Which group of trees gives almost all structural timber?

Exogenous trees, which add annual rings under the bark

Endogenous trees such as bamboo and palm

Any tree, as long as it is kiln-seasoned

Only conifers, because hardwoods have no annual rings

Compared with a softwood, a typical hardwood is:

slow growing, heavy and usually stronger

fast growing, light and lower in strength

from conifers such as pine and deodar

used mainly for roofing and formwork

Timber is anisotropic. What does this mean for placing a structural member?

Load it along the grain, where strength is much greater than across the grain

Load it across the grain, where it is strongest

Orientation does not matter if the moisture content is low

Always use sapwood so that strength is the same in every direction

Practice · 5 marks

Classification and characteristics of structural timber

Short answer

Timber is classified by tree growth (exogenous softwoods and hardwoods, endogenous), by strength groups and grades, and by seasning/treatment. Structurally suitable timbeer requires high strength-to-weight ratio, adequate stiffness, durability, straight grain with few defects, low and stable moisture content, good workability, and must be loaded along the grain due to anisotropy.

Explain how timber is classified. State the important characteristics that make a timber suitable for structural use.

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

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.
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.
By seasoning and treatment: green, air-seasoned, kiln-seasoned, and preservative-treated timber.

Point	Softwood	Hardwood
Source	Conifers	Broad-leaved trees
Growth	Fast	Slow
Density	Light	Heavy
Strength	Lower (usually)	Higher
Use	Roofing, formwork, light framing	Columns, beams, heavy trusses

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.

✓ Answer checked by Claude Sonnet 5.5

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Classification and characteristics of structural timber

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