--- title: "Structural timber and factors affecting strength" subject: "Design of Steel and Timber Structure" chapter: 9 key: "semester_6/design_of_steel_and_timber_structure:structural-timber-factors-strength" source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/structural-timber-factors-strength site: BCT Engineering --- # Structural timber and factors affecting strength Design of Steel and Timber Structure · Chapter 9: Timber Structures and Design Methods - Asked in: 2075 Bhadra, 4+2 marks - Video: [Factors affecting wood strength](https://www.youtube.com/watch?v=PkLdBHNVr9o) — Tonya Nilsson - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/structural-timber-factors-strength - Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-9 ## Question Explain about structural timber and factors affecting the strength of timber. Write the design basis. ## Short answer Structural timber is graded species (Sal, Teak, Deodar) per IS 883:1994 for load-bearing use. Strength depends on species/density, moisture content, defects (knots, grain slope), load direction, duration, size, temperature, and treatment. Design uses working stress method: permissible stresses from Table 1 modified by grade, location, duration, slope-of-grain, and form factors; loads from IS 875; deflection limits span/360 or span/240. ## Answer #### 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is structural timber graded? A. Select, Grade I or Grade II, according to the defects present B. By colour and smell C. By age of the tree only D. Group A, B, C, by length Answer: A. Grade depends on defects (knots, slope of grain, etc.); species groups A, B, C are separate and based on E and fibre stress. (Key point: Grading of structural timber) 2. How does moisture content affect timber strength? A. Wet timber is stronger B. Strength rises as timber dries below the fibre saturation point (about 25 to 30%) C. Moisture has no effect D. Strength falls as timber dries below fibre saturation Answer: B. Drying below the fibre saturation point increases strength, so wet locations need lower permissible stresses. (Key point: Moisture and strength) 3. How does duration of load enter the permissible stress? A. Wind loads reduce stress by 0.75 B. Duration has no effect C. Short-term loads such as wind allow a higher stress via modification factor K2 (1.33) D. Permanent loads allow higher stress Answer: C. Timber can resist higher stress for a short time, so K2 = 1.33 is applied for wind. (Key point: Load duration) 4. What is the design basis of IS 883? A. Limit state with partial safety factors B. Ultimate load factor only C. Elastic collapse load D. Working stress method: actual stress under service loads must not exceed permissible stress Answer: D. Permissible stresses come from basic strength divided by a safety factor and adjusted by grade, location, duration, grain and form factors. 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