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

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Presentation on theme: "Physiology of timber fibres, cells, grain, growth rings Moisture content emc and shrinkage Creep and duration of load effects Natural growth characteristics."— Presentation transcript:

1 Physiology of timber fibres, cells, grain, growth rings Moisture content emc and shrinkage Creep and duration of load effects Natural growth characteristics Structural properties of timber Characteristics of timber - Relationship to properties Knowledge of Properties and Performance Intuitive understanding of timber behaviour Maximise performance of timber

2 Performance of Timber Appearance/Structural/Durability Appearance Grain and colour Feature Dimensional stability & emc% Structural Essential e.g. strength and stiffness Utility e.g. dimensional stability - shrinkage/emc Straightness - bow, spring, cup and twist Durability Biological hazards Natural resistance / treatment

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

4 Cells Chemical components of wood - products of photosynthesis Cellulose - network of molecules cell walls - microfibrils - fibrous Lignin - ‘gel’ - acts as bonding agent which ‘glues’ cells together Hemicellulose - cross linking - binds cellulose into the cell Straight fibres Spirally wound fibres

5 Direction of Strength and Stiffness Direction of grain Strong parallel to grain & Stiff parallel to grain Weak perpendicular to grain

6 Moisture in Wood Cells removed bound water Seasoned timber 15% 100% Unseasoned timber Growing tree free water 25% fibre saturation bound water Partially seasoned timber

7 Seasoning - process of removing moisture from timber Kiln drying (steam, LPG gas, solar) Air drying Other - chemical, microwave. Moisture in Timber Moisture content (mc) = weight water weight wood in growing tree - mc = 50% to > 100% felled tree - mc begins to decrease Fibre saturation point (fsp) (~25%) above fsp - moisture in cell cavities lost -> little change in dimension below fsp - moisture in cell wall lost -> shrinkage perp to grain

8 Wet atmosphere / Dry wood  moisture moves to wood Equilibrium Moisture content (emc) Dry Atmosphere / Wet wood  moisture moves from wood Wood at emc  no moisture movement to / from wood Moisture in wood at equilibrium with moisture in atmosphere Typical emc Indoor air conditionedemc 8% - 10% Indoor heatedemc 8% - 12% External - coastalemc 14% - 18% External - inlandemc 10% - 15% Indoor air conditionedemc 8% - 10% Indoor heatedemc 8% - 12% External - coastalemc 14% - 18% External - inlandemc 10% - 15%

9 Specification of Moisture Content Seasoned timber: mc < 15% - close to emc indoors will shrink & swell slightly as humidity changes Usually specified as Seasoned or Unseasoned dimensions A decrease in dimensions b & d (shrinkage mainly perp. to grain) Everything else: sold as Unseasoned timber shrinks on further drying Effect of mc on properties: reducing mc causes an increase in strength stiffness (reduced creep) durability (reduced risk of attack) effectiveness of coatings

10 Shrinkage from 25% to 12% Radiata PineRad 3.5%Tang 4% Hoop PineRad 2.5%Tang 3.5% CypressRad 3.5%Tang 4% Spotted GumRad 4.5%Tang 6% KarriRad 4.5%Tang 10% Sydney Blue GumRad 5%Tang 9% Grey Iron BarkRad 5.5%Tang 7.5% Mountain AshRad 6.5%Tang 13% Softwood Hardwood Shrinkage Loss of moisture in range mc <25% Reduction in cell wall thickness Longitudinal shrinkage Tangential shrinkage Radial shrinkage Reduction in cross-sectional dimensions Reduction in cross-sectional dimensions

11 Shrinkage Specify correctly Detail to avoid problems Specify correctly Detail to avoid problems Large timber - large splits Restraint of seasoned timber - splits

12 Knots - part of a branch extending from pith Checks - small surface cracks, often caused in drying Included bark - pockets with no wood fibres Others - pith, resin pockets, shakes... Natural growth characteristics Natural Growth Characteristics Appearance enhanced - timber shows its character Strength decreased: dependent on size and location of characteristic Application dictates selection of ‘clear’ ( few characteristics) Clear Feature ‘feature’ ( conspicuous characteristics)

13 Natural features in Sawn Timber Slope of grain Especially at edges - low strength perpendicular to grain decreases strength at angle to grain Knots contain weak juvenile wood, cause slope of edge Centre knots Arris knot edge knot

14 Natural features and Properties l Knots discontinuity of grain at edge cause slope of grain at an edge often reduce strength and stiffness l Gum and resin veins less connection across grain lower shear strength and stiffness l Pith and core wood contain weak juvenile wood Included bark l Checks less connection across grain reduced shear strength and stiffness

15 Producers minimise problems by good cutting practice quality control - grading Utility of Sawn Timber Trees are prestressed Cutting boards from trunks causes stress relief & slow change in shape of boards Bent trees can cause slope of grain in products Spring is a problem for all timber cup bow twist spring

16 Summary - Properties of Timber  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)  Structural: Strength (tension, compression, bending, shear, bearing) - stronger parallel to grain Stiffness (MoE) - stiffer parallel to grain Reflect grain structure, slope of grain, features in timber Specification: structural grade and species


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