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Properties of Timber
Lec: Wood 3
Moisture Content
Moisture content (M.C.) =
Mwet – Mdry
Mdry
x 100%
Where Mwet is the mass of the wet wood and Mdry is the mass of the sample
after oven drying at 105oC (to constant mass).
The moisture content of green wood (in living
trees) may range between 30 to 200%
depending on the species and the season.
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Equilibrium Moisture Content
Wood is a hygroscopic material. It looses or absorbs water until its
moisture content is in equilibrium with the surrounding air. This is
called the equilibrium moisture content (EMC) of wood. It is a
function of temperature and relative humidity.
Moisture Content
Moisture in wood may exist as free
water in the cavities of the cell or it
may be held (bound water) within the
cell walls by adsorption forces.
The fibre saturation point (FSP)
Young et al. 1998
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Moisture Content
Changes in the moisture
content above the FSP do
not generally affect the
dimensions of the wood.
In other words, changes in
the amount of free water in
the cell cavities does not
lead to significant volume
changes.
Moisture Content
As the moisture content decreases below the FSP and water is lost
from the cell walls, there is a reduction in volume – i.e. shrinkage
occurs. The process is reversible.
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Free water: liquid filling
the wood cell cavities
Bound water: liquid or
vapour chemically
bound by hydrogen
bonding to the cellulose
of the wood cell walls
http://timber.ce.wsu.edu/Supplements/Moisture/moisture%20page2.htm
http://timber.ce.wsu.edu/Supplements/Moisture/moisture%20page2.htm
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http://timber.ce.wsu.edu/Supplements/Moisture/moisture%20page2.htm
http://timber.ce.wsu.edu/Supplements/Moisture/moisture%20page2.htm
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http://timber.ce.wsu.edu/Supplements/Moisture/moisture%20page2.htm
Shrinkage
The volumetric shrinkage that occurs is approximately
proportional to the volume of water lost below the FSP and can
be predicted from the following equation:
S M 2  M1 
M 2  M1
 SG  D
FSP
S M 2  M 1 = shrinkage between moisture contents M2 and M1
SG  D
= shrinkage between green and oven-dry conditions
FSP
= fibre saturation point (assume ~ 30% if no data)
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Species
Radiata
Douglas Fir
Rimu
Beech
% shrinkage from green to
12% m.c.
Tangential Radial
3.9
2.1
4.9
2.8
4.2
3.0
7.1
3.3
SM2-M1 =
M2 – M1
FSP - 12
Fibre
Saturation
Point
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27
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xS
S M 2  M 1 = shrinkage between moisture contents M2 and M1
S
= shrinkage between green and stated conditions
FSP
= fibre saturation point (assume ~ 30% if no data)
Shrinkage
The loss of water from the cell walls results in compaction of the
microfibrils, which causes them to bunch together. The greatest
amount of shrinkage occurs in the tangential direction and the total
shrinkage in this direction (from green to oven dry) ranges from 5 to
12%.
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Shrinkage
Slightly less shrinkage occurs in the radial direction (about 2/3
of the tangential shrinkage).
Shrinkage
Very little shrinkage (< 0.5%) occurs in the longitudinal
direction.
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Effect of Moisture Conditions
Lumber Processing: Sawing
– Live (plain) sawing – most rapid and economic
– Quarter sawing – maximum amount of prime (vertical)
cuts
– Combination – provides range of products
Live (Plain) Sawing
Quarter Sawing
Combination
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•Three types of board cut
–Flat-sawn (grain is <45o from flat side)
•worst quality, most problems and defects
–Rift-sawn (45o-80o)
–Quarter-sawn (vertical- or edge-sawn) (80o-90o)
•best quality, least shrinkage problems
Flat-Sawn
Rift-Sawn
Quarter-Sawn
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Differential Shrinkage
The different shrinkage behaviour in the radial and tangential
directions can result in considerable distortion (warping, twisting)
of the wood as it dries. The extent of distortion depends on how
the timber was cut in relation to the grain.
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Lumber Processing: Seasoning (Drying)
•Green wood has 30-200% moisture content
•~12-15% when it leaves the mill
•Methods of Seasoning
–air drying (cheap & slow)
–kiln drying (fast & expensive)
–usually a combination
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Lumber Processing: Surfacing (Planing)
•Surfacing takes approximately 2-5 mm (or more) from
each side
•Nominal sizes refer to the rough-sawn (unsurfaced)
dimensions of the lumber
•For example, the actual dimensions of a (50 x 100 mm) are
(45 x 90 mm)
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Features of Sawn Timber
Visible features in sawn timber caused by:
–natural wood growth
–seasoning too fast
–wood diseases
–animal parasites
–faulty processing
Affect both appearance & mechanical properties. In
order of importance:
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Knots – result of wood grain flowing into the branches of a
living tree.
Will result in a reduction in strength and stiffness due to:
•
loss of load carrying cross section
•
Fibres in area of knots are distorted
•
Checking or slitting often occurs around knots
•
Greatest impact on tensile strength
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Knot resulting in localized
tensile stresses perpendicular
to grain
Sloping grain – Will reduce strength and stiffness but has
largest effect on tensile strength. Not always easy to
detect. Has several causes:
•
grain was disturbed locally in growing tree due to
branch
•
board sawn parallel to pith but log had a significant
taper
•
log had fibers growing in a spiral direction about the
trunk
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Corewood (juvenile wood). Refers to the wood within
the first 5 to 10 growth rings from the centre. Corewood
is less dense than the rest of the wood and may contain
many small knots.
Not a problem in all
species of tree but it is
significant in radiata pine.
wood
heartwood
sapwood
Reaction wood – Also known as compression wood in
softwoods, forms on the lower side of a leaning trunk.
Density may be 30 – 40% greater than normal wood.
After sawing can lead to significant shrinkage and
warping.
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Checks (cracks) are an almost unavoidable consequence of
differential drying in larger pieces of timber.
Shake is a separation occurring between annual growth
rings.
Wane a lack of wood at the corner of a board.
Grading for structural timber
Sawn timber generally has large variability in strength and
stiffness between individual pieces.
Two types of grading system:
•Pre-sorted – timber boards are sorted to some preestablished rules. Typically visual grading and design
stresses determined based on representative testing.
•Strength Class – strength categories established and mill
chooses method for segregating product into different
classes.
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Visual Grading
Widely used practice of segregating timber for different
uses. Grading rules limit size of knots and other visible
defects according to the reduction they make in the
properties of clear defect free timber.
Consider NZS 3603 - No. 1 framing grade requires no knot
or combination of knots should occupy more than 1/3rd
the cross section of the piece.
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In radiata pine there is poor correlation (R2 = 0.13)
between knot cross section and strength.
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Machine Grading
Uses a machine to assess the strength of each piece of
board. Either apply a constant load and measure
deflection or apply a constant deflection and measure
load.
Constant load plank grader – Timber bent on the flat with
application of a constant load and the deflection is
measured.
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Alternative: Acoustic grader – Sends a sonic stress wave
through the piece and computes the MoE.
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