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Engineering materials -Timber

Engineering Materials
WEEN 334
Cross section of a tree
Classification of Timber
• Mode of Growth
Exogens (Exogenous)- these trees grow outwards
» Conifererrous Trees
» Decidous Trees
b) Endogens (Endogenous)- These trees grows inwards
Classification of Timber
• Modulus of Elasticity
• Group A: E=12.5 GPa
• Group B: E=9.8GPa to 12.5GPa
• Group C: E=5.6 GPa to 9.8GPa
• Durability
• High durability
• Moderate durability
• Low durability
Classification of Timber
• Seasoning Characteristics
• High refractory (Class A
• Moderately refractory (Class B
• Non refractory (Class C)
• Grading
• Select Grade
• Grade I
• Grade II
Identification of Timber
• General Properties
• Color- a darker color in wood indicates greater durability
• Odor-it is present only on freshly cut trees
• Hardness- is the ability of wood to withstand indentations
caused by harder bodies
• Density- denser woods are generally stronger
• Grain- depending on the actual alignment the grain may be
straight, spiral, interlocked, wavy or irregular
• Texture- in hardwoods, the texture depends upon the size and
distribution vessels and rays. In softwoods, it is determined by
the size and distribution of tracheid.
Structure of softwoods
• Tracheids- these are hollow, needle shaped units closely packed together
so that a cross section through them resembles a honeycomb.
• Storage tissues (Rays and wood parenchyma cells-these are too small to
be seen individually and are not of much value in wood identification.
• Resin Canals- these are cavities in the wood lined by the parenchyma cells
• Growth Rings- these are normally distinct in softwoods.
Structure of Hardwoods
• Pores or vessels- these are tubular dements running in a vertical direction
and serve to conduct water and nutrients
• Inclusions- the pores or vessels are normally open to enable conduction
but many times particularly in the heartwood, they become filled with
inclusions of various types
• Parenchyma or soft tissues- these are storage tissues and consists of small,
thin walled, rectangular or brick-shaped cells. They are vertically aligned.
• Rays- are groups of horizontally aligned parenchyma cells running in a
radial direction and their function is horizontal conduction and storage.
• Fibres- these are vertically aligned thick walled elements with pointed
tapering ends which makes up the bulk of the weight of the wood.
• Intercellular cells- these are long tubular cavities seen in the horizontal or
vertical directions.
Felling and Conversion of Timber
• Felling and Transportation of Timber
• Felling is done by a clean saw cut as close to the base as possible.
• Transportation of logs to saw mills depends on the infrastructure
available. In Kenya logs are transport by road. In areas with large
rivers such as the Amazon, logs are floated down the river. For long
distance hauling, railway transport is preferred.
• Conversion (Sawing)
• There are 2 main Methods of sawing
1. Ordinary or flat sawing
2. Quarter sawing
Others include
Tangential sawing
Radial sawing
iii. Combination sawing
Conversion of Timber
• Some advantages of Plain sawing
• Shrinks and swells less in thickness
• Surface appearance less affected by round or oval knots compared
to effects of spike knots in quarter sawn boards
• (boards with round or oval knots not as weak as boards with spike
• Shakes and pitch pockets when present extends through fewer
• Figure patterns resulting from annual rings and some other types
of figure brought out more conspicuously
• Is less susceptible to collapse in drying
• Costs less as it is easy to obtain
Conversion of Timber
• Some advantages of Quarter sawing
• Shrinks and swells less in width
• Cups, surface-checks and splits less in seasoning and in use.
• Raised grain caused by separation in annual rings does not
become as pronounced
• Figure patterns resulting from pronounced rays, interlocked and
wavy grain are brought out more conspicously
• Does not allow liquids to pass through readily in some species
• Holds paint better in some species
• Sapwood appears in boards at the edges and the width of such
sapwood is limited by the width of the log
Sawing of Timber
Moisture of Timber
• Moisture Content- it is one of the most important
characteristics of timber that affects its use and its site
• Wood is hygroscopic i.e it picks up or gives up moisture
relative to the humidity and temperature of the surrounding
atmosphere. Changes in MC causes
– changes in strength e.g. Bending strength can increase by 50% in
going from green to a moisture content (MC) found in wood members
in a residential structure.
– Changes in size- shrinks as it dries and swells as it picks moisture with
concomitant warpage potential.
Moisture of Timber
• Moisture content of wood is defined as the weight of water in
wood expressed as a fraction, usually a percentage, of the
weight of ovendry wood.
• Weight, shrinkage, strength, and other properties depend
upon the moisture content of wood
• In trees, moisture content can range from about 30% to more
than 200% of the weight of wood substance.
• In softwoods, the moisture content of sapwood is usually greater than
that of heartwood.
• In hardwoods, the difference in moisture content between heartwood and
sapwood depends on the species.
Green Wood and Fiber Saturation Point
• Moisture can exist in wood as
– liquid water (free water) or water vapor in cell lumens and cavities
– water held chemically (bound water) within cell walls.
• Green wood is often defined as freshly sawn wood in which
the cell walls are completely saturated with water; however,
green wood usually contains additional water in the lumens.
• Fiber Saturation Point is the moisture content at which both the cell
lumens and cell walls are completely saturated with water= the
maximum possible moisture content
• Specific gravity is the major determinant of maximum moisture
• Lumen volume decreases as specific gravity increases, so maximum
moisture content also decreases as specific gravity increases
because there is less room available for free water.
Equilibrium Moisture Content
• The moisture content of wood below the fiber
saturation point is a function of both relative
humidity and temperature of the surrounding air.
• Equilibrium moisture content (EMC) is defined as
that moisture content at which the wood is neither
gaining nor losing moisture
Seasoning of Timber
• Seasoning is the controlled process of reducing the
moisture content of timber so as to make it suitable for
intended use
• Objectives of Seasoning
• To reduce movement of timber, to reduce the
tendency to split
• Make it immune to Fungal and insect attacks
• To increase strength, durability, workability and
• Make the timber receptive to finish like
preservatives, paints and varnish
• Reduce weight and minimize cost of transport
• Reduce Distortion and warping
• Make timber burn more readily if used as fuel
Seasoning of Timber
% Moisture Content
Situation of use
Limit of air seasoned timber
Limit for the occurrence of dry rot
Outdoor furniture
Occasionally heated areas
Heated areas
Very heated areas
Seasoning of timber
• Principles of seasoning
– Factors that control seasoning
1. Relative humidity
2. Rate of air circulation
3. Temperature
• Methods of seasoning
1) Natural/air seasoning
2) Artificial seasoning
i. Kiln seasoning
ii. Chemical seasoning
iii. Electric seasoning
3) Water seasoning
Natural seasoning
• Timber is dried by direct action of air, wind and sun.
• Timber is roughly sawn to size and stacked in open air using
spacers called stickers
• Vertical spacing is achieved by using timber battens
(25mm) of the same species. The piling sticks should be
spaced close enough to prevent bowing (600-900mm) but
to allow free flow of air
• The final stack should be protected from the direct
influence of the elements
• The ends of the beams should ideally be painted to prevent
• Generally, natural seasoning requires a few months up to a
Natural seasoning
• Advantages
– Cheap
– Small labour cost once stack is made
– Environmentally friendly
• Disadvantages
– Very slow
– Large area of space required for a lot of timber
– Only dries timber to approximately 20% MC therefore
leaving it open to some insect and fungal attacks
– Timber so seasoned is only suitable for outdoor joinery
– Splitting is common at the end of the logs causing wastage.
Kiln seasoning
• There are two main types of kilns used in artificial seasoning
– Compartmental kilns
– Progressive kilns
• Both methods rely on a controlled environment to dry out the
timber and require the following factors
– Forced air circulation (fans, blowers etc)
– Humidity control (steam jets)
• The amount and duration of air, heat, and humidity depends
– Species
– Size
– Quantity
• As a general rule, the atmosphere in the kiln will at first be
cool and moist. The temperature is gradually increased and
the humidity decreased until the required MC is attained.
Compartmental Kilns
• This is a single enclosed container or building
• The timber is stacked in the same manner as in natural
• Whole stack is seasoned using a program of settings
(temperature and humidity) until the whole stack is reduced
to the required MC.
Progressive Kilns
• A progressive kiln has the stack on trolleys that progressively
travel through a sequence of chambers
• Each chamber has varying conditions that change the MC of
the timber as it passes through
• Advantage of this system is the availability of a continuous
flow of seasoned timber.
Kiln seasoning
• Advantages
– Faster due to higher temperatures, controlled ventilation
and air circulation
– Achieves lower MC
– Defects associated with drying can be controlled
– Allows more precise rates of drying for various timber
species and thickness of boards
• Disadvantages
– Expensive
– Requires skilled labour
– Uses a lot of energy.
Chemical seasoning
• In chemical seasoning, carbon dioxide, ammonium carbonate
or urea are used as agents for seasoning.
• They are applied in dry state
• These chemicals are more hygroscopic than timber. They
absorb the moisture from the outer layers of the logs. This
creates a vapour pressure gradient and the moisture is
gradually drawn from the inner layers of timber.
• Advantage is that it ensures uniform seasoning.
Electric seasoning
• In this method an electric current is passed through the
timber logs.
Water Seasoning
• In this method, timber logs are kept immersed whole in a
flowing stream of water.
• The sap present in the timber is washed out.
• The logs are then taken our and air dried to remove the free
Seasoning characteristics of timber
• Class A (Highly refractory wood)- these timbers are slow and
difficult to dry if the final product is to be free from cracks and
• Class B (Moderately refractory woods)- these timbers shows a
moderate tendency to crack and split during seasoning
• Class C (non refractory woods)- these timbers can be rapidly
seasoned to be free of defects.
Finding the MC
• A moisture meter is most commonly used to establish the MC of a
particular batch of timber
• These meters are usually attached to 2 probes which send an
electrical signal through the wood.
• Water is a conductor of electricity and the more water present, the
higher the conductivity.
• Manual method of establishing the MC is to remove random
samples from the stack. Each sample is weighed, then placed in an
oven until the moisture has evaporated. The sampled is weighed
• The % MC is calculated from the two weights as
Wet weight-dry weight X 100 =%MC
Dry weight
Seasoning and Shrinkage
• Seasoning causes positive changes such as increase in
strength but also distortion and shrinkage.
• The greatest amount of shrinkage takes place tangentially
along the grain with little loss over the radial direction and
along the length of the board.
• Because of these varying shrinkage rates, tangential boards
tend to cup because of the geometry of the annual rings.
• Outer rings are longer than inner rings and therefore shrinks
Physical properties of Timber
• Specific Gravity (SG):
– Generally, specific gravity (SG) and the major strength
properties of wood are directly related. SG for the usually
used structural species ranges from roughly 0.30 to 0.90.
Higher allowable design values are assigned to those
pieces having narrower growth rings (more rings per mm)
or more dense latewood per growth ring and, hence,
higher SG
• Moisture Content (MC) and Shrinkage:
– wood’s reaction to moisture provides more problems than
any other factor in its use.
– Wood is hygroscopic, which causes changes in strength;
– E.g bending strength can increase by about 50% in going
from green to a moisture content (MC) found in wood
members in a residential structure
Physical properties of Timber
• Moisture Content (MC) and Shrinkage:
– Wood shrinks as it dries, or swells as it picks up moisture,
The swelling and shrinkage processes are reversible and
approximately linear between fiber saturation point and
0% MC.
– Wood decay or fungal stain does not occur when the MC is
below 20%.
– There is no practical way to prevent moisture change in
– Most wood finishes and coatings only slow the process
down. Thus, vapor barriers, adequate ventilation,
exclusion of water from wood, or preservative treatment
are absolutely essential in wood construction.
Physical properties of Timber
• Thermal Properties/Temperature Effects:
• Although wood is an excellent heat insulator, its strength and
other properties are affected adversely by exposure for
extended periods to temperatures above about 40°C.
• The combination of high relative humidity or MC and high
temperatures, as in unventilated attic areas, can have serious
effects on roof sheathing materials and structural elements
over and above the potential for attack by decay organisms.
• At temperatures above 100°C, wood takes on a thermoplastic
• This characteristic, which is rarely encountered in normal
construction, is an advantage in the manufacture of some
reconstituted board products, where high temperatures and
pressures are utilized.
Physical properties of Timber
• Environmentally friendly
• Strong and lightweight
• Safe-Timber has low toxicity and therefore requires
no special safety precautions to work with it
• Easy to install
• Durable
• Comfortable
• Flexible
Mechanical Properties
• Tensile strength-Timber is stronger in tension along the grain
• Compressive Strength- the strength along the grain is
important for columns, props and posts.
• Shear Strength- Shear strength is important in the case of
beams and slabs
• Brittleness is used to describe the property of suffering little
deformations before breaking.
• Cleavability- High resistance for cleavage is important for
nailing and screwing while low spliting strenghth is important
for wood used as firewood
• Hardness- is important when timber is used as paving blocks,
floor bearings and other similar purposes.
Defects in Timber
• Knots- are dark, hard pieces occurring as signs
of branches broken or cut off. They are
classified according to
– Size (pin, small, medium and large)
– Form (round, oval, splay and spike)
– Quality (live, sound, tight and dead)
– Quality (loose, knot hole, decayed knot)
Timber defects-shakes
• Shakes- or cracks caused by rupture of tissues
resulting in partial or complete seperation of
the fibers along the grain
– Star shakes
– Heart shakes
– Cups
– Ring
– Radial
Timber defects
• Rind Galls-these are typical curved swelling
formed upon the trunk of the tree by
successive layers of the sapwood
• Upsets- are clipping or buckling of fibers
caused by crushing of fibers when the trunk is
felled violently over a rocky surface.
Timber defects
• Twisted or spiral grain- fibers or wandering
hearts are caused by the prevalent wind
turning or twisting the trees at its young age
constantly in one direction
• Wind cracks- are shakes on the outside of a
log due to the shrinkage of the exterior
surface caused by the atmospheric influences
Timber defects-due to seasoning
• Bow- a curvature of the timber in the
direction of its length.
• Cup- a curvature in the transverse direction of
the timber
• Twist- a spiral distortion along the length of
the timber
• Spring- a curvature of the timber in its own
Timber defects-due to seasoning
• Case hardening- caused by unequal drying of the
exterior surface under compression and the
interior surface under tension due to rapid drying
• Split- separation of the fibers along the grain and
extends from one end of the plank to the other
• Honey Combing- separation of the fibers in the
interior due to drying stresses
• Collapse- the cells of the timber are flattened due
to excessive shrinkage.
Defects due to manufacturing or use
• Chipped or Torn grain- a defect caused by the
breaking away of timber below the surface of the
dressing by the action of the planning tool
• Chip Mark- indentation on the finished surface of
the timber caused by the chips or other small
pieces of the timber being carried around on the
planning knife edges
• Wane- the original round surface of the tree that
remains on the finished prank.
• Boxed heart- the pith at the centre fully enclosed
within one piece in the process of conversion
Evaluation of defects
• Knots- tend to weaken timber in tension but may
improve the strength in compression
• Checks, splits and shakes- these reduce shearing
resistance of the timber
• Compression wood- increases density and shrinkage
along the grain and decreases shock resistance.
• Tension Wood- it has high longitudinal shrinkage
increasing its tendency to warp and split
• Sap Rot- this results in decreased shock resistance and
increased absorption
• Slope or Grain- this lowers the tensile strength parallel
to the grain and the modulus of rupture which
increases in the case of radial slope of grain
Evaluation of defects
• Loose Grain- weaker than closed grain timber
• Wane- this reduces the mechanical properties and volume
• Worm hole deep large worm holes spoil the appearance
and reduce the mechanical properties of timber
• Pith Pockets-affects the appearance and mechanical
• Boxed heart-in cut sizes of timber, this defect has little or
no effect on quality
• Discoloration of heartwood- has no effect on quality
• Mould-spoils appearance of the wood.
• Sap Stain does not affect the quality
Decay of Timber
• Decay or Rot- is usually caused by the
presence of sap or by the alternate dry and
wet conditions in timber
– Forms of decay
Wet rot
Dry rot
Fungal attack
Insect attack- such as termites, beetles and borers
Preservation of Timber
• Three main methods of preservative application
– Painting
– Dipping
– Pressure/Empty cell process
• In painting and dipping preservatives rarely ingress beyond
1/16’’. Ingress is determined by duration of contact and
temperature of preservative
• In pressure/empty cell process, timber is placed in an air
tight container and a vacuum. Preservative is then pumped
in at 100-200psi. Very good penetration is achieved.
• Timber treated in this manner is mostly used as piles in
water or for railway sleepers.
Name of
Preservative. Guards internal
framing against insect attack
Self diffusing. Water soluble.
Preservative. Guards timber used
externally against fungal and
insect attack.
Water soluble in formulations.
Fixes in wood. Broad spectrum
Low toxicity. Stable in aqueous
formulations in which it forms a
TBTO (tri-butyl
tin oxide)
Preservative. Guards above ground Colourless. Difficult to leach.
timber (e.g. piles) against insect
and fungal attack.
Preservative. Guards timber used
externally (e.g. railway sleepers)
against insect and fungal attack.
Broad spectrum biocide. Good use
of an otherwise undesirable
Preservation of timber
• Characteristics of a good preservative
– Should be toxic to insects and fungi
– Should disperse readily into the timber
– Should be cost effective
– Should be easy to apply
– Should form stable solutions in water (should not
react with water)
– Should be difficult to leach
– Should not increase flammability of timber
Availability and uses of timber
• Advantages of timber
– In terms of specific strength, it can be used for both load
bearing and non load bearing structures
– Can easily be converted to any size and shape
– With the advent of superior adhesives, wastage in timber
construction can be minimized
– Has high resale and salvage value
– It has better insulating properties than most other
construction materials
– When properly seasoned and preserved, it is highly
– It is light weight
– has good aesthetics