Characteristics of timber Relationship to properties

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					    Characteristics of timber -
    Relationship to properties
       Maximise performance of timber


      Intuitive understanding of timber behaviour

     Knowledge of Properties and Performance

•   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
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
   Microstructure of Timber
                  vessels
                                                earlywood
                            fibres
                                             rays

                                     cells


                                      latewood
                                                                   Grain
                                      rays
                                                                   direction


                                             softwood       rays
                   hardwood

• Cells - fibres - mainly longitudinal orientation
• Bound together with rays
• Higher strength and stiffness parallel to grain
                          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
       Spirally                              fibres
       wound fibres
Direction of Strength and
        Stiffness

                                       Direction of grain




   Strong parallel to grain & Stiff parallel to grain




           Weak perpendicular to grain
Moisture in Wood Cells
             100%                 Growing
                                  tree

Unseasoned
  timber
                                free water

             25%                 bound water
 Partially
seasoned           fibre saturation
  timber
                                 removed
                                 bound
Seasoned                         water
             15%
 timber
 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
• Seasoning - process of removing moisture from
               timber
   • Kiln drying
     (steam, LPG gas, solar)
   • Air drying
   • Other - chemical, microwave.
    Equilibrium Moisture content
               (emc)
• Wet atmosphere / Dry wood  moisture moves to wood



• 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 conditioned   emc 8% - 10%
                         Indoor heated            emc 8% - 12%
                         External - coastal       emc 14% - 18%
                         External - inland        emc 10% - 15%
           Specification of Moisture Content
       Usually specified as Seasoned or Unseasoned
• Seasoned timber:
   • mc < 15% - close to emc indoors
   • will shrink & swell slightly as humidity changes
• Everything else:
   • sold as Unseasoned timber
   • shrinks on further drying
• Effect of mc on properties: reducing mc causes
    an increase in
   • strength
                                             •    dimensions

   • stiffness (reduced creep)                 A decrease in
   • durability (reduced risk of attack)       dimensions b & d
                                               (shrinkage mainly
   • effectiveness of coatings
                                               perp. to grain)
   Shrinkage                            Radial
                                       shrinkage                  Tangential
                                                                  shrinkage
Loss of moisture in range mc <25%


Reduction in cell wall thickness

                                                       Longitudinal
        Reduction in                                    shrinkage
 cross-sectional dimensions
                                    Shrinkage from 25% to 12%

                               Radiata Pine      Rad 3.5%   Tang 4%
                               Hoop Pine         Rad 2.5%   Tang 3.5%
                               Cypress           Rad 3.5%   Tang 4%

                               Spotted Gum       Rad 4.5%   Tang 6%
                               Karri             Rad 4.5%   Tang 10%
                               Sydney Blue Gum   Rad 5%     Tang 9%
                               Grey Iron Bark    Rad 5.5%   Tang 7.5%
                               Mountain Ash      Rad 6.5%   Tang 13%
    Shrinkage

Large timber -
large splits
                     Restraint of
                 seasoned timber
                          - splits
   •Specify correctly
•Detail to avoid problems
   Natural growth characteristics
• Application dictates selection of
  ‘clear’ (few characteristics)
   ‘feature’ (conspicuous characteristics)
• Natural Growth Characteristics
   • Appearance enhanced - timber shows its          Clear
     character
   • Strength decreased: dependent on size and
     location of characteristic

   • 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...        Feature
         Natural features in Sawn Timber
Knots
contain weak juvenile wood,
cause slope of grain @ edge

                                              Centre knots



   edge knot




 Arris knot                                   Slope of grain
        Especially at edges - low strength
        perpendicular to grain decreases strength at angle to grain
    Natural features and Properties
                                Knots
                                 •discontinuity of grain at edge
                                 •cause slope of grain at an edge
                                 •often reduce strength and stiffness
                                                         Included bark
   Gum and resin veins
     •less connection across grain
     •lower shear strength and stiffness
   Checks
    •less connection across grain
    •reduced shear strength and stiffness

                                               Pith and core wood
                                                •contain weak juvenile wood
         Utility of Sawn Timber
                     • Trees are prestressed
                     • Cutting boards from trunks
                       causes stress relief & slow
                       change in shape of boards
cup                  • Bent trees can cause slope of
                       grain in products
bow                  • Spring is a problem for all
                       timber
twist
                    Producers minimise
spring              problems by
                    • good cutting practice
                    • quality control - grading
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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posted:3/28/2011
language:English
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