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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