Timber : As A Material Of Construction
What Is Timber?
The word Timber is derived from an old English word Timbrian which means to build. 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 circumference of the trunk.
Timber as a building material possess number of valuable properties like :-
- low heat conductivity
- amenability to mechanical work
- small bulk density
- relatively high strength
However it also has its own drawbacks such as :-
- susceptibility to decay
- fluctuation in properties due to variation in moisture content
- variation in strength
These shortcomings can be greatly reduced by using some modern wood processing techniques.
Types Of Timber
- Converted timber : which is sawn and cut into suitable commercial sizes.
- Rough timber : which is obtained after felling a tree.
- Standing timber : timber contained in living tree.
Classification Of Trees
For engineering purposes, trees are classified according to their mode of growth.
These trees increase in bulk by growing outwards and distinct rings are formed in the horizontal cross section of their trunks. These rings are called annual rings because one such ring is added every year and are useful in predicting the age of the tree. These trees are subdivided into two groups.
- Conifers : these are also known as ever-green trees and bear cone shaped fruits. These trees yield soft wood which is generally light colored, resinous, light in weight and relatively weak. They show distinct annual rings.
- Deciduous : also known as broad leaf trees. The timber for engineering purposes is generally derived from these trees. These trees yield hard wood which is generally close grained, dark colored, heavy, durable and non resinous. They do not show distinct annual rings.
Examples of soft woods are Chir, Pine, Spruce, Deodar etc. and those of hard woods are Oak, Sal, Teak etc.
2. Endogenous trees
These trees grow inwards and fibrous mass is seen in their longitudinal section. The timber obtained from these trees has very limited engineering applications. Examples of these trees are Bamboo, Cane, Palm etc.
Structure Of A Tree
Following are its components:
- Pith: The innermost central portion or the core of the trunk is called pith or medulla. It consists entirely of cellular it nourishes the plant in its young age. When plant gets old, pith dies and the sap is then transmitted by the woody fibres deposited round the pith.
- Heart Wood: Inner annual rings surrounding the pith constitute the heart wood. It is usually dark in color. It doesn't take active part in the growth of a tree. But it imparts strength, rigidity and durability to the timber.
- Sap Wood: The outer annual rings between heart wood and cambium layer is called as sap wood. It is usually light in color and weight. Its annual rings are not sharply defined which indicates recent growth and it contains sap. It takes active part in the growth of the tree and the sap moves in an upward direction through it.
- Cambium Layer: The thin layer of sap between sap wood and inner bark is known as cambium layer. It indicates sap which has yet not been converted into sap wood.
- Inner Bark: The inner layer or skin covering the cambium layer is called inner bark. It gives protection against injury.
- Outer Bark: The outer skin or cover of the tree is known as the outer bark. It is the outermost protective layer and sometimes contains cracks and fissures. It is also known as cortex.
- Medullary Rays: These are thin radial fibres extending from pith to cambium layer. They hold the annual rings of heart wood and sap wood together.
The structure of wood apparent only under great magnification is called microstructure. Wood consists dead and living cells of various sizes and shapes. The living cell consists of four parts, namely, membrane, protoplasm, sap, and core
The cells according to their functions can be categorized as:-
- Conductive Cells: mainly transmit nutrients from roots to branches and leaves.
- Mechanical Cells: these cells are elongated, thick walled and have tightly interconnected narrow interior cavities. they impart strength to the wood.
- Storage Cells: store and transmit nutrients to living cells and are usually located in the medullary rays.
Processing of timber
Processing of timber consists of following stages:-
1. Felling of trees
2. Seasoning of timber
3. Conversion of timber
4. Preservation of timber
Now let us discuss the stages in detail.
1. Felling of trees
the following three things are taken care of:
- Age of the tree- Age of the tree should lie between 50 to 100 years at the time of cutting. If under aged tree is cut, it's Sap wood will be weaker than Heart wood. And in an over aged tree, the decay of Heart wood would have taken place. Both cases are useless.
- Method of cutting- Trees should be cut from a section so as to get maximum quantity of wood. Cutting should be done by the method of parallel sawing and cuts should be made at some angle from the horizontal.
- Season of cutting- The tree should be cut a season in which movement of nutrient is minimum. Trees in the plain areas should be cut in winters to avoid twisting and warping if cut in summers, while, the trees in hilly areas should be cut in summers.
2. Seasoning of timber
This is the process of reducing the moisture content in wood so as to increase it's durability. Generally two types of moisture is present in wood.
- Free water
- Bound water
During seasoning, we remove free water. The point at which entire free water gets removed is called as Fiber Saturation point.
Advantages of seasoning
- Wood can be burnt readily if used as fuel.
- It reduces the weight of the timber which in turn lowers the transportation cost and dead load of the structure.
- Imparts hardness and strength.
- . Workability of timber gets increased at the time of conversion.
- It increases the resistance of wood against the attack of fungi and insects.
- vi. It increases the adhesive capacity the protective and decorative solutions over the timber.
Methods of seasoning
There are two methods of seasoning of timber.
a- Natural method
In this method, logs are stacked upon one another in assemblies with space provided in between the logs to allow circulation of air. The width and height of each assembly is restricted to about 2 m and 3 m respectively. The method takes about 3 years to complete. Water content gets reduced to about 10% - 20%.
b- Artificial methods
Following are the artificial methods of seasoning of timber:-
- Boiling- Timber is boiled to remove the moisture by raising it's surrounding temperature.
- Water seasoning- the logs of wood are drowned longitudinally in a flowing stream of water with the larger cross section of log facing the upstream direction. This flows out all the sap from it. Then wood is seasoned naturally to remove the water. This is an economical method.
- Chemical seasoning- the chemicals which increase the rate of evaporation are used.
- Electrical seasoning- wood is a bad conductor of electricity. Hence when electricity is passed through it, heat develops owing to the resistance and moisture gets evaporated.
- Kiln seasoning- In kiln seasoning timber is placed in achamber with some special heatingarrangement.In this process one thing should bekept in mind that heating systemshould be under control, other wisetimber will be crack or wrap. The timerequired for this seasoning is 3 to 12days. This is quick process.
3- Conversion of timber / Sawing
There are mainly two methods of sawing.
- Thorough and through sawing- the log is cut lengthwise into pieces of suitable dimensions, one after another, without giving it any rotation. Hence, all the cut surfaces are essentially parallel.
- Selective seasoning- in this method, log is rotated after one or more successive cuts in a particular direction. The log is first cut into four equal parts longitudinally. This can be done to eliminate defects during sawing or for obtaining wood boards of a particular grain. Two varieties are:-
- Slash cut- Boards are cut in a direction tangential to annual rings. In such boards, annual rings can make an angle less than 450 to the face.These give a flat appearance and are also termed as flat grained.
- Rift cut- In these boards, annual rings make an angle more than 450 to the face.
3. Quarter sawing- It is a process of selective sawing of logs. It's main aim is to obtain specially attractive boards. Boards are cut in such a way that each board has one edge from the center of the log and the other edge from the peripheral part of that log. The process consists of cutting the log into two parts by central cut, cutting each half again to get four of them, parallel cutting from each of them to obtain boards.
4- Preservation of Timber
Preservation of timber is done for the following reasons:-
- to increase the age of timber structure
- to increase it's durability
- to make it resistant against the attack of fungi and insects.
Wood preservative chemicals are classed under the following classes:-
a. Oil soluble salts- such preservatives are soluble in oils.The most common preservative of this class is coal tar creosote oil. The most important properties of this type of preservative are:-
- high degree of permanence, i.e., it stays within the cells for long time
- it penetrates quickly and easily into the wood tissue
- highly destructive for fungi
b. Water soluble preservative- these salts make easy solution with water. It can be easily dissolved and used, which is it's advantage, but it also has a disadvantage of getting easily washed away if timber gets in moist conditions. Zinc chloride, copper sulfate, sodium fluoride etc. are the examples of this class.
c. Volatile base salts- are those substances which make solutions with substances like petroleum. The creosote petroleum blend are typical examples.
Methods of preservation
- Brush application- timber is given one or two coats of preservative with the help of a brush. This method is used for painting coal tar at the ends of beams that are embedded in walls or base of poles and posts that go into the ground.
- Dipping application- Timber parts are made to dip in the preservative and kept immersed for various periods from a few hours to few days.
- Open tank application- timber is kept immersed in a suitable metallic tank of proper size till a proper saturation is obtained. The tank containing timber is then heated at about 700 - 800 for several hours.This ensures deeper penetration of preservative into the timber.
- The pressure process- This is the best and commonly applied method for preserving costly varieties of timber. It involves passage of preservative though the timber under pressure. This is achieved by either of the following:-
i. The full cell process- the timber is placed in a large steel cylinder acting as a pressure vessel. Vacuum is first created and maintained for about one hour or more.After this, coal tar creosote oil or any other suitable preservative, preheated to a specified temperature is forced into the cylinder under sufficient pressure. This is continued till the required quantity of preservative has been introduced into the timber. Thereafter pressure is reduced and after giving some vacuum, timber is taken out.
ii. The empty cell method- no vacuum is created in the beginning. Instead, timber placed in the pressure vessel is subjected to initial pressure while preservative is being introduced into the cylinder. Once the vessel is full of preservative, sufficient pressure is applied which forces the preservative from tank into the timber.
Causes of timber decay
There are basically two causes of timber decay, by fungus and insects.
1. Types of Fungal decay
In this, the lignin part of the cell tissue gets destroyed by the fungus, leaving behind the fluffy light colored cellulose part in rotten form.
The fungus eats the cellulose part of the cell tissue leaving behind the lignin part in rotten form.
The fungi causing this kind of decay lives on sugars and starch in the cell tissue. Such fungus discolorizes the original timber.
It is cotton like growth of fungus over the timber. It may not spoil the inner portion of the timber the outer appearance gets displeasing.
It is the term given to brown rot caused by certain fungi that deteriorate timber in buildings and other wooden construction seemingly without an apparent source of moisture, mainly in the places where there is no circulation of air.
It is the decay of the timber caused by some fungi in the presence of high moisture levels.
In trees, heart rot is a fungal disease that causes the decay of wood at the center of the trunk and branches. Fungi enter the tree through wounds in the bark and decay the heartwood. The diseased heartwood softens resulting in trees being structurally weaker and prone to breakage.
Termite Damage To Timber
A few types of insects are known to spoil huge amount of timber within a short time of few years. The White Ants may be found attacking the wood furniture in homes. These belong to a group of insects called Termites. Other insects are Pole-Borers, Marine-Borers etc. Marine-Borers attack the wooden base in the sea.
Defects in Timber
The following defects may occur in timber:-
- Natural Defects
- Artificial Defects
Let us take a look at them.
The natural defects are as follows:-
It is the most common type of natural defect. As the diameter of the tree increases, it covers the bases of lateral branches. The enclosed portion of branches within the wood are called knots. It affects the technical properties of timber and spoils the appearance.
Classification of Knots:
(i) Pin knots: less than 6.5 mm in diameter.
(ii) Small knots: 6.5 mm to 20 mm in diameter
(iii) Medium knots: 20 mm to 40 mm in diameter
(iv) Large knots: above 40 mm in diameter
some of the common types of knots are as follows:-
- Live Knots- It is the part of the tree that becomes completely enclosed within the growing trunk of a tree. In this, there is a complete structural continuity between the fibers of the main branch and the main tree.
- Dead Knot- In this, the continuity of structure is only up to 25 percent.
- Intergrown knot- This is a knot in which almost three-fourths of the fibers of the knot show continuation with the main tree.
- Encased Knot- This knot is entirely surrounded by bark.
These are the cracks or fissures in the timber. These may develop due to shrinkage on aging of the tree, movement caused by wind action on growing tree, freezing sap in the cells during its ascent etc. The various types of shakes are as follows
- Heart Shake- These are the cracks which develop in heart wood. They indicate the beginning of decay of the tree.
- Star Shake- They are also called radial shakes when fine in nature. They generally arise in the sapwood and extend towards the inner regions. Hence, they are broader outwards and get narrow inwards.
- Cup Shakes- These are also called ring shakes.These develop parallel to annual rings When fully developed, they may separate portions of timber longitudinally along the rings.
Grain in timber that does not lie in straight lines but it changes it's slope in relation to the plank.
These are typical enlarged swellings and occur due to branches cut-off.
In growing trees strong winds commonly cause shaking of standing trees again and again. As a reaction to such shaking action, wood in these trees develops additional strength in those parts where bending action is most severe repeated very often. The reaction wood is further distinguished into tension and compression wood depending on which side of a bending tree it developed.
Following are the artificial defects in timber.
These are the cracks or clefts developing longitudinally in the logs after their felling. These are often the cuttings across the annual rings of the log. In most cases, their origin is due to simple cause of unequal drying. Thus, when a log is exposed to atmosphere only on one side, the upper side will lose more moisture than that under side. This will result in shrinkage in the exposed region to such an extent that tensile forces sufficient to cause a crack or cracks in the wood will be easily developed.
It refers to the deformation in the original plane surface of a board cut from the timber.This is also attributed to unequal shrinkage. It generally develops in those boards which are cut from green or unseasoned timber.
Warping deformations can be of many forms. Of these, following are more common:
- Bow- It is a curvature of the board developed in a longitudinal direction. The thin board takes the shape resembling a bow.
- Cupping- It is distortion developed transversely. Length of the board remains unaffected, but width suffers distortion.
- Twisting- It is a complex distortion in the board in which one end of the board is so much distorted that it comes to lie over a part of the other edge.