Characteristics of timber - Relationship to properties

1. 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 • Natural growth characteristics Knowledge of Properties and Performance

2. Grain direction Microstructure of Timber • Cells - fibres - mainly longitudinal orientation • Bound together with rays • Higher strength and stiffness parallel to grain

3. Desired performance • Appearance • Structural • Durability Properties Specification Microstructure Performance of Timber • Material / species • Grade • Size • Protective treatment

4. 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

5. 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

6. Straight fibres Spirally wound fibres Duration of Load - Creep Deformation Creep important in choosing deflection limits, & allowing for deformation

8. 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

9. Instantaneous loads Wind loads k1 Duration of load Duration of Load - Loss of Strength

10. 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

11. 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)

12. 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