Knots in timber are classified as a structural defect because:
Choose the correct answer
They improve the appearance of wood
They cause local grain deviation and stress concentration, reducing tensile and bending strength of the member
They increase the density of wood
They prevent insect attack
Correct Answer
B. They cause local grain deviation and stress concentration, reducing tensile and bending strength of the member
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Knots (base of branch embedded in tree stem): grain deviates around knot (cross-grain), reducing tensile strength. IS 883 limits knot sizes for structural timber. Other defects: shakes (separation along grain, from growth stresses/drying), checks (surface cracking), warping, blue stain (fungal, not structurally harmful).
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