Characteristics of timber - Relationship to properties

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					    Characteristics of timber -
    Relationship to properties
         Maximise performance of timber
         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



                                                     Grain
                                                     direction




• 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
             Typical emc Indoor air conditioned   emc 8% --10%
                                                   emc 8% 10%
                         Indoor heated
                          Indoor heated           emc 8% --12%
                                                   emc 8% 12%
                         External --coastal
                          External coastal        emc 14% --18%
                                                   emc 14% 18%
                         External --inland
                          External inland         emc 10% --15%
                                                   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%
 Loss of moisture in range mc <25%


Reduction in cell wall thickness
Reduction in cell wall thickness

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


                                              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
     •Specify correctly
•Detail to avoid problems
 •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
                             l   Knots
                                  •discontinuity of grain at edge
                                  •cause slope of grain at an edge
                                  •often reduce strength and stiffness
                                                          Included bark
l   Gum and resin veins
     •less connection across grain
     •lower shear strength and stiffness
l   Checks
     •less connection across grain
     •reduced shear strength and stiffness

                                             l   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:7/12/2013
language:English
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Jun Wang Jun Wang Dr
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