Timber Structures and Design Methods

Structural timber

Structural timber is timber of 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 in three groups by modulus of elasticity and extreme fibre stress: Group A (E above 12600 N/mm², $f_b$ above 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 grain is many times that perpendicular to grain, and the angle of the load to the grain changes the permissible stress.
5. Duration of load: timber carries a higher stress for a short time (wind, earthquake) than for permanent loads; 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 and preservative treatment, and location (inside, outside, wet).

Design basis (IS 883:1994)

• Working stress (permissible stress) method: 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 from IS 875 (dead, imposed, wind); wind and earthquake are not applied together.
• Net sections are used for tension members; gross section 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.

Types of timber

1. By tree: Hardwood from broad-leaved (deciduous) trees such as Sal, Teak, Shisham; strong, durable, heavy. Softwood from conifers (needle-leaved) such as Deodar, Chir pine; light, easier to work, less durable.
2. By structural use (IS 883): Group A (strongest, e.g. Sal), Group B (e.g. Teak, Sissoo), Group C (e.g. Deodar).
3. By form: logs, baulks, planks, scantlings, battens, boards, plywood, laminates.
4. By treatment: seasoned or unseasoned, treated or untreated.

Defects of timber

Defects reduce strength, durability and appearance. Their size and number decide the grade of structural timber (Select, Grade I, Grade II). They are controlled by proper felling, conversion, seasoning (air or kiln), good storage, ventilation and preservative treatment.

Design methods for timber structures

1. Working stress (permissible stress) method (IS 883:1994, used in Nepal and India): the stresses produced by service loads must not exceed the safe permissible stresses of the species and grade. It is simple and is the method in the course.
2. Limit state method: partial safety factors on loads and material strength, used in codes such as Eurocode 5 (and modern codes) with checks for strength and serviceability; it is more rational, but IS 883 does not use it.
3. Load factor / ultimate strength approach and tests on prototypes (IS 883 allows design by calculation or by prototype testing).

Design basis (IS 883:1994)

• Permissible stresses from Table 1 for the species for Grade I, modified: Select grade $\times1.16$, Grade II $\times0.84$; low-durability species outside $\times0.8$; slope of grain ($K_1$), duration of load ($K_2$, e.g. 1.33 for wind), form factors ($K_3$ to $K_6$), bearing length ($K_7$).
• Loads from IS 875 (dead, imposed, wind, snow); wind and seismic loads are not combined.
• Beams: check bending, horizontal shear, bearing and deflection (limits span/360 with brittle finish, span/240 otherwise, doubled dead load for initially dry timber); minimum width 50 mm or span/50; depth not over 3 times the width unless laterally stiffened.
• Columns: classified short, intermediate and long by slenderness $S/d$; $S/d \le 50$ for solid columns; combined bending and axial compression by $f_{ac}/f_c + f_{ab}/f_b \le 1$.
• Tension members: net section; notches may not remove more than one quarter of the section.
• Moisture content, grading, preservation as per IS 287, IS 3629 and IS 401.