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Intuitive understanding of timber behaviour. Characteristics of timber - Relationship to properties. Maximise performance of timber. Knowledge of: Physiology of timber fibres, cells, grain, growth rings Moisture content emc and shrinkage Creep and duration of load effects

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characteristics of timber relationship to properties
Intuitive understanding of timber behaviourCharacteristics of timber - Relationship to properties

Maximise performance of timber

Knowledge of:

  • Physiology of timber fibres, cells, grain, growth rings
  • Moisture content emc and shrinkage
  • Creep and duration of load effects
  • Natural growth characteristics

Knowledge of Properties and Performance

microstructure of timber
Grain

direction

Microstructure of Timber
  • Cells - fibres - mainly longitudinal orientation
  • Bound together with rays
  • Higher strength and stiffness parallel to grain
performance of timber
Desired performance
    • Appearance
    • Structural
    • Durability

Properties

Specification

Microstructure

Performance of Timber
  • Material / species
  • Grade
  • Size
  • Protective treatment
performance of timber4
Performance of Timber
  • Appearance
    • Grain and colour
    • Feature
    • Dimensional stability & emc%
  • Structural
    • Essential e.g. strengthand stiffness
    • Utility e.g. dimensional stability

- shrinkage/emc

    • Straightness - bow, spring, cup and twist
  • Durability
    • Biological hazards
    • Natural resistance / treatment
duration of load creep deformation
Duration of Load - Creep Deformation

Stiffness:

Creep (extra deformation under load)

  • recoverable - deformation slowly comes out after load removed
  • irrecoverable - deformation remains after load removed
  • function of moisture movement, magnitude & duration of load
  • occurs at all loads
  • modelled with j2 factor for deflections of beams
duration of load creep deformation6
Straight

fibres

Spirally

wound

fibres

Duration of Load - Creep Deformation

Creep important in choosing deflection limits, & allowing for deformation

duration of load loss of strength
Duration of Load - Loss of Strength
  • increase in duration & magnitude of loadcauses decrease in strength
  • irreversible and cumulative loss of strength
  • function of duration of peak load over lifetime of structure
  • Different load combinationshave different duration ofload effects

All composite materials show this effect

evaluation of structural properties
Evaluation of Structural Properties
  • Small clear specimens - data only reflects wood fibre strength
    • For timber beams, we must reduce small clear strengths significantly to allow for strength reducing natural features
  • In-grade testing - commercial sized timber under realistic loading conditions
  • Commercial timber
    • tensile strength < compression strength
    • tensile failures - splintery, brittle, sudden, loud
    • compression failures - wrinkles, ductile, slow, quiet
summary
Summary
  • Appearance:
    • Colour, grain, features, smoothness of surface
    • Reflect species, growth patterns, history of tree
    • Specification: species, durability, appearance graded
  • Utility:
    • Dimensional stability (shrinkage, twist, bow, cup, spring),surface hardness
    • Reflect stress changes with moisture loss, creep
    • Specification: moisture content (best close to equilibrium moisture content)
summary12
Summary
  • Structural:
    • Strength - stronger parallel to grain
      • tension,
      • compression,
      • bending,
      • shear,
      • bearing
    • Stiffness (MoE) - stiffer parallel to grain
    • Reflect grain structure, slope of grain, features in timber
    • Specification: structural grade and species
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