Introduction
The word timber is derived from an old English word nimbesan which means to build. Following three terms are to be noted in connection with the timber.
● Converted timber: This indicates timber which is sawn and cut into suitable commercial sizes.
●Rough timber: This indicates timber which is obtained after feeling a tree.
●Standing timber: This indicates timber contained in a living tree. The timber or wood as a building material possesses anumber of valuable properties such as low heat conductivity amenability to mechanical working, small bulk density, relatively high strength etc.
Uses of Timber in Construction
1. It can be used in form of piles, vertical posts, beams, lintels, doors and windows.
2. It can also be used as members of roofing trusses, ridge, truss and rafter.
3. It is used as formwork for cement concrete struc-tures. It is very much used in timbering the deep trenches.
4. It is an important material for furniture making. It is very much used in making sports goods, musical instruments, well curbs, agricultural implements etc.
5. It is very much used for railway sleepers.
It can be said that there is no engineering field, where timber is not used one way or the other.
Types of Timber
1. Teak: Its colour is yellow to dark brown. It is used for ship building, railway sleepers, railway carriages and furniture. It can be used for structural or deco-rative purposes. It is a costly timber and hence used for costly and specialized works. It can be easily sea-soned. It shrinks very little. It can be worked easily and finally varnished. Its fibres are straight. It does not warp due to shrinkage. It contains such resinous materials which acts as white ant resistant. Its colour is yellow to dark brown.
2. Sal: Its colour is brown. It is hard, fibrous and close grained. It requires slow and careful seasoning. It is durable under, ground and water. It weight after seasonings is 8000N/m3.
3. Mango: This tree is found practically in all over India. It is used for cheap fiirniture, packing boxes,cabinet work, panel for doors and windows. It is moderately strong and weighs 6550 N/m’
4. Jack: Its colour is yellow when freshly cut and it darkens with age. It is compact and even grained. It is moderately strong. It is easy to work. It takes a good finish. It maintains its shape well. Its weight after seasoning is 5950 N/m³. It is used for plain fur-niture, boat construction, well curbs, doors and win-dows, panels, cabinet making etc.
Defects in Tmber
Defects due to natural forces: The main natural forces responsible for causing defects abnormal growth and rupture of tissues.
1. Burls
2. Knots
3. Rindgalls
4. Shakes
5. Twisted fibers
6. Wind cracks
1. Burls: These are also known as the excrescenses and they are particularly formed when a tree has received shock or injury in its young age.
2. Knots: These are the bases of branches or limb.The portion from which the branch is removed receives nour-ishment from the stem for a pretty long time and it ultimately results in the formation of dark, hard rings which are known as knots. As con-tinuity of wood fibres is broken by knots, they form a source of weak-ness.
3. Rind Galls: Rind galls are due to wrong cut of a tree. They develop at points from where branches are improperly cut and removed
4. Shakes: These are cracks which partly or complete ly separate the fibres of wood. Cup Shakes: These are caused by the rupture of tis sues in circular direction: It is a curved crack and it separates partly one annual ring from the other. It develope due to non-uniform growth.
5. Wind Cracks: If wood is exposed to atmospheric agencies, exterior surface shrinks. Such a shrinkage results into cracks and these are known as wind cracks.
Seasoning of Timber
When a tree is newly felled it contains about 50 percent or more of its own by weight as water. Water is to be removed, before timber can be used for any engineer. ing purpose. Removal of water from timber is known as “Seasoning of timber.
Objective of Seasoning:
Seasoning of timber is carried to achieve the follow-ing objects.
● To decrease the weight of timber and thereby to lower the cost of transport and handling.
● To impart hardness, stiffness and strength to timber.
● To maintain the shape and size of the components of the timher articles which are expected to remain unchanged in form.
● To make timber easily workable and to facilitate operations during conversion.
● To reduce the tendency of timber to crack, shrink and warp.
Methods of Seasoning of Timbers
1. Natural seasoning
2. Artificial seasoning.
1. Natural Seasoning
this method, timber logs are sawn into planks or ether marketable sizes immediately after felling the tree. The sawn timber is stacked under a covered shed. Sawn timber is stacked in such a way that suf ficient space is left around each sawn piece, so that free circulation of air may take place without any dif ficulty. Timberpieces may be stacked horizontally or vertically. But horizontally stacking arrangement is the most common method. The platform where stack is to be erected should be raised from the adjoining ground by atleast about 300 mm. The stack is pre pared by laying layers of sawn pieces in cross wise directions in alternate layers, length of the stack is equal to length of timber pieces. Width and height of the stack are restricted to about 1.5 m and 3 m respectively. A number of such stacks may be con-structed under the same shed. Minimum distance between adjacent stacks should be kept about 600 mm. This method of seasoning is also called “air sea-soning Natural air remains circulating around each piece of the stacks and in due course of time, which depends upon the climatic conditions and the typeand size of the timber, seasoning is brought about
. 2. Artificial Seasoning
1. Boiling
2. Chemical seasoning
3. Electrical seasoning
4. Kiln seasoning
5. Water sensoning
Boiling:In this method, timber is immersed in waterand the water is then boiled for about three or fourhours. It is then dried very slowly. Instead of boilingin water, timber may be exposed to the action of hot steam. This method of seasoning proves to be costly.
Electrical seasoning: In this method, use is made of high frequency alternating current issued on tim ber. When it is green, it offers less resistance to the flow of electric current. is not economical but very costly.
Kiln seasoning: This method of seasoning is carried out in air tight chambers or ovens. Converted timber pieces are stacked inside the chamber such that spaces are left for free circulation of air. Now air, ful ly saturated with moisture and heated about 40°C is forced inside of the chamber. The heated air gradual-ly enters the inside of the timber pieces and the mois-ture content in the timber is gradually reduced.
Wood Based Products or Industrial Timber
1. Veneers: The primary process in the manufacture of The log is then cut to veneers. Depending on the cutting process, the veneers are classified as rotary veneers and sliced veneers.
2. Plywoods: Plywood is in the form of boards from thin layers of wood of veneers. Veneers are placed one above the other with the direction of grains of successive lay-ers at right angles to each others. To paste this wet and dry adhesives are used pasted plywood boards are cut with the help of hydraulic machine.
Uses of plywood
1. It gives decorative appearance
2. It will not expand under weathering conditions because it is manufactured as a flexible material.
3. It is used to make doors and it is easy to handle.
3. Fibre Boards: The resulting material is called the fibre board. These are rigid boards of thickness vaiy-ing from 3mm to 12mm. They are also known as “pressed wood” or “reconstructed wood”.
4. Laminated Board: Laminated board is a board having a core of strips 1 cm to 5 cm in thickness, glued together face to face to form a slab which in turn is glued to two or more outer veneers, with direction of the grain of the core strips running at right angles to that of the adjacent outer veneers.
5. Straw Board: These are pre-pared with chipped straw and gum in the form of boards. They are used for ceiling and the inside fac-ing wall of auditorium as acoustic materials.
