Structural timber and factors affecting strength

Structural timber

Structural timber is timber of a species, size and quality graded to carry loads in a structure (beams, joists, columns, trusses). In Nepal and India it is chosen from the species listed in IS 883:1994 Table 1 (Sal, Teak, Deodar and others), seasoned, and graded as Select, Grade I or Grade II according to the defects present (IS 3629, IS 1331). The species are classed into three groups by modulus of elasticity and extreme fibre stress: Group A (E above about 12.5 kN/mm², $f_b$ above about 18 N/mm²), Group B and Group C.

Factors affecting the strength of timber

1. Species and density: denser, heavier timber is generally stronger (Sal is stronger than Deodar).
2. Moisture content: strength rises as timber dries below the fibre saturation point (about 25 to 30%); wet timber is weaker, so the permissible stresses are lower for wet locations.
3. Defects: knots, slope of grain, shakes, checks, decay and insect attack reduce strength; the grade (Select, I, II) takes this into account.
4. Direction of grain: strength parallel to the grain is many times that perpendicular to the grain, and the angle of the load to the grain changes the permissible stress.
5. Duration of load: timber can carry a higher stress for a short time (wind, earthquake) than under permanent loads; this is allowed for by the modification factor $K_2$ (e.g. 1.33 for wind).
6. Size and shape: the form factor for deep beams, and the effect of size on the section.
7. Temperature, age, preservative treatment and location (inside, outside, wet).

Design basis (IS 883:1994)

• Working stress (permissible stress) method: the actual stress under service loads must not exceed the safe permissible stress given in Table 1, which is obtained from the basic strength by dividing by a factor of safety and applying grade, location, duration, slope-of-grain and form factors.
• Loads are taken from IS 875 (dead, imposed, wind); wind and earthquake are not applied together.
• Net sections are used for tension members; the gross section is used for columns.
• Deflection limits: span/360 for members supporting brittle finishes and span/240 otherwise; permanent loads are doubled when checking deflection in timber that is initially dry.
• Effective span, minimum bearing (75 mm on masonry), notching limits and the slenderness limits of columns are as given in the code.