--- title: "Classification and characteristics of structural timber" subject: "Design of Timber and Masonry Structures" chapter: 1 key: "semester_5/design_of_timber_and_masonry_structures:timber-classification-characteristics" source: https://www.bctengineer.com/q/semester_5/design_of_timber_and_masonry_structures/timber-classification-characteristics site: BCT Engineering --- # Classification and characteristics of structural timber Design of Timber and Masonry Structures · Chapter 1: Structural Timbers - Asked in: year not given, 5 marks - Video: [03 06 Masonry and Timber Design Timber](https://www.youtube.com/watch?v=61RafkPXHZ0) — JDM Academy - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/design_of_timber_and_masonry_structures/timber-classification-characteristics - Whole chapter: https://www.bctengineer.com/md/semester_5/design_of_timber_and_masonry_structures/chapter-1 ## Question Explain how timber is classified. State the important characteristics that make a timber suitable for structural use. ## 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. ## Answer 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. | 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which group of trees gives almost all structural timber? A. Endogenous trees such as bamboo and palm B. Exogenous trees, which add annual rings under the bark C. Any tree, as long as it is kiln-seasoned D. Only conifers, because hardwoods have no annual rings Answer: B. Exogenous trees (softwoods and hardwoods) grow outward by annual rings and supply nearly all structural timber; endogenous trees are used only for light work. (Key point: Exogenous vs endogenous) 2. Compared with a softwood, a typical hardwood is: A. fast growing, light and lower in strength B. from conifers such as pine and deodar C. slow growing, heavy and usually stronger D. used mainly for roofing and formwork Answer: C. Hardwoods come from broad-leaved trees (sal, teak), grow slowly, are dense and usually stronger, so they suit columns, beams and heavy trusses. (Key point: Softwood vs hardwood) 3. Timber is anisotropic. What does this mean for placing a structural member? A. Load it along the grain, where strength is much greater than across the grain B. Load it across the grain, where it is strongest C. Orientation does not matter if the moisture content is low D. Always use sapwood so that strength is the same in every direction Answer: A. Strength along the grain is much greater than across it, so members must be placed to load the wood along the grain. 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