In this article we will discuss about:- 1. Meaning of Timber 2. Trees for Life 3. Fire-Resistance 4. Qualities 5. Factors Affecting the Strength 6. Decay 7. Storage 8. Market Forms 9. Advantages 10. Uses.
Meaning of Timber:
The word timber is derived from an old English word timbrian which means to build. The timber thus denotes wood which is suitable for building or carpentry or various other engineering purposes and it is applied to the trees measuring not less than 600 mm in girth or circumference of the trunk.
The timber or wood, as a building material, possesses a number of valuable properties such as low heat conductivity, amenability to mechanical working, small bulk density, relatively high strength, etc.
However it has also its own drawbacks such as susceptibility to decay and inflammability, fluctuations in properties due to changes in moisture content, variations in strength in length and across fibres, etc. These shortcomings of timber can greatly be reduced by the application of some of the modern wood processing techniques.
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At present, it has become possible to have effective utilization of wood waste e.g. sawdust and shavings are used with admixture of organic glues to make fibre-slabs, Fiber boards, etc.
In addition to the above, the wood is also used in the manufacture of various other products such as organic acids, rosin, paper, cardboard, cellulose, etc. Thus the consumption of wood in building industry should be carried out in the best possible economic way.
The wood is the most unusual and valuable raw material of the building industry. There is acute shortage of this critical building material in the country. It is necessary that instead of curtailing or banning its use in the building industry and advocating usage of energy intensive materials like steel, aluminium and plastic, suitable alternatives should be found out to augment the availability of wood for the construction industry.
For instance, the traditional forest species take about 50 years or even more to yield tree of adequate girth capable of producing quality timber. On the other hand, there are several fast grown species which mature in 10 to 20 years and can produce wood to augment wood supply. These species are Babul, Eucalyptus, Poplar and Sissoo which have been found suitable under various climatic and soil conditions.
Trees for Life:
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The trees are the tallest living things and the man chops down forests without replanting for tomorrow. The forests are vital for the economic prosperity of land. The man and trees are closely linked and without trees, the life on our planet would be endangered.
The powers of tree can be briefly enumerated as follows:
(i) It conserves soil moisture.
(ii) It precipitates rainfall and lowers temperature. A single tree can cool the summer heat for an entire day and night and is found better than 20 air-conditioners running for 20 hours.
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(iii) It prevents depletion of the ozone layer of the atmosphere.
(iv) It prevents soil erosion.
(v) It produces oxygen and purifies the air. It is estimated that a hectare of trees will produce about 10 tonnes of oxygen which is enough for 45 persons to live for one year.
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For the engineering purposes, the trees are classified according to their mode of growth.
Following is the classification:
(1) Exogenous Trees:
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These trees increase in bulk by growing outwards and distinct consecutive rings are formed in the horizontal section of such a tree. These rings are known as the annual rings because one such ring is added every year and these rings are useful in predicting the age of tree. The timber which is mostly used for engineering purposes belongs to this category.
The exogenous trees are further subdivided into two groups:
(i) Conifers; and
(ii) Deciduous.
The conifers are also known as the ever-green trees and leaves of these trees do not fall till new ones are grown. As these trees bear cone-shaped fruits, they are given the name conifers. These trees yield soft woods which are generally light coloured, resinous, light in weight and weak. They show distinct annual rings.
The deciduous trees are also known as the broad-leaf trees and leaves of these trees fall in autumn and new ones appear in spring season. The timber for engineering purposes is mostly derived from deciduous trees. These trees yield hard woods which are usually close-grained, strong, heavy, dark coloured, durable and non- resinous. They do not show distinct annual rings.
Soft Woods and Hard Woods:
The soft woods form a group of ever-green trees. The hard woods form a group of broad-leaf trees. It is to be noted that the terms soft woods and hard woods have commercial importance only. It is quite likely that some variety of soft wood may prove to be stronger than some variety of hard wood. Table 9-1 is prepared to show points of differences between soft woods and hard woods.
The examples of soft woods are chir, deodar, fir, kail, pine, spruce, etc. and those of hard woods are babul, mahogany, oak, sal, teak, etc.
(2) Endogenous Trees:
These trees grow inwards and fibrous mass is seen in their longitudinal sections. The timber from these trees has very limited engineering applications. The examples of endogenous trees are bamboo, cane, palm, etc.
Structure of a Tree:
A tree basically consists of three parts, namely, trunk, crown and roots. The function of the trunk is to support the crown and to supply water and nutrients from the roots to the leaves through branches and from the leaves back to the roots. The roots are meant to implant the trees in the soil, to absorb moisture and the mineral substances it contains and to supply them to the trunk.
From the visibility aspect, the structure of a tree can be divided into two categories:
(I) Macrostructure
(II) Microstructure.
(I) Macrostructure:
The structure of wood visible to the naked eye or at a small magnification is called the macrostructure. Fig. 9-1 shows the transverse section of the trunk of an exogenous tree.
Following are its different components:
(1) Pith:
The innermost central portion or core of the tree is called the pith or medulla. It varies in size and shape for different types of trees. It consists entirely of cellular tissues and it nourishes the plant in its young age. When the plant becomes old, the pith dies up and decays and the sap is then transmitted by the woody fibres deposited round the pith. The pith of branches is nothing but merely a prolongation of the pith of stem.
(2) Heart Wood:
The inner annual rings surrounding the pith constitute the heart wood. It is usually dark in colour. As a matter of fact, it indicates dead portion of tree and as such, it does not take active part in the growth of tree. But it imparts rigidity to the tree and hence it provides strong and durable timber for various engineering purposes.
(3) Sap Wood:
The outer annual rings between heart wood and cambium layer is known as the sap wood. It is usually light in colour and weight. It indicates recent growth and it contains sap. The annual rings of sap wood are less sharply defined than those of heart wood. It takes active part in the growth of tree and the sap moves in an upward direction through it. The sap wood is also known as the alburnum.
(4) Cambium Layer:
The thin layer of sap between sap wood and inner bark is known as the cambium layer. It indicates sap which has yet not been converted into sap wood. If the bark is removed for any reason, the cambium layer gets exposed and the cells cease to be active resulting in the death of tree.
(5) Inner Bark:
The inner skin or layer covering the cambium layer is known as the inner bark. It gives protection to the cambium layer from any injury.
(6) Outer Bark:
The outer skin or cover of the tree is known as the outer bark. It is the outermost protective layer and it sometimes contains cracks and fissures. It consists of cells of wood fibre and is also known as the cortex.
(7) Medullary Rays:
The thin radial fibres extending from pith to cambium layer are known as the medullary rays. The function of these rays is to hold together the annual rings of heart wood and sap wood. These rays are sometimes broken and in some varieties of trees, they are not very prominent.
(II) Microstructure:
The structure of wood apparent only at great magnifications is called the microstructure. When studied under a microscope, it becomes evident that wood consists of living and dead cells of various sizes and shapes.
A living cell consists of four parts, namely, membrane, protoplasm, sap and core. The cell membrane consists mainly of cellular tissue and cellulose. The protoplasm is a granular, transparent, viscous vegetable protein composed of carbon, hydrogen, oxygen, nitrogen and sulphur. The core of cell differs from protoplasm merely by the presence of phosphorus and it is generally oval.
The cells, according to the functions they perform, are classified into the following three categories:
(1) Conductive cells
(2) Mechanical cells
(3) Storage cells.
(1) Conductive Cells:
These cells serve mainly to transmit nutrients from roots to the branches and leaves.
(2) Mechanical Cells:
These cells are elongated, thick-walled and have tightly interconnected narrow interior cavities. These cells impart strength to the wood.
(3) Storage Cells:
These cells serve to store and transmit nutrients to the living cells in the horizontal direction and they are usually located in the medullary rays.
Fire-Resistance of Timber:
As a general rule, the structural elements made of timber ignite and get rapidly destroyed in case of a fire. Further, they add to the intensity of a fire. But the timber used in heavy sections may attain high degree of fire-resistance because the timber is a very bad conductor of heat. This is the reason why time is required to build up sufficient heat so as to cause a flame in the timber.
With respect to the fire-resistance, the timber is classified as refractory timber and non-refractory timber. The refractory timber is non-resinous and it does not catch fire easily. The examples of refractory timbers are sal, teak, etc. The non-refractory timber is resinous and it catches fire easily. The examples of non- refractory timbers are chir, deodar, fir, etc.
To make timber more fire-resistant, the following methods are adopted:
(1) Application of Special Chemicals:
The timber surface is coated with the solution of certain chemicals. It is found that two coats of solution of borax or sodium arsenate with strength of 2 per cent are quite effective in rendering the timber fire-resistant.
These special chemicals are known as the fire protection compounds or antipyrines and they are more reliable. When the temperature rises, they either melt or give off gases which hinder or forbid combustion. When the wood is treated with antipyrine, it does not inflame even at high temperature, but it merely smoulders i.e. burns slowly without flame. The antipyrines containing salts of ammonium or boric and phosphoric acids are considered to be the best in making the timber fire-resistant.
(2) Sir Abel’s Process:
In this process, the timber surface is cleaned and it is coated with a dilute solution of sodium silicate. A cream-like paste of slaked fat lime is then applied and finally, a concentrated solution of silicate of soda is applied on the timber surface. This process is quite satisfactory in making the timber fire-resistant.
Qualities of Good Timber:
In general, the quality of timber depends on the following factors:
(a) Environmental conditions of the locality,
(b) Maturity of the tree,
(c) Method of seasoning,
(d) Nature of the soil,
(e) Process of preservation, and
(f) Time of felling.
Following are the characteristics or qualities of a good timber:
(1) Appearance:
A freshly cut surface of timber should exhibit hard and shining appearance.
(2) Colour:
The colour of timber should preferably be dark. The light colour usually indicates timber with low strength.
(3) Defects:
A good timber should be free from serious defects such as dead knots, flaws, shakes, etc.
(4) Durability:
A good timber should be durable. It should be capable of resisting the actions of fungi insects, chemicals, physical agencies and mechanical agencies. If wood is exposed to the actions of acids and alkalies for a prolonged period, it is seriously damaged. The weak alkali and acid solutions usually do not affect wood to a considerable extent.
(5) Elasticity:
This is the property by which timber returns to its original shape when load causing its deformation is removed. This property of timber would be essential when it is to be used for bows, carriage shafts, sport goods, etc.
(6) Fibres:
The timber should have straight fibres.
(7) Fire Resistance:
The timber is a bad conductor of heat. A dense wood offers good resistance to the fire and it requires sufficient heat to cause a flame. The heat conductivity of wood is low and it depends on various factors such as porosity, moisture content, surrounding temperature, orientation of fibres, bulk density, etc.
(8) Hardness:
A good timber should be hard i.e. it should offer resistance when it is being penetrated by another body. The chemicals present in heart wood and density of wood impart hardness to the timber. The mere resistance offered to chisel or saw does not usually indicate hardness of timber.
(9) Mechanical Wear:
A good timber should not deteriorate easily due to mechanical wear or abrasion. This property of timber would be essential for places where timber would be subject to traffic e.g. wooden floors, pavements, etc.
(10) Shape:
A good timber should be capable of retaining its shape during conversion or seasoning. It should not bow or warp or split.
(11) Smell:
A good timber should have sweet smell. An unpleasant smell indicates decayed timber.
(12) Sound:
A good timber should give out a clear ringing sound when struck. A dull heavy sound, when struck, indicates decayed timber. The velocity of sound in wood is 2 to 17 times greater than that in air and hence the wood may be considered high in sound transmission. The sound conductivity is faster along the fibres, is lower in the radial direction and is slowest along the chord of a cross-section.
(13) Strength:
A good timber should be strong for working as structural member such as joist, beam, rafter, etc. It should be capable of taking loads slowly or suddenly. It should also possess enough strength in direct and transverse directions.
(14) Structure:
It should be uniform. The fibres should be firmly added. The medullary rays should be hard and compact. The annual rings should be regular and they should be closely located.
(15) Toughness:
A good timber should be tough i.e. it should be capable of offering resistance to the shocks due to vibrations. This property of timber would be essential when it is to be used for tool handles, parts of motor cars and aeroplanes, etc.
(16) Water Permeability:
A good timber should have low water permeability which is measured by the quantity of water filtered through a unit surface area of specimen of wood. The water permeability is greater along the fibres than in other directions and it depends on initial moisture content, character of cut, type of wood, width of annual rings, age of wood, etc.
(17) Weathering Effects:
A good timber should be able to stand reasonably the weathering effects. When timber is exposed to weather, its colour normally fades and slowly turns grey. A good timber should show the least disintegration of the surface under adverse weather conditions such as drying and wetting, extreme heat and extreme cold, etc.
(18) Weight:
The timber with heavy weight is considered to be sound and strong.
(19) Working Condition:
The timber should be easily workable. It should not clog the teeth of saw and should be capable of being easily planed or made smooth.
Factors Affecting the Strength of Timber:
It may be mentioned that the chief factors affecting the strength of timber are as follows:
(1) Abnormalities of growth,
(2) Faults in seasoning,
(3) Invasion of insects,
(4) Irregularities of grain, the log, etc.
(5) Moisture content,
(6) Presence of knots, shakes, etc.,
(7) way in which a timber piece is cut from
Decay of Timber:
The timber is said to be decayed when it is so deteriorated that it loses its value as an engineering material. When these defects are in excess, the timber decays and such timber is not used for engineering purposes.
Following are the various causes or situations which favour the early decay of timber:
(i) Alternate dry and wet conditions,
(ii) Bad storage or stacking of timber,
(iii) Fungi which are responsible for developing diseases in timber such as blue stain, brown rot, dry rot, heart rot, sap stain, wet rot and white rot,
(iv) Improper seasoning,
(v) Insects such as beetles, marine borers, termites, etc.,
(vi) Keeping timber in contact with damp wall, damp earth, etc.,
(vii) Shocks or impacts received during young age from natural forces such as fast blowing wind, etc.
(viii) Use of timber without taking out sap wood from its structure,
(ix) Using seasoned timber without applying suitable preservative on its surface, and
(x) Using unseasoned wood with the application of protective coat of paint or tar.
Storage of Timber:
The structural timber should be properly stored so as to avoid any further development of defects. For the purpose of storage, suitable stacks of timber pieces are formed. The stacks are prepared on similar lines to the stacks for air seasoning as shown in fig. 9-16. The length of stack depends on length of timber pieces. Its width and height are usually limited to about 1.50 m to 2 m respectively.
The material is arranged in layers and the layers are separated by wooden battens which are known as the crossers or spacers. The stack should be protected from direct sun, dry wind and rain. If necessary, a sloping roof of suitable material may be provided over the stack.
The important facts to be remembered for storage of timber are as follows:
(i) In each layer, an air space of about 25 mm should be maintained between adjacent members.
(ii) The crossers or spacers should be of sound wood, straight and uniform in thickness.
(iii) The ends of all members should be coated with suitable material to prevent end-cracking.
(iv) The longer pieces should be placed in bottom layers and the shorter pieces should be placed in top layers.
(v) The platform of stack should be made at least 150 mm higher than ground.
(vi) There should be a minimum distance of at least 300 mm between adjacent stacks.
Market Forms of Timber:
The timber is converted into suitable commercial sizes.
Following are various forms in which the timber is available in the market:
(1) Batten:
This is a timber piece whose breadth and thickness do not exceed 50 mm.
(2) Baulk:
It is a roughly squared timber piece and it is obtained by removing bark and sap wood. One of the cross-sectional dimension exceeds 50 mm, while the other exceeds 200 mm.
(3) Board:
It is a plank i.e. a timber piece with parallel sides. Its thickness is less than 50 mm and width exceeds 150 mm.
(4) Deal:
It is a piece of soft wood with parallel sides. Its thickness varies from 50 mm to 100 mm and its width does not exceed 230 mm.
(5) End:
This is a short piece of batten, deal, scantling, etc.
(6) Log:
It is the trunk of tree obtained after removal of branches.
(7) Plank:
It is a timber piece with parallel sides. Its thickness is less than 50 mm and its width exceeds 50 mm.
(8) Pole:
It is a sound long log of wood. Its diameter does not exceed 200 mm. It is also known as a spar.
(9) Quartering:
It is a square piece of timber, the length of side being 50 mm to 150 mm.
(10) Scantling:
This is a timber piece whose breadth and thickness exceed 50 mm, but are less than 200 mm in length. These are the pieces of miscellaneous sizes of timber sawn out of a log.
Advantages of Timber Construction:
The timber has been probably the first material to be adopted in the construction of engineering structures.
It possesses the following distinct advantages in preference to other engineering materials:
(i) It can be easily handled and can be planed, sawn and joined with ordinary tools of the carpenter.
(ii) It can be used either for load bearing members or for non-load bearing members.
(iii) It combines light weight with strength and hence it is generally preferred for the buildings in the earthquake-prone regions.
(iv) It is easy to provide connections in the timber construction.
(v) It is economical and cheap. This is due to the fact that the smallest piece of wood can be put to one or other use and the wastage of material is thereby considerably minimized.
(vi) It is possible to realise some value even after timber construction has completed its useful life.
(vii) It is used to prepare furniture of decent appearance and comfortable design.
(viii) The heavy timber construction presents a massive appearance.
(ix) The houses with timber construction are found to be cool in summer and warm in winter. This is due to the fact that the wood is a non-conductor of heat.
(x) The other forms of present-day such as plywoods, Fiber boards, etc. have made timber construction to match with the present-day requirements.
(xi) The timber construction is quite durable, if properly protected against moisture, rain, wind, etc.
(xii) The additions, alterations and repairs to the timber constructions can be carried out easily.
(xiii) It is found to be superior to the cement concrete and steel in respect of thermal insulation, sound absorption and electrical resistance. For instance, the thermal insulation of wood is 15 times better than concrete, 6 times better than bricks and 1770 times better than aluminium.
(xiv) It is also highly energy efficient. The amount of electricity or its equivalent required for the production of one unit of timber is nearly 6.25 times less than that required for steel and about 40 times less than that required for aluminium.
(xv) It is easily available and can be speedily transported by simple means of communication.
It should however be remembered that every effort should be made to use timber as economically as possible and thus to minimize its wastage. The production processes and working conditions should be judiciously decided and scientifically designed.
The important steps to be taken to increase the durability of wood and its structures are adequate temperature and moisture, proper inspection and maintenance, treatment of exposed surfaces, provision of favourable working conditions, etc.
The disadvantages of timber construction are as follows:
(i) It is likely to crack, warp and decay, if not properly seasoned and not treated with the preservatives.
(ii) It is not suitable for cyclonic weather and for places subjected to the natural calamities.
(iii) It requires careful regular maintenance.
(iv) It is subjected to the risk of fire.
(v) If not locally available, it proves to be costly.
Uses of Timber:
There is hardly any material other than timber which can be used as an all-round substitute in construction work and its various uses can be summarized as follows:
(i) It is used for door and window frames, shutters of doors and windows, roofing materials, etc.
(ii) It is used for formwork of cement concrete, centering of an arch, scaffolding, etc.
(iii) It is used for making furniture, agricultural instruments, sport goods, musical instruments, etc.
(iv) It is used for making railway coach wagons.
(v) It is used for making toys, engraving work, matches, etc.
(vi) It is used for railway sleepers, packing cases, etc.
(vii) It is used for temporary bridges and boat construction.