Timber – Its Properties, Classifications, Parts, Seasoning, Preservation, Uses of Timber in Construction Works and Defects in Timber

Timber refers to wood used for construction works. In fact the word timber is derived from an old English word ‘Timbrian’ which means ‘to build’. A tree that yields good wood for construction is called ‘Standing Timber’. After felling a tree, its branches are cut and its stem is roughly converted into pieces of suitable length, so that it can be transported to timber yard. This form of timber is known as rough timber.

By sawing, rough timber is converted into various commercial sizes like planks, battens, posts, beams etc. Such form of timber is known as converted timber. Timber was used as building material even by primitive man. Many ancient temples, palaces and bridges built with timber can be seen even today.

 

PROPERTIES OF A GOOD TIMBER

The Properties of good timbers are:

[1] COLOUR

A good timber should have a uniform Colour.

[2] ODOUR

A good Timber should have a pleasant Odour when cut freshly.

[3] SOUNDNESS

A clear ringing sound when struck indicates the timber is good.

[4] TEXTURE

The Confidence is when you’re able to compliment people without feeling insecure of good timber is fine and even.

[5] GRAINS

A good timber has closed grains.

[6] DENSITY

The higher the Timber Density the Stronger the Timber.

[7] HARDNESS

Harder timbers are strong and durable.

[8] WARPING

A Good timber do not warp under changing environmental conditions.

[9] TOUGHNESS

Timber should be capable of resisting shock loads.

[10] ABRASION

Good timber do not deteriorate due to wear. This property should be looked into, if timber is to be used for flooring.

[11] STRENGTH

Timber should have high strength in bending, shear and direct compression.

[12] MODULUS OF ELASTICITY

Timber with higher modulus of elasticity are preferred in construction.

[13] FIRE RESISTANT

A good timber should have high resistance to fire.

[14] PERMEABILITY

A Good timber has low water permeability.

[15] WORKABILITY

Timber should be easily workable. It should not clog the saw.

[16] DURABILITY

Good timber is one which is capable of resisting the action of fungi and insects attack.

[17] DEFECTS

A Good timber is free from defects like dead knots, shakes and cracks.

 

CLASSIFICATIONS OF TIMBERS

Various bases are considered for the classification of timbers.

[1] CLASSIFICATION BASED ON MODE OF GROWTH

The Mode of Growth of Timber can also be divided into the following

{1} EXOGENOUS

These trees grow outward by adding distinct consecutive ring every year. These rings are known as annual rings. Hence it is possible to find the age of timber by counting these annual rings. These trees may be further divided into

  • CONIFEROUS TREES

Coniferous trees are having cone shaped leaves and fruits. The leaves do not fall till new ones are grown. They yield softwood.

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  • DECIDUOUS TREES

Deciduous trees are having broad leaves. These leaves fall in autumn and new ones appear in springs. They yield strong wood and hence they are commonly used in building construction. The classification as softwood and hardwood have commercial importance.

{2} ENDOGENOUS

These trees grow inwards. Fresh fibrous mass is in the innermost portion. Examples of endogenous trees are bamboo and cane. They are not useful for structural works.

[2] CLASSIFICATION BASED ON MODULUS OF ELASTICITY

Young’s modulus is determined by conducting bending test. On this basis timber is classified as:

  • Group A: MOE = 12.5 kN/mm²
  • Group B: MOE = 9.8 kN/mm² to 12.5 kN/mm²
  • Group C: MOE = 5.6 kN/mm² to 9.8 kN/mm²
[3] CLASSIFICATION BASED ON DURABILITY

Durability tests are conducted by the forest research establishment. They bury test specimen of size 600 × 50 × 50 mm in the ground to half their length and observe their conditions regularly over several years. Then timbers are classified as:

{1} High Durability

If average life is more than 10 years.

{2} Moderate durability

Average life between 5 to 10 years.

{3} Low durability

Average life less than 5 years.

[4] CLASSIFICATION BASED ON GRADING

IS 883-1970 classifies the structural timber into three grades-select grade, grade I and grade II. The classification is based on permissible stresses, defects etc.

[5] CLASSIFICATION BASED ON AVAILABILITY

Forest departments classify timbers based on the availability as

  • X — Most common. 1415 m³ or more per year.
  • Y — Common. 355 m³ to 1415 m³ per year
  • Z — Less common. Less than 355 m³ per year.

 

PARTS OF A TIMBER

[1] PITH

Pith is the innermost part of the tree and hence the oldest part of exogeneous tree when the plant becomes old, the pith dies and becomes fibrous and dark. It varies in size and shape.

[2] HEART WOOD

This is the portion surrounding pith. It is dark in colour and strong. This portion is useful for various engineering purpose. This is the dead part of wood. It consists of several annular rings.

[3] SAPWOOD

It is the layer next to heart wood. It denotes recent growth and contains sap. It takes active part in the growth of trees by allowing sap to move in upward direction. The annual rings of sapwood are less sharply divided and are light in colour. The sapwood is also known as alburnum.

[4] CAMBIUM LAYER

It is a thin layer of fresh sap lying between sapwood and the inner bark. It contains sap which is not yet converted into sapwood. If the bark is removed and cambium layer is exposed to atmosphere, cells cease to be active and tree dies.

[5] INNER BARK

It is a inner skin of tree protecting the cambium layer. It gives protection to cambium layer.

[6] OUTER BARK

It is the outer skin of the tree and consists of wood fibres. Sometimes it contains fissures and cracks.

[7] MEDULLARY RAGS

These are thin radial fibres extending from pith to cambium layer. They hold annular rings together. In some of trees they are broken and some other they may not be prominent.

 

SEASONING OF TIMBER

Seasoning is a process by which moisture content in a freshly cut tree is reduced to a suitable level. By doing so the durability of timber is increased. The various methods of seasoning used may be classified into:

[1] NATURAL SEASONING

 

Natural Seasoning may be air seasoning or water seasoning. Air seasoning is carried out in a shed with a platform. On about 300 mm high platform timber balks are stacked as shown in Care is taken to see that there is proper air circulation around each timber balk. Over a period, in a natural process moisture content reduces. A well seasoned timber contains only 15% moisture.

This is a slow but a good process of seasoning. Water seasoning is carried out on the banks of rivers. The thicker end of the timber is kept pointing upstream side. After a period of 2 to 4 weeks the timber is taken out. During this period sap contained in the timber is washed out to a great extent. Then timber is stalked in a shed with free air circulation.

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[2] ARTIFICIAL SEASONING

In this method timber is seasoned in a chamber with regulated heat, controlled humidity and proper air circulation. Seasoning can be completed in 4 to 5 days only. The different methods of seasoning are:

{1} BOILING

In this method timber is immersed in water and then water is boiled for 3 to 4 hours. Then it is dried slowly. Instead of boiling water hot steam may be circulated on timber. The process of seasoning is fast, but costly.

{2} KILN SEASONING

Kiln is an airtight chamber. Timber to be seasoned is placed inside it. Then fully saturated air with a temperature 35°C to 38°C is forced in the kiln. The heat gradually reaches inside timber. Then relative humidity is gradually reduced and temperature is increased, and maintained till desired degree of moisture content is achieved.

The kiln used may be stationary or progressive. In progressive kiln the carriages carrying timber travel from one end of kiln to other end gradually. The hot air is supplied from the discharging end so that temperature increase is gradual from charging end to discharging end. This method is used for seasoning on a larger scale.

{3} CHEMICAL SEASONING

In this method, the timber is immersed in a solution of suitable salt. Then the timber is dried in a kiln. The preliminary treatment by chemical seasoning ensures uniform seasoning of outer and inner parts of timber.

{4} ELECTRICAL SEASONING

In this method high frequency alternate electric current is passed through timber. Resistance to electric current is low when moisture content in timber is high. As moisture content reduces the resistance reduces. Measure of resistance can be used to stop seasoning at appropriate level. However it is costly process. This technique has been tried in some plywood industries but not in seasoning of timber on mass scale.

 

PRESERVATION OF TIMBER

Preservation of timber means protecting timber from fungi and insects attack so that its life is increased. Timber is to be seasoned well before application of preservatives. The following are the widely used preservatives:

[1] TAR

Hot coal tar is applied to timber with brush. The coating of tar protects the timber from the attack of fungi and insects. It is a cheapest way of protecting timber. Main disadvantage of this method of preservation is that appearance is not good after tar is applied it is not possible to apply other attractive paints. Hence tarring is made only for the unimportant structures like fence poles.

[2] PAINTS

Two to three coats of oil paints are applied on clean surface of wood. The paint protects the timber from moisture. The paint is to be applied from time to time. Paint improves the appearance of the timber. Solignum paint is a special paint which protects the timber from the attack of termites.

[3] CHEMICAL SALTS

These are the preservatives made by dissolving salts in water. The salts used are copper sulphate, masonry chloride, zinc chloride and sodium fluoride. After treating the timber with these chemical salt paints and varnishes can be applied to get good appearance.

[4] CREOSOTE

Creosote oil is obtained by distillation of coal tar. The seasoned timber is kept in an air tight chamber and air is exhausted. Then creosote oil is pumped into the chamber at a pressure of 0.8 to 1.0 N/mm² at a temperature of 50°C. After 1 to 2 hours timber is taken out of the chamber.

[5] ASCO

This preservative is developed by the Forest Research Institute, Dehradun. It consists of 1 part by weight of hydrated arsenic pentoxide (As2O5, 2H2O), 3 parts by weight of copper sulphate (CuSO4 ⋅ 5H2O) and 4 parts by weight of potassium dichromate (K2Cr2O7) or sodium dichromate (Na2Cr2O7 ⋅ 2H2O). This preservative is available in powder form. By mixing six parts of this powder with 100 parts of water, the solution is prepared. The solution is then sprayed over the surface of timber. This treatment prevents attack from termites. The surface may be painted to get desired appearance.

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USES OF TIMBER IN CONSTRUCTION WORKS

Timber is used for the following works:

[1] For heavy construction works like columns, trusses, piles.

[2] For light construction works like doors, windows, flooring and roofing.

[3] For other permanent works like for railway sleepers, fencing poles, electric poles and gates.

[4] For temporary works in construction like scaffolding, centering, shoring and strutting, packing of materials.

[5] For decorative works like showcases and furnitures.

[6] For body works of buses, lorries, trains and boats

[7] For industrial uses like pulps (used in making papers), card boards, wallpapers

[8] For making sports goods and musical instruments.

 

DEFECTS IN TIMBER

Various defects which are likely to occur in timber may be grouped into three:

[1] DUE TO NATURAL FORCES

The following defects are caused by natural forces:

{1} KNOTS

When a tree grows, many of its branches fall and the stump of these branches in the trunk is covered. In the sawn pieces of timber the stump of fallen branches appear as knots. Knots are dark and hard pieces. Grains are distorted in this portion. If the knot is intact with surrounding wood, it is called live knot. If it is not held firmly it is dead knot.

{2} SHAKES

The shakes are cracks in the timber which appear due to excessive heat, frost or twisting due to wind during the growth of a tree. Depending upon the shape and the positions shakes can be classified as star shake, cup shake, ring shakes and heart shakes

{3} WIND CRACKS

These are the cracks on the outside of a log due to the shrinkage of the exterior surface.

{4} UPSETS

This type of defect is due to excessive compression in the tree when it was young. Upset is an injury by crushing. This is also known as rupture.

[2] DEFECTS DUE TO DEFECTIVE SEASONING AND CONVERSION

If seasoning is not uniform, the converted timber may warp and twist in various directions. Sometimes honey combining and even cracks appear. This type of defects are more susceptible in case of kiln seasoning. In the process of converting timber to commercial sizes and shapes the following types of defects are likely to arise: chip marks, torn grain etc.

[3] DEFECTS DUE TO FUNGI AND INSECTS ATTACK

Fungi are minute microscopic plant organism. They grow in wood if moisture content is more than 20°C and exposed to air. Due to fungi attack rotting of wood, takes place. Wood becomes weak and stains appear on it. Beetles, marine borers and termites (white ants) are the insects which eat wood and weaken the timber. Some woods like teak have chemicals in their compositions and resist such attacks. Other woods are to be protected by chemical treatment.

 

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