7 TIMBER
7.1 Timber: source, types, classification, characteristics, advantages and uses
Timber is a material that has many characteristics that make it a good material for construction
of buildings. The material has a very high strength, especially when compared with its low
weight. The material is, however, very anisotropic with different properties in different
directions due to its make-up of oriented fibers. The strength parallel to the fiber direction is
very good while the strength when loaded perpendicular to the fiber direction is very low. This
low strength perpendicular to the fiber direction needs to be addressed when designing timber
structures.
Properties of timber/wood
Physical Properties :
1. Density
- light — very light: density less than 550 kg/m3
, eg. bamboo
- moderately heavy: density 550 kg/m3 — 750 kg/m3
, eg. deodar
- heavy-very heavy: density greater than 750 kg/m3
, eg. Sal
2. Bulk Density (Defined asthe mass of many particles of the material divided by the total
volume they occupy)
- Depends on air voids present in wood.
3. Moisture Movement
- Wood is liable to shrink or swell with the movement of moisture.
4. Shrinkage :
- Is a Reduction in linear and volumetric dimension due to drying of moisture.
5. Swelling (Increase in size)
- It is increase in linear and volumetric dimension on absorbing moisture.
6. Sound conductivity
- Has high Sound Conductivity
- Sound velocity in wood is 2-17 times greater than in air.
7. Thermal Conductivity
- Has very low thermal conductivity
Mechanical and thermal properties of timber/wood:
As wood is natural material there is different mechanical properties for different wood
Mechanical properties are governed by soil type, climate and other factors
Properties affected by specific gravity, moisture content, knots and other defects and
slope of grain.
Wood can’t resist fire and burnt easily
It will be better to take precautions to save timber structure from thermal action
Characteristics of good timber
It should be free from sap and be from heart of a sound tree.
It should have straight and close fibres.
It should give a clear ringing sound when struck. Dull heavy sound is a sign of internal
decay.
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It should be of uniform dark colour. Light colour usually indicates timber with low
strength.
It should have regular annual rings.
Timbers with narrow annual rings are generally the strongest.
Freshly cut surface should give sweet smell.
It should have bright and smooth surface when planed. Dull appearance is a sign of
defective timber.
Teeth of saw should not get clogged while sawing.
Out of same variety of timber, darker and heavier pieces are stronger.
It should be free from dead knots, from too many knots, shakes or other defects.
It should have firm adhesion of fibers and compact medullary rays.
A good timber should be durable. It should be capable of resisting the actions of fungi,
insects, chemicals etc.
A good timber should be capable of retaining its shape during conversion or seasoning.
It should not bow or warp or split.
A good timber should be capable of offering resistance to shocks due to vibrations.
Advantages of timber
availability: source and transportation
strength: light weight and higher strength
easy to handle and planed easily
joining: simple
Repair and modification: easy
superior, thermal insulation, sound and electrical resistive
uses: furniture and decorative purpose
strong and flexible
corrosion resistance: excellent
higher shock and impact strength .
Quick and easy to build.
Cheaper than other types of building materials.
Easy to heat and cool depending on the weather.
More eco-friendly than other materials
Can last many years if proper maintenance is carried out
Uses of timber
It is mainly used for following categories of works
It is used for construction purposesincluding building construction, houseposts, beams,
rafters, bridges, piles, poles etc.
It is used for furniture and cabinet making.
It is used for light packing cases.
It is also used for heavy packing cases such as machinery and similar stores.
It is used for manufacturing agricultural implement sand tool handles.
It is used for manufacturing veneers, plywoods, boards etc.
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Disadvantages of timber:
Defects in timber can reduce the life of wood.
Affected by relative humidity, fungi, insects
Modification of general arrangement drawings if based on masonry construction
Transportation and carriage access
Exposure to weather before enclosed
vulnerability to decay of timber when exposed to excessive moisture.
If left untreated, timber become water logged and crack or chip.
Timber is more susceptible to fire than other building materials unless treated and
maintained.
Types of timber/wood
Wood is a product obtained from the tree.
In simple classification of tree, it can be classified as:
a. Standing timber - if a tree is living then the timber is termed as Standing Timber.
b. Rough timber - if a tree is fallen then the timber is termed as Rough timber.
c. Converted timber - If a tree is modified into desired size and shape then the timber is
termed as Converted timber.
On the basis of mode of growth, the types of trees are:
1. Endogenous Tree
2. Exogenous Tree
Endogenoustree
Trees grow in inward direction with the addition of fibrous (like of jute) layer at tip
portion, i.e the growth is longitudinal.
Trees are hollow at the core and have flexible stem. e.g-Bambo, coconut, Sugarcane
etc.
For construction purpose only bamboo can be used.
Are very flexible due to which they are not generally used for construction material.
Exogenoustree
Trees increase in bulk by growing outwards and distinct consecutive rings are formed
in the horizontal section of such trees.
These rings are called annual rings, because one such ring added every year, such trees
grow bigger in diameter as well.
Life of trees can be find by counting the annual rings.
e.g – Sisham, Sal, Pine etc.
Timber from these suitable for construction purpose
These are further classified as
1. Deciduous tree
2. Coniferous tree
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Deciduoustree
Are characterized by flat and broad leaves which fall in autumn season and new one
appear in spring season
Hard wood is obtained by such tree which can be used for superior structural and
engineering work.
e.g – Sal, Sisham, Teak etc.
Coniferous tree
Trees are characterized by needle shaped thin leaves which are evergreen
Trees bear conical shaped fruits and yield resins ( viscous substance )
Soft variety of woods are obtained
Used is general constructional purpose
ex Pine, deodar etc
Hard Wood
A hard wood possesses the following characteristics:
(a) The wood is comparatively heavier and is darker in colour.
(b) The annual rings are not distinct.
(c) It contains a large percentage of acid.
(d) It is hard and difficult to work upon.
(e) It resists shearing stresses.
(f) It is close-grained and strong.
(g) It is non resinous.
Shisham, Sal, Teak, Oak, Mahagony and Babul are examples of hard wood.
Soft Wood
A soft wood possesses the following characteristics.
(i) It is light in weight and colour.
(ii) Annual rings are very distinct.
(iii) It is comparatively weaker and splits easily.
(iv) It has straight fibres.
(v) It is resinous, i.e. contains resins and turpentine. It has a peculiar fragrance.
(vi) It is strong for resisting tensile forces.
(vii) It is weak in directions across the grains.
(viii) Its texture is soft and regular.
Spruce, Deodar, Chir, Kail and Walnut etc., are examples of soft wood.
7.2 Growth of a tree
The roots of the tree suck a solution of salts from the soil in spring season. These salts are food
for the tree and roots transmit the same through the trunk of tree to its branches and leaves.
This solution of salts looses some of the moisture because of evaporation and absorbs carbon
dioxide from the air. This action in the presence of sun makes the solution a bit viscous. This
transformed viscous solution is known as sap.
This viscous sap descends below the bark and leaves a thick layer in autumn. Layer of sap left
below the bark getstransformed to wood and is known as cambium layer. A fresh layer is thus
added on the outside of the tree every year forming a new annual ring. The new ring represents
a year’s growth of tree. Medullary rays carry the sap from below the bark to the interior thereby
nourishing the tree.
Structure of a Exogenous tree
A tree basically consists of three parts, viz, trunk, crown and roots.
From the visibility aspects, the structure of a tree can be divided into two categories:
Macrostructure
Microstructure
Macrostructure
The structure of wood visible to the naked eye or at a small magnification is called
macrostructure. Figure 9-1 shows the macrostructure of exogenous tree.
These parts of the cross-section are described below:
Pith or Medulla
It is the first formed portion of the stem of tree and it consists entirely of cellular tissues. The
pith contains a large amount of fluid and nourishes the plant when the plant is young. It dies
up and decays when the plant becomes old and sap is then transmitted by the woody fibres
deposited round the pith.
Annual Rings
The rings of woody fibre arranged in concentric circle around the pith are known as annual
rings because one such ring is added every year.
Heart Wood
Inner most rings surrounding the pith constitute the heart wood. This wood is darker in colour,
stronger, more compact and durable.
Sap Wood
The outer annual rings between heart wood and cambium layer of the tree constitute the sap
wood which transmits the sap from roots to branches. Compared with heart wood, sap wood is
lighter in colour, weaker and more liable to decay. Sap wood is also known as alburnum.
Cambium Layer
Outermost ring between the bark and sap wood which is not yet converted into wood is known
as the cambium layer.
Inner Bark
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The inner skin or layer covering the cambium layer is known as inner bark. It gives protection
to cambium layer from any injury.
Outer Bark
The outer skin or cover of the tree is known as outer bark. It is the outermost protective layer
and it sometimes contains cracks and fissures. It consists of cells of woody fibre and is also
known as cortex.
Medullary Rays
These are thin horizontal veins radiating from the pith towards the bark. They carry sap from
outside to the inner parts of tree and nourish it. They keep the annual rings tightly gripped
together.
Microstructure
The structure of wood apparent only at great magnifications is called microstructure. When
studied under a microscope, it becomes evident that wood consists of living and dead cells of
various sizes and shapes.
3. Defects in timber
Defects are either due to growing phases of tree or defect occurring after cutting trees. Defects
reduce the usefulness of wood as aesthetic appearance as well as strength
1. Heart Shake
This defect usually occurs in over matured trees due to shrinkage of heart wood
These cracks are mostly confined to heart wood portion and diminishing towards
outward.
2. Star Shake
It is confined in the outward portionand diminishing towards center.
It occurs due to frost action and strongheat of sun.
It is the most serious defect because ittends to separate single log into smallpieces.
3. Radial Shake
These are radial cracks extending fromcentre towards bark .
These are similar to star shake but areirregular and numerous.
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It occurs due to drying of sap or moisture.
4. Cup or Ring Shake
Is the separation of tissue along annual rings
If the shake is whole round the annual ring then it is called ring shake and if it is
half way round the ring then it is called cup shake.
It is due to unequal seasoning and change in humidity.
5. Rind Gall
Is the curved irregular swelling on surface.
Due to development of new sap layer on wound left after branches have been
irregularly cut.
New layer fails to unite with old one and decay starts from that point
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6. Knots
This defect occurs due to formation of annual rings at right angles to surface and
also due to twisting of fibers.
The roots of the branches yet embedded(Enclosed firmly) by the tissue and knots
are formed
Knots reduce the aesthetic value and effects tensile strength
Knots may be round or oval shaped
7. Twisted fibers
This defect occurs due to wind velocity constantly turning trees inone direction
only.
8. End Splits
Occurs only at the ends of log due to exposed conditions duringseasoning.
May be prevented by painting the ends of log or covering withmetallic cap.
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9. Druxiness and Foxiness
Druxiness is a term that is defined as a disease that isfound in timber or wood. It is
caused or brought about bya wound located in the layer known as the cambium
which is attacked by fungus.
Foxiness is caused due to poor ventilation during storageor due to over maturity of
the tree
These are stains or patches on the surface of timberwhich destroy the appearance and
may cause decay of timber. Stain may be in form of white patches or mottled reddish
colour.
7.4 Seasoning of timber
The art of seasoning is to extract the moisture under controlled conditions as nearly as possible
at a uniform rate from all parts of timber and to leave the remaining moisture that cannot be
extracted, uniformly distributed throughout the mass.
Irregular drying will cause irregular shrinkage resulting in the setting up of internal stresses
between the fibres. When these stresses become strong enough to overcome the cohesion of
the fibres then the timber warps and shakes are formed.
Objectives of Seasoning
(i) Seasoning makes timber resistant to decay.
(ii) Seasoning makes timber lighter.
(iii) It becomes easier to paint and polish seasoned timber.
(iv) It is easier to treat seasoned timber with preservatives.
(v) Seasoned timber becomes stronger and more stable.
(vi) Seasoning stopsshrinkage of timber on drying.
(vii) Seasoned timber has better electrical resistance.
Methods of Seasoning
1. Natural seasoning or Air Seasoning
2. Artificial seasoning or Kiln Seasoning
3. Water Seasoning
4. Boiling or Steam Seasoning
5. Chemical or Salt Seasoning
6. Electrical Seasoning
Natural Seasoning or Air Seasoning
The log is converted by sawing it into battens and planks etc. as soon as possible after felling
of tree. These are then stacked on a well drained place in the shade. Care should be taken to
ensure free circulation of fresh air all around each piece while stacking. The stacking should
be done on masonry or concrete supports a few centimeters above the ground.
Care should be taken not to expose the freshly converted timber stacked for seasoning to
severe winds or to sun.
This process of seasoning timber is the best as it gives very strong and durable timber, but it
is extremely slow. It takes more than six months for timber to season in moderate climates.
Kiln Seasoning or Artificial Seasoning
This method of seasoning speeds up the seasoning process. This method of seasoning is a
must for large scale production of seasoned timber.
Kiln seasoning is done in a chamber equipped with arrangements for heating and humidifying
the air to required conditions of relative humidity and temperature and for its circulation
across the timber stacked in the chamber for seasoning. Usually, it is steam that is used for
heating and humidifying the air in the kiln. The seasoning of the timber is started at a
comparatively lower temperature and high humidity. As the timber dries, these conditions are
gradually altered until at the end of the seasoning. The temperature of the air inside the
chamber is fairly high and the humidity is low. The kiln charge is allowed to cool inside the
kiln to within 15 to 20o C of the outside temperature before removal. Seasoning of timber by
this method takes about four to five days under normal conditions.
Water Seasoning
Log of wood is kept completely immerged in running stream of water. Sap, sugar etc are
leached out of wood and replaced by water. Then log is kept out of water after 2 to 4 weeks
and placed in air. Quick process and also remove organic matters materials. Elasticity may
reduce and make timber brittle. Boiling of timber in water is another rather quick process. It
takes long time to be dried but less than air seasoning.
Boiling or Steam seasoning
Submersion in boiling water or the application ofsteam speed the drying of wood. This method
is said to cause less shrinkage but it is expensive to use, and reduces the strength and elasticity
of the timber.
Chemical or salt seasoning
Salt seasoning is the submersion of wood in a solution of urea, sodium nitrate or sodium
chloride, all of which act as dehydrating agents. Then the wood is air dried.
Electrical seasoning
Electrical seasoning involves running an electrical current through the timber causing heat to
build up drying the wood. This method is expensive but is fast and uniform quality.
7.5 Preservation of timber
Timber has to be protected from the attack of insects, e.g. white ants etc., and from internal
decay due to dry and wet rots.
Perfect seasoning is the most effective means of preservation. Timber should be so used that
either it is wholly dry and well ventilated or is wholly under water. It will not decay when kept
under water but it will become soft and weak.
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Proper damp proofing of the building and providing free circulation of air around the built in
portions of timber are essential for the preservation of the timber used. However, when these
conditions cannot be obtained then preservatives have to be applied for preservation.
Timber should be well seasoned before the application of preservatives as otherwise the
preservatives would block the pores of timber thereby causing its decay due to the entrapped
moisture. Direct contact with lime mortar should be avoided while using preservative with
masonry.
Methods of Preservation of Timber
Following are some of the common methods of preservation adopted
(i) Charring
(ii) Tarring
(iii) Painting
(iv) Creosoting
(v) Wolman salt
(vi) Ascu treatment
(vii) Fire proofing of timber
Charring
Lower ends of the posts that are to be embedded in ground are generally charred with a view
to prevent dry rot and attack of worms. It is done by quenching the ends of posts in water after
they are charred on wood fire to a depth of 1.5 cm.
Tarring
It consists in coating with tar or tar mixed with pitch. Embedded portions of timber fence posts,
ends of door and window frames, battens and beams built in wall are usually tarred. Tarring is
not done in case of those portions of structural members that are open to view, because of
unsightly black colour.
Painting
A paint when applied to timber acts not only as a good preservative but also it enhances the
appearance of the surface so treated. Only well seasoned timber should be painted as otherwise
the moisture entrapped in the timber, because of the closing of timber bores by paint, would
cause decay. Paints however, protect seasoned timber against moisture thereby prolonging its
life, e.g. soligum paints have excellent preservative properties and protect timber against the
attack of white ants.
Creosoting
Creosote oil is a dark brown thick oily liquid. Thoroughly seasoned timber dried for 24 hrs
before its treatment is placed in an airtight chamber. After the air has been exhausted from this
chamber, the creosote oil is then pumped in at a pressure of 9 kg/ cm2 at a temperature of 50o
Cso long as the timber is not fully saturated with oil. The oil preserves the timber from rot and
from the attacks of white ant.
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Uses
•It is used in case of railway sleepers, piles and transmission poles.
Limitations
•Undesirable colour and smell, inability to take paint well and the tendency to stainplaster
limit its use.
Wolman Salt
This salt consists of creosote and sodium fluoride and is soluble in water.
It is odour less and leaves no stain on wood. After treatment, timber could be painted or
varnished. These salts destroy many kinds of fungi that cause timber to rot. This renders the
timber extremely fire resistant too.
Treatment of timber with zinc chloride, sodium fluoride, magnesium, silico fluoride or copper
sulphate renders the timber immune from the attacks of fungi. The timber so treated is capable
of being painted on drying.
Ascu Treatment
Ascu is available in the form of powder and is made up of three chemicals mixed in the ratios
given below :
(i) 1 part by weight of hydrated arsenic pentaoxide (As2 O5.2H2 O)
(ii) 3 parts by weight of blue vitriol (CuSO4 .5H2 O)
(iii) 4 parts by weight of potassium dichromate (K2 Cr2 O7 .2H2O)
Six parts of this powder are mixed with 100 parts by weight of water. Ascu solution can be
applied or sprayed in two coats.
To achieve better results, timber may be soaked in the solution and impregnated with it under
pressure. The timber should be allowed to dry for three to six weeks. This treatment renders
timber immune to the attacks of white ant.
Ascu treated timber may be painted, varnished, polished or waxed. The solution is odourless.
Fire Proofing of Timber
Timber cannot be made completely fireproof, however, by treating as below it can be made fire
resistant to a sufficient extent.
Soaking timber in ammonium sulphate, ammonium chloride, ammonia phosphate, sodium
arsenate, zinc chloride etc. orspraying on timber, solution ofsodium silicate, potassium silicate
or ammonia phosphate etc. imparts fire resisting properties. Abel’s methods of fire proofing
timber is painting the surface first with a dilute solution of sodium silicate (Na SiO ) then with a
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cream like paste of slaked fat time and in the end with a concentrated solution of silicate of soda.
6. Properties and uses of bamboo
Bamboo, like true wood, is a natural composite material with a high strength-to-weight
ratio useful for structures.
In China and India, bamboo was used to hold up simple suspension bridges, either by
making cables of split bamboo or twisting of sufficiently pliable(workable) bamboo
together.
Bamboo has also long been used as scaffolding; the practice has been banned in China
for buildings over six storeys, but is still in continuous use for skyscrapers in Hong
Kong.
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In Japanese architecture, bamboo is used primarily as a supplemental and/or decorative
element in buildings such asfencing, fountains, grates and gutters, largely due to
theready abundance of quality timber.
(Grates - A barrier that has parallel or crossed bars blocks a passage but admitting air.)
(Gutters - A channel along the eaves or on the roof; collects and carries away rain water)
Can be cut and laminated into sheets and planks (like board, plyboard).
Bamboo intended for use in construction should be treated to resist insects and rot. The
most common solution for this purpose is a mixture of borax and boricacid.
Bamboo has been used as reinforcement for concrete in those areas where it is plentiful,
though dispute exists over its effectiveness in the various studies done on the subject.
Bamboo does have the necessary strength to fulfil this function, but untreated bamboo
will swell with water absorbed from the concrete, causing it to crack. Several
procedures must be followed to overcome this shortcoming.
Tensile strength: Bamboo has higher tensile strength than steel because its fibers run
axially.
Fire Resistance: Capability of bamboo to resist fire is very high and it can withstand
temperature up to 4000 C. This is due to the presence of high value of silicate acid and
water.
Elasticity: Bamboo is widely preferred in earthquake prone regions due to its elastic
features.
Weight of bamboo: Bamboos due to their low weight are easily displaced or installed
making it very easier for transportation and construction.
Unlike other building materials like cement and asbestos, bamboo poses no danger to
health.
They are cost effective and easy to use.
They are especially in great demand in earthquake prone areas.
7.7 Wood based products
Timber which is prepared scientifically in a factory is termed as industrial timber and such
Timber possesses desired shape, appearance strength.
Veneers:
These are thin sheets or slices of 0.40 to 6mm wood of superior quality. Indian timbers,
which are suitable for veneers, are mahagony, oak, rosewood, sissoo, teak etc. The process of
preparing a sheet of veners is known as veneering. Veneers are used to produce plywoods
batten boards and lamin boards. Veneers used for making plywood are known as plies.
Plywoods:
Plywoods are boards, which are prepared from thin layers of wood or veneers. Three or more
veneers in odd number are pressed using adhesives such that the direction of the grains are at
Right angles to each other. The odd numbers of the veneers are used in order that the shrinkage
is symmetrical about the middle ply. The plywoods are used for various purposes such as
ceilings, doors, furniture, partitions, panelling walls, packing cases, railway coaches,
formwork for concrete etc. Thickness may vary from 6 to 25mm.
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Lamin Board
A lamin board is a board having a core of strips, each not exceeding 7 mm in thickness glued
together face to face to form aslab which in term is glued between two or more veneers, with
the direction of the grain of the core strips running at right angles to that of the adjacent outer
veneers. The lamin boards are light, strong and do not split or crack easily. They are used for
walls, ceilings, partitions and packing cases.
Block Board
A block board is constructed in the same way as lamin board. In this case core consists of
smaller timber block upto 25 mm in width. These blocks are cemented edge to edge and on
each face plies upto 3 mm thickness are glued. These are extensively used for construction of
railway carriages, busbodies, marine and river crafts and for furniture making, partitions,
panelling, prefabricated houses etc.
Batten Boards
The batten board is a board having a core made up of strips of wood usually 80 mm wide, each
laid separately or glued or otherwise joined to form a slat which is glued between two or more
outer veneers with the direction of the core running at right angles to that of the adjacent outer
veneers. These boards are used for door panels, table tops etc.
Fibre boards:
These are rigid boards and they are also known as pressed wood or reconstructed wood. The
thickness varies from 3mm to 12mm. These are available in lengths from 3 to 4.5m and width
varying from 12 to 18m. The weight of fibre boards depends on the pressure applied during
manufacture. These are used for:
(i) For internal finish of rooms such as wall panelling; suspended ceilings.
(ii) To construct form work for cement concrete.
(iii) To construct partitions.
(iv) To prepare flush doors, tops of tables etc.
(v) To provide an insulating material of heat and sound.
(vi) To work as paving or flooring material. Impreg timber /impregnated timber Timber which has been made flame-resistant, fungi resistant, or insect-proof by forcing into it under vacuum or pressure a flame retardant or a fungal or insect poison. Compreg timber Compreg timber are processed wood whose cells are impregnated with a resin and compressed, to reduce shrinking and swelling and to increase density and strength. Compreg is a very special wood composite material with exquisite mechanical properties. Compreg indicates that it is both impregnated and compressed. Used in applications where long durability, hardness, and dimensional stability is required e.g. gears, rolls and wear strips of industrial conveyors, woodworking machine tables. Regardless of its hardness, it is comparatively easy to machine, which makes it an excellent choice for manufacturers of exclusive woodworking products.