7. Size and position of the member: larger sections contain more defects and have lower strength per unit area.

(b) Permissible stresses

Small clear specimens are tested to failure and the mean and standard deviation of strength are found. The basic stress is taken near the 5 percent exclusion value and then divided by a factor of safety:

$$\sigma_{\text{permissible}} = \frac{\text{Basic (clear) strength}}{\text{Factor of safety}} \times \text{strength ratio (grade)} \times \text{modification factors}$$

The factor of safety allows for variability, reduction due to long-term loading and local defects.

Grading

Timber is visually graded according to the size and number of defects permitted (knots, slope of grain, shakes).

The permissible stresses of the select grade are listed in the code for each strength group; stresses for lower grades are obtained by multiplying by the grade ratio. Modification factors are then applied for moisture, load duration and size of the member.