Factors affecting strength and permissible stresses

(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.