Timber, being a natural product available in abundance in India, is used for the construction of doors, windows, roofs, partitions, beams, posts, cupboards, shelves, etc. The strength characteristics of various types of timbers, generally used in India for engineering works.

Generally, the trees from which timber is obtained are classified in two groups:

(i) Exogenous or outward growing trees,
(ii) Endogenous or inward growing trees.

The former is the class of trees which includes all the commercial timbers used for building purposes, and the latter class includes the trees, such as palms and bamboos, not suitable for engineering use and are found in the tropics. The exogenous trees are further divided into two main groups: Hard woods and soft woods. One should not misunderstand this nomenclature used in classification (i.e., hard wood is not necessarily possessing more strength ) because it is purely a botanical classification. It has been found that many hard woods are softer than certain soft woods.

Hard woods have broad leaves, dark colour, are generally dense and have narrow and well defined annular rings. They are heavy, strong and hard. Hardwoods are capable of resisting all stresses equally well.

Softwoods have long and narrow pointed leaves and are characterized by distinct annular rings ; have straight grains, more uniform texture and light colour. Soft woods are very strong for direct pull weak in resisting thrust or shear.

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Timber
Structure of a Timber Tree
Properties or Characteristics of Timber
Defects of Timber
Diseases of Timber - Dry and Wet Rot
Decay of Timber
Seasoning of Timber
Methods of Seasoning of Timber
Preservation of Timber

Structure of a Timber Tree

The trees which grow outwards about the centre are called exogenous trees. The cross sectional details of such a tree are shown. Exogenous trees such as Sal, Babul, Teak, etc. are used for all engineering purposes. If trunk of tree is cut in transverse direction, the annual rings can be seen which are useful in predicting the age of tree. The cross- sectional parts of tree and their functions are described below (parts from outer to inner).

(i) Bark or skin:

It is the outermost portion of tree. The outer bark protects the tree from high temperature and mechanical harm. The inner bark covering the cambium layer protects cambium layer from injury and also supplies food to those parts where it is needed most.

(ii) Cambium layer:

This layer of sap between inner bark and sapwood is known as cambium layer. This layer makes bark on the outer side and wood on inner side. It is an immature sap wood.

(iii) Sap wood:

The outer annual rings between cambium layer and heart wood are known as sap wood. It is usually light in colour and weight as compared to heart wood. Sap (i.e., plant juice and moisture ) actually is the food of tree which is made available to the tree from the ground through roots. Sapwood is not used for any engineering work because it contains large amount of moisture and is liable to quick decay.

(iv) Medullary rays:

These are thin radial fibres between cambium layer and pith of tree. The function of these rays is to hold together the annual rings of sapwood and heartwood. In addition to this, these rays store the food of the tree and distribute to different parts according to the necessity.

(v) Heart wood:

The annual rings between sapwood and pith form the heart wood. The rings of heartwood are closer than rings of sapwood. Heartwood is darker in colour and is strong and durable. Actually, the sapwood gets converted to heartwood in due course of time, usually 10 to 30 years. Heartwood is dead portion of tree and does not take part in the growth of tree. Heartwood is used for all engineering works.

(vi) Medullary sheath:

It is a layer between heart wood and the pith. It covers the pith from all around.

(vii) Pith:

It is the innermost portion of the tree. Pith is very soft portion and varies in size and shape for different types of trees. It consists entirely of cellular tissues.

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Timber
Structure of a Timber Tree
Properties or Characteristics of Timber
Defects of Timber
Diseases of Timber - Dry and Wet Rot
Decay of Timber
Seasoning of Timber
Methods of Seasoning of Timber
Preservation of Timber

Properties or Characteristics of a Good timber:

(i) Strength:

The timber should be strong enough to withstand the loads whether being applied slowly or suddenly. It should possess enough strength in direct compression and transverse direction.

(ii) Durability:

A good timber should be capable of resisting the various actions due to fungj, insects, chemicals, physical and mechanical agencies. On the basis of durability, the timbers are classified into three classes:

(a) Class I – Timbers having average life of 120 months and over,
(b) Class II – Timbers having average life of 60 to 119 months, and’
(c) Class III - Timbers having average life of 59 months and below.

N.B. These grades of durability have been mentioned for various Indian timbers.

(iii) Weather resistance.:

A good timber should possess adequate resistance against weathering effects such as alternate drying and wetting, alternate heating and cooling because of temperaturw variations, wind effects, etc.

(iv) Fire resistance:
The timber should offer sufficient resistance against fire so that it does not easily ignite. It helps in fire protection of buildings.

(v) Elasticity:

The timber should be capable of regaining its original shape when load causing deformation is removed. The property is important when timber is to be used for bows, carriage shafts, sport goods, wooden beams, wooden floors, etc.

(vi) Workability:

The timber should be easily workable and should not clog the teeth of saw. It should also be capable of being easily planed or made smooth.

(vii) Toughness and Abrasion:

A good timber should be capable of offering resistance to shocks due to mechanical wear.

(viii) Other properties of a good timber are:

(a) Timber should have sufficient weight. A timber with heavy weight is considered to be sound and strong.

(b) The structure of timber should be uniform, hard and compact.

(c) A timber should have sufficient hardness, i.e., resistance against penetration.

(d) A timber should have favourable physical characteristics such as a dark colour (sign of strength); straight fibres; shining appearance; free from defects; sweet smell; good sound when struck, etc.

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Timber
Structure of a Timber Tree
Properties or Characteristics of Timber
Defects of Timber
Diseases of Timber - Dry and Wet Rot
Decay of Timber
Seasoning of Timber
Methods of Seasoning of Timber
Preservation of Timber

Several natural defects occur in all kinds of timber depending upon the soil upon which they grow and climatic conditions to which they are subjected while growing. These defects should be avoided or removed as far as possible during conversion for use. The following are the most common defects in timber.

(i) Heart shakes:

These are the cracks or splits in the centre, starting from the pith and extending in the direction of the medullary rays in one or opposite directions towards sap wood. Such defects are found in over matured trees but sometimes they may be caused by quick drying of the central part of the tree, if a tree nearing maturity, is felled and left unbaked for a long time. This defect is caused due to shrinkage of heart wood.

(ii) Star shake:

These are cracks or splits which extend from bark towards sap wood. This defect is caused when the tree is subjected to severe heat or frost during its growth. The width of these cracks or shakes is more at the outer ends and reduces when they extend towards centre.

(iii) Cup-shakes and Ring-shakes:

These are formed by the rupture of tissues in a circular direction across the cross- section of a log, usually along annual rings. When the rupture extends only a part round, it is called a cup shake and when the whole way round or almost so, it is called a ring shake. This defect is caused either due to unequal growth or due to sudden contraction of timber under atmospheric changes assisted by twisting action due to wind.

(iv) Radial shakes:

These are similar to star shake but occur due to exposure to sun when felled timber is placed for seasoning. These cracks are fine, irregular and numerous. Many cracks or split run for a short distance from bark towards centre, then follow the course of annual ring and lastly go towards the centre radially.

(v) Rind galls:

These peculiar curved swellings found on living or dead tree, generally caused by the growth of layers over the wounds left after branches have been imperfectly cut off or removed.

(vi) Upsets or Ruptures:

These are ruptures of fibres of wood caused due to some sort of impact, injury or pressure. They are also caused due to unskillful felling and violent wind effect.

(vii) Twisted fibres or Wandering hearts:

This defect is due to the tree being twisted constantly in one direction by the force of prevalent wind. This wood is unsuitable for squaring or sawing and hence can be used for posts or poles in unsawn condition or for fuel purposes.

(viii) Wind cracks:

These are shakes or splits on the sides of a bark of timber due to the shrinkage of the exterior surface exposed to atmospheric agencies like sun, wind, etc.

(ix) Knots:

They are the roots or bases of small branches of the tree. They may be alive or dead but break the continuity of fibres. In the beginning, the base portion gets food from stem but finally results in formation of dark and hard rings known as knots. These knots are not very harmful when small, hard and round, and used for members other than tie bars and beams. Knots are classified on the basis of diameter. If the diameter of knot is less than 6 mm, it is called a ‘nail knot’ , if the diameter ranges between 6.5 and 20 mm, then it is called ‘small knot’ , and if diameter ranges between 20 and 40 mm then it is called ‘medium knot’ and knots with diameter more than 40 mm are called ‘large knots’ . Timbers containing large dead knots or many smaller ones should be rejected as they are poorer in appearance and strength.

(x) End splits:

These are cracks extending from one face to another.

(xi) Dead wood:

Timber obtained from dead standing tree (i.e., after maturity) is known as dead wood. It is light in weight, deficient in strength and reddish in colour.

(xii) Druxiness;

It is an early decay of healthy wood indicated by whitish spots. It is due to fungi giving access and setting decay.

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Timber
Structure of a Timber Tree
Properties or Characteristics of Timber
Defects of Timber
Diseases of Timber - Dry and Wet Rot
Decay of Timber
Seasoning of Timber
Methods of Seasoning of Timber
Preservation of Timber

Diseases of Timber:

A perfectly seasoned timber obtained from a sound matured tree and placed in a dry and well ventilated place or entirely immersed in water can last for several years. But when timber is confined to a place where it is liable to be subjected to alternate dry and wet conditions or when used in a dark, damp and unventilated position, then it gives birth to the following two common diseases:

(i) Dry rot

(ii) Wet rot

(i) Dry rot:

This disease in timber is caused due to the lack of ventilation. Such situation promotes the growth of fungus which eats the fibres of timber and reduces them to dry powder. The growth of fungi accelerates the progress of dry rot but the origin of the disease is due to the decomposition of sap in the timber first and then this decomposition supplies food for the growth of fungi. This disease is highly infectious and causes tremendous destruction of timber. This disease may develop in seasoned timber, unseasoned timber or unseasoned timber treated with preservatives. This disease is common in warm cellers, unventilated wooden floors, basements, ends of timber built into walls without protection or ventilation, timber piles, etc. This disease can be detected by tapping at one end of timber post or log and hearing the sound on the other end.

If a distinct sound is heard then it indicates a sound timber and if a dull sound is heard then it is a sign of decayed or diseased timber.

The following measures can be taken to prevent or eradicate this disease:

(a) By following free circulation of fresh air.

(b) The portion of the wood which is attacked by dry rot should be cut off and remaining portion painted with copper sulphate.

(c) By exposing the infected portion to sunlight and sweeping off fungus.

(ii) Wet rot:

This disease is the result of decomposition of timber caused by damp and moisture. This disease is further induced by alternate wet and dry conditions. The portion of timber attacked by wet reduces to a grayish brown powder. In this disease, the timber is first decomposed while wet and then it scales off when it dries. This disease can develop in a timber whether dead or alive. All types of timber used in uncovered situations is likely to be exposed to rain and hence subjected to wet rot.

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Timber
Structure of a Timber Tree
Properties or Characteristics of Timber
Defects of Timber
Diseases of Timber - Dry and Wet Rot
Decay of Timber
Seasoning of Timber
Methods of Seasoning of Timber
Preservation of Timber

Timber is said to be decayed when it is deteriorated to the extent that it losses its value as an engineering material. Timber generally deteriorates in strength when subjected to excessive defects or diseases or attack by certain categories of insects. The insects damage the appearance and reduce the strength in search of their food and shelter.

Following insects generally attack the timber.

(a) Pin hole borers – They attack the standing tree or recently felled timber.

(b) Powder – post beetles - They attack timber while it is stacked for seasoning, sale or awaiting use or has been used for years.

(c) Termites or white ants - They attack timber used for structural purposes in buildings, bridges, poles, sleepers, fence, posts, etc.

The magnitude of destruction caused depends upon following situations favouring decay:

(i) Alternate dry and wet conditions.

(ii) Improper stacking or storage of timber.

(iii) Improper seasoning or unseasoned timber.

(iv) Use of unseasoned timber treated with preservatives.

(v) Use of seasoned timber without treating with preservatives.

(vi) Presence of moisture, sap and different types of insects.

(vii) Development of defects during growth, conversion, improper use, etc.

(viii) Improper positioning of structural members of timber.

(ix) Development of timber diseases.

(x) Effect of natural agencies.

The decay of timber due to attack of insects can be prevented by adequate ventilation, exclusion of dampness and impregnation of properly seasoned timber with wood preservatives.

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Timber
Structure of a Timber Tree
Properties or Characteristics of Timber
Defects of Timber
Diseases of Timber - Dry and Wet Rot
Decay of Timber
Seasoning of Timber
Methods of Seasoning of Timber
Preservation of Timber

A newly felled tree contains water and sap (i.e., plant juice) both about 50% of its oven dry weight. For using this timber for any engineering purpose, all the moisture and sap are removed out of it so that it is not liable to decay by the fermentation of the sap or warp or bend owing to uneven expansion or contraction of the moisture with rise or fall of temperature. Thus “Seasoning is the process of drying out the timber to a moisture content approximately equal to average humidity of situation where it is to be used”.

Advantages of Seasoning of Timber.

The basic objective of seasoning is to reduce the moisture content of timber in order to prevent it from possible fermentation and decay. The following are the objectives and advantages of seasoning of timber:

(i) To decrease the volume and weight of timber and thereby to lower the cost of transport and handling.

(ii) To improve strength, hardness and stiffness of timber.

(iii) To improve the resisting power of timber so that it is less liable to attack by insects and fungus.

(iv) To improve the working qualities of timber so that it easily works under saw or plane during conversion.

(v) To maintain the size and shape of the timber articles even after leaving the hands of carpenters and joiners.

(vi) To make timber fit for receiving treatment of paints, preservatives, varnishes, etc.

(vii) To reduce the tendency of timber to crack, warp, bend or shrink.

(viii) To allow timber to burn readily, if used as fuel.

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Timber
Structure of a Timber Tree
Properties or Characteristics of Timber
Defects of Timber
Diseases of Timber - Dry and Wet Rot
Decay of Timber
Seasoning of Timber
Methods of Seasoning of Timber
Preservation of Timber

Broadly speaking, seasoning is of two types viz.,

(i) Natural or Air Seasoning,
(ii) Artificial Seasoning.

Further, artificial seasoning is of various types namely., water seasoning, seasoning by boiling, kiln seasoning, chemical seasoning, electrical seasoning, smoke-drying, charrying, etc.

The various methods of seasoning of timber are briefly described below:

(i) Natural or Air seasoning.

In this method, converted timber in the form of planks or posts or scantlings is stacked horizontally or vertically above 30 cm of ground under a covered shed. Care should be taken to see that sawn timber scantlings are placed in cross-wise directions in alternate layers to permit free circulation of air and they should also be protected from the sun and rain. The time of seasoning depends upon the type of wood, thickness of sawn members and weather. Generally, for every 25 cm thickness of sawn timber, soft wood takes about 2 to 3 months and hard wood about 12 months.

This method of seasoning s simple, cheap and requires little supervision. However, the rate of drying is slow and it is difficult to exercise controls over temperature and humidity, and hence desired seasoning quality is not attained.

(ii) Artificial Seasoning.

The various methods of artificial seasoning are adopted against natural seasoning because of the following reasons:
(a) The process of drying is controlled and hence not liable to be attacked by insects and fungi.
(b) Period of seasoning can be reduced depending upon the need.
(c) Since the desired moisture content in timber can be attained, so it monomises the defects such as shrinkage, cracking, wraping, etc.
(d) Drying of different surfaces is even and uniform due to better control of air, humidity and temperature.

The different methods of artificial seasoning are described below:

A. Water seasoning.

In this process, timber after felling in the form of logs of suitable sizes, is totally immersed in a running stream of water for a period of 3 to 4 weeks. The thicker end of the log is kept pointing upstream. The sap in timber is then washed away by water. The log is then kept out in the free access of air to dry out. Timber is thus seasoned. It is a quick method and renders timber less liable to warp and crack. It also removes organic materials contained in sap of timber. Although it is a quick process but it reduces the elasticity and durability of timber and makes it weak and brittle.

B. Seasoning by boiling or steaming.

In this method, the timber is first immersed in water and then water is boiled for 3 to 4 hours. In steam seasoning, timber is exposed to the action of steam spray. The timber is thus dried out slowly and seasoned. This is very quick method of seasoning and reduces the shrinkage. However, it is expensive method and also affects the strength and elasticity of timber.

C. Kiln seasoning or Hot air seasoning.

In this method, timber can be seasoned to any moisture content and hence is commonly adopted for rapid seasoning of timber on a large scale. In this process, timber is stacked inside the chamber and hot air is forced for free circulation for about three days. The sap and moisture content are dried out to the desired extent. This process of drying out can be carried either in stationary kilns(by stacking timber in kiln) or in progressive kiln (by moving the timber from one end to the other end of kiln). This method of kiln seasoning gives a well seasoned timber as it controls three important conditions, namely, air circulation, relative humidity and temperature. However, this method is expensive and can be used for timber sawn to small pieces under skilled supervision.

D. Chemical seasoning or Salt seasoning.

In this method, the timber is immersed in solution of suitable salt which absorbs the moisture content from inside of timber. The timber is then taken out and seasoned by ordinary way i.e. by air seasoning. Since the interior surface of timber dries in advance of exterior one and, therefore, the chances of formation of external cracks are reduced.

E. Electric seasoning.

It is an established fact that a green timber offers less resistance to the flow of electric current as compared to a dry timber. In other words, a hard timber offers more resistance to the flow of current than a soft wood since resistance of current is inversely proportional to the moisture content present in the timber. Therefore, high frequency alternating currents which produce heat are used to dry out the timber. This is the most rapid method of seasoning but being very costly cannot be used for seasoning of timber on a large scale.

F. Smoke-drying.

In this method, timber is dried out over a fire of straw or twig. Care should be taken in applying heat gradually to prevent splitting. Timber seasoned by this method is hardened, more durable and proof against worm attacks. This method is generally used for bending planks in boat building.

G. Charring or Scorching.

In this process the ends of piles or posts are burnt to charcoal in order to remove moisture content. This method is used for piles, posts and other members that are to be kept under water or ground. However, this produces dry-rot in green timber.

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Timber
Structure of a Timber Tree
Properties or Characteristics of Timber
Defects of Timber
Diseases of Timber - Dry and Wet Rot
Decay of Timber
Seasoning of Timber
Methods of Seasoning of Timber
Preservation of Timber

Different variety of glues/adhesives are now available for use with wood, depending on the application. The older adhesives include animal glue, casein glue, and a different types of vegetable glues, of which soybean is most popular even today. Animal glues provide strong, and tough joints, which however, are not . Casein mixed with cold water, when properly formulated, provides highly moisture resistant glue joints, although they cannot be called waterproof. The vegetable glues have good dry strength but are not moisture resistant.

The common high strength glues that are available today are synthetic resins, of which phenol formaldehyde, urea formaldehyde, resorcinol formaldehyde, melamine formaldehyde, and epoxy are the most common one. Phenol, resorcinol, and melamine provide glue joints that are completely waterproof and will not separate when properly made even on boiling. Urea formaldehyde provides a glue joint of high moisture resistance, although it is not good as the other three. Phenol and melamine require application of heat, as well as pressure, to cure the adhesive. Urea and resorcinol, however, can be formulated to be mixed with water at ordinary temperatures and hardened without application of heat above room temperature. Waterproof plywood is commonly made in hot-plate presses with phenolic or melamine adhesive.

Resorcinol is used where heat cannot be applied, as in a variety of assembly operations and the manufacture of laminated parts like ships’ keels, which must have the maximum in waterproof qualities. Epoxide resins provide strong joints.

Adhesives containing an elastomeric material, such as natural or synthetic rubber, may be classified as contact or mastic. The former, applied to both mating surfaces and allowed to partly dry, permit adhesion on contact. Mastics are highly viscous and applied with a trowel or putty knife. They may be used to fix wooden flooring. An emulsion of polyvinyl acetate serves as a general purpose adhesive, for general operations where maximum strength and heat or moisture resistance are not required. This emulsion is merely applied to the surfaces to be bonded, after which they are pressed together and the adhesive is allowed to harden.

This utilizes metal clamps bonded to tempered plate glass at the top edge, with vertical glass supports at right angles for resistance to wind pressure. The vertical supports, called stabilizers, have their exposed edges polished. The joints between the large plates and the stabilizers are sealed with a bonding cement. The bottom edge or sill is held in position by a metal channel, and sealed with resilient waterproofing. Suspended glazing offers much greater latitude in use of glass and virtually eliminates visual barriers.

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These glasses are made of plastics such as acrylic or polycarbonate, plastic glazing is used for urban school buildings and in areas where high vandalism might be anticipated. These plastics have substantially higher impact strength than glass or tempered glass. Allowance should be made in the framing and installation for expansion and contraction of plastics, which may be about 8 times as much as that of glass. Note also that the modulus of elasticity (stiffness) of plastics is about one-twentieth that of glass. Standard sash, however, usually will accommodate the additional thickness of plastic and have sufficient rabbet depth.

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This appears as a mirror when viewed from a brightly lighted side, and is transparent to a viewer on the darker opposite side. This one way- vision glass is available as a laminate, plate or float, tinted, and in tempered quality.

Find more about, Types of Glasses