characteristics of timber - relationship to properties

8/6/2019 Characteristics of Timber - Relationship to Properties

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Physiology of timber fibres, cells, grain, growth rings

Moisture content emcand 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


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Performance of TimberAppearance/Structural/Durability

Appearance

Grain and colour

Feature

Dimensional stability & emc%

Structural

Essential e.g. strengthand stiffness

Utility e.g. dimensional stability

- shrinkage/emc Straightness - bow, spring, cupand twist

urability

Biological hazards

Natural resistance / treatment


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Microstructure of Timber

Cells - fibres - mainly longitudinal orientation

Bound together with rays

Higher strength and stiffness parallel to grain

rays

rays

cells

fibres

vessels

hardwood

earlywood

rays

latewood

softwood

Graindirection


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Cells

Chemical components of wood - products of photosynthesis

Cellulose - network of molecules

cell walls - microfibrils - fibrous

Lignin - gel - acts as bonding agent which glues cellstogether

Hemicellulose - cross linking - binds cellulose into the cell

StraightfibresSpirally

wound fibres


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Direction of Strength and

Stiffness

Direction of grain

Strong parallel to grain & Stiff parallel to grain

Weak perpendicular to grain


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Moisture in Wood Cells

removed

bound

waterSeasoned

timber

15%

100%

Unseasoned

timber

Growing

tree

free water

25%

fibre saturation

bound waterPartially

seasonedtimber


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Seasoning - process of removing moisture fromtimber

Kiln drying

(steam, LPG gas, solar)

Air drying

Other - chemical, microwave.

Moisture in Timber

Moisture content (mc) = weight waterweight wood

in growing tree - mc= 50% to > 100%

felled tree - mcbegins 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


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Wet atmosphere / Dry wood p moisture moves to wood

Equilibrium Moisture content(emc)

Dry Atmosphere / Wet woodp moisture moves from wood

Wood at emcp no moisture movement to / from wood

Moisture in wood atequilibrium with

moisture in atmosphere

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

Indoor heated emc 8% - 12%

External - coastal emc 14% - 18%

External - inland emc 10% - 15%

Indoor air conditioned emc 8% - 10%

Indoor heated emc 8% - 12%

External - coastal emc 14% - 18%

External - inland emc 10% - 15%


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Specification of Moisture Content

Seasoned timber:

mc< 15% - close to emcindoors

will shrink & swell slightly as humidity changes

Usually specified as Seasoned or Unseasoned

dimensionsA decrease in

dimensions b & d

(shrinkage mainly

perp. to grain)

Ev

erything else: sold as Unseasoned timber

shrinks on further drying

Effect ofmcon properties: reducing mc causes

an increase in

strength

stiffness (reduced creep)

durability (reduced risk of attack)

effectiveness of coatings


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Shrinkage

Specify correctly

Detail to avoid problems

Large timber -

large splits

Restraint ofseasoned timber

- splits


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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 andlocation of characteristic

Application dictates selection of

clear (few characteristics)

Clear

Feature

feature (conspicuous characteristics)


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Natural features in Sawn Timber

Slope of grainEspecially at edges - low strength

perpendicular to grain decreases strength at angle to grain

KnotsKnotscontain weak juvenile wood,

cause slope of grain @ edge

Centre knots

Arris knot

edge knot


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Natural features and Properties

Knots

discontinuity of grain at edge

cause slope of grain at an edge

often reduce strength and stiffness

Gum and resin veins

less connection across grain

lower shear strength and stiffness

Pith and core wood

contain weak juvenile wood

Included bark

Checksless connection across grain

reduced shear strength and stiffness


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Producers minimiseproblems 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 ofgrain in products

Spring is a problem for all

timber

cup

bow

twist

spring


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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

characteristics of timber - relationship to properties

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