Timber defects — identification of knots In timber grading, “knots” are best described as:

Difficulty: Easy

Correct Answer: Visible indications of branches cut off (embedded branch bases)

Explanation:


Introduction / Context:
Knots are among the most common and impactful timber features, affecting strength, appearance, and machining. Accurate identification guides placement in structural members and decisions on allowable grades.



Given Data / Assumptions:

  • Knots originate where branches grew from the stem.
  • They may be sound (tight) or loose/decayed.
  • Their size and location significantly influence permissible stresses.



Concept / Approach:
During tree growth, branches integrate with the trunk, forming areas of complex grain orientation. When sawn, these show as ovoid or circular features called knots. They interrupt straight grain, reducing tensile and bending capacity, and can complicate machining and finishing.



Step-by-Step Solution:
Identify origin: branch base incorporated into the trunk wood.Relate to grading: standards limit knot size/position for structural grades.Therefore, the correct definition is “signs of branches cut off”.



Verification / Alternative check:
Structural design codes provide modification factors or rejection criteria based on knot size and distribution; visual grading rules illustrate knot types.



Why Other Options Are Wrong:
Crushing of fibres and raised grain are handling/machining issues; splits from seasoning are checks/shakes, not knots; speckled stains indicate biodeterioration, not embedded branch bases.



Common Pitfalls:
Assuming decorative knots are structurally harmless; ignoring the impact of knot clusters near maximum bending zones; failing to seal knots before finishing to prevent resin bleed.



Final Answer:
Visible indications of branches cut off (embedded branch bases)

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