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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

• Structural properties of timber 

Characteristics of timber -

Relationship to properties

nowledge of Properties and Performance

!ntuiti"e understanding of timber beha"iour 

Maximise performance of timber 

Maximise performance of timber 

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Performance of #imber  $ppearance%Structural%&urability

• Appearance• 'rain and colour • (eature• &imensional stability ) emc *

• Structural• +ssential eg strength

and stiffness• tility eg dimensional stability

 - shrinkage%emc 

• Straightness - bow, spring, cupand twist

• Durability• .iological ha/ards

• Natural resistance % treatment

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Microstructure of #imber 

• Cells - fibres - mainly longitudinal orientation

• .ound together with rays

• 0igher strength and stiffness parallel to grain

  rays

  rays

cells

fbres

vessels

hardwood

earlywood

rays

latewood

sotwood

'raindirection

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Cells

  Chemical components of wood - products of photosynthesis

•  Cellulose - network of molecules

cell walls - microfibrils - fibrous

•   Lignin - 1gel2 - acts as bonding agent which 1glues2 cellstogether 

•  Hemicellulose - cross linking - binds cellulose into the cell

 

Straight fibresSpirally

wound fibres

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&irection of Strength and

Stiffness

&irection of grain

Strong parallel to grain & Stiff parallel to grain

Weak perpendicular to grain

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Moisture in 3ood Cells

remo"ed

bound

water Seasoned

timber 

15%

455*

Unseasoned

timber 

Growing

tree

free water 

25%

fibre saturation

bound water Partially

seasoned timber 

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• Seasoning - process of remo"ing moisture from  timber 

• iln drying6steam, 7P' gas, solar8

• $ir drying

• 9ther - chemical, microwa"e

Moisture in #imber 

• Moisture content 6mc 8 : weight water   weight wood

• in growing tree - mc : ;5* to < 455*

• felled tree - mc  begins to decrease

• ibre saturation point 6fsp8 6=>;*8• abo"e fsp - moisture in cell ca"ities lost -< little change

in dimension

• below fsp - moisture in cell wall lost -< shrinkage perpto grain

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• 3et atmosphere % &ry wood → moisture mo"es to wood

+?uilibrium Moisture content6emc 8

• &ry $tmosphere % 3et wood→

 moisture mo"es from wood

• 3ood at emc  → no moisture mo"ement to % from woodMoisture in wood at

equilibrium with

moisture in atmosphere

Typical emcTypical emc Indoor air conditioned   emc  8% - 10%

Indoor heated   emc  8% - 12%

External - coastal  emc

 1% - 18%External - inland   emc 10% - 1!%

Indoor air conditioned   emc  8% - 10%

Indoor heated   emc  8% - 12%

External - coastal   emc 1% - 18%

External - inland   emc 10% - 1!%

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Specification of Moisture Content

• Seasoned timber :

•  mc @ 4;* - close to emc  indoors

• will shrink ) swell slightly as humidity changes

  Usually specified as Seasoned  or Unseasoned 

•  dimensions $ decrease in

dimensions b ) d  

6shrinkage mainly

perp to grain8 

• !"eryt#ing elseA

• sold as nseasoned timber 

• shrinks on further drying

• !ffect of  mc  on properties$ reducing mc causes

an increase in

•   strength

•   stiffness 6reduced creep8

•   durability 6reduced risk of attack8

•   effecti"eness of coatings

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Shrinkage from 25% to 12%

"adiata #ine "ad $!% Tan& %

'oop #ine "ad 2!% Tan& $!%(ypress "ad $!% Tan& %

)potted *um "ad !% Tan& +%

,arri "ad !% Tan& 10%

)ydney lue *um "ad !% Tan& .%

*rey Iron ar/ "ad !!% Tan& !%

Mountain sh "ad +!% Tan& 1$%

      S    o      f     t    w    o    o      d

      0

    a    r      d    w    o    o      d

  Shrinkage

7oss of moisture in range mc @>;*7oss of moisture in range mc @>;*

Reduction in cell wall thicknessReduction in cell wall thickness

7ongitudinalshrinkage

#angential

shrinkage

Radial

shrinkage

Reduction in

cross-sectional dimensions

Reduction in

cross-sectional dimensions

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Shrinkage

•Specify correctly

•Detail to a"oid problems

•Specify correctly

•Detail to a"oid problems

7arge timber -

large splits

Restraint ofseasoned timber

- splits

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• nots & part of a branch eBtending from pith• '#ec(s - small surface cracks, often caused

in drying

• )ncluded bar( - pockets with no wood fibres

• 9thers - pit#* resin poc(ets* s#a(es+++

Natural growth characteristics

• ,atural Growt# '#aracteristics

•  $ppearance enhanced - timber shows itscharacter 

• Strength decreasedA dependent on si/e andlocation of characteristic

• Application dictates selection of 

 ‘clear’ (few characteristics8 

Clear

Feature

‘feature’ (conspicuous characteristics8

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 atural 3eatures in )awn Timber 

Slope of grain!specially at edges & low strengt#

perpendicular to grain decreases strengt# at angle to grain

notsnots

contain weak u"enile wood,cause slope of grain D edge

Centre knots

 $rris knot

edge knot

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Natural features and Properties

  nots •discontinuity of grain at edge

•cause slope of grain at an edge

•often reduce strength and stiffness

  Gum and resin "eins

•less connection across grain

•lower shear strength and stiffness 

  -it# and core wood

•contain weak u"enile wood

!ncluded bark

  '#ec(s

•less connection across grain

•reduced shear strength and stiffness 

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Producer minimiepro!lem !"4  good cutting practice4  #ualit" control $ grading

tility of Sawn #imber 

• .rees are prestressed

• 'utting boards from trun(s

causes stress relief / slow

c#ange in s#ape of boards

• 0ent trees can cause slope ofgrain in products

• Spring is a problem for all

timber 

cup

 bow

twist

sprin&

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  Summary -Properties of #imber 

Appearance$•  Colour, grain, features, smoothness of surface•  Reflect species, growth patterns, history of tree•  SpecificationA species, durability, appearance graded

 Utility$

• &imensional stability 6shrinkage, twist, bow, cup, spring8,surface hardness

• Reflect stress changes with moisture loss, creep• SpecificationA moisture content

6best close to e?uilibrium moisture content8 Structural$

• Strength 6tension, compression, bending, shear, bearing8 -stronger parallel to grain

• Stiffness 6Mo+8 - stiffer parallel to grain• Reflect grain structure, slope of grain, features in timber • SpecificationA structural grade and species