Timber

UNIT - II

Chapter - 14 (H)

TIMBER

 

TIMBER

Timber refers to wood used for construction purposes. The word timber is derived from an old English word 'timbrian' which means 'to build'. A tree that yields good wood for construction is known as standing timber. After felling the tree, its branches are cut and its stem is roughly converted into pieces of suitable length so that they can be transported to timber yards. This form of timber is known as rough timber. Rough timber is sawed and converted into various commercial sizes like planks, battens, posts and beams. This form of timber is known as converted timber.

 

1. USES OF TIMBER

Timber was used as a building material even by primitive man. Ancient temples and palaces were built with timber. Timber is used in construction for the following:

1. Load bearing structures like beams, columns, trusses and piles.

2. Light construction works like doors, windows, flooring and roofing.

3. Temporary works like scaffolding and centering.

4. Permanent works like railway sleepers, fencing poles, electric poles and gates.

5. Making furniture, cabinets and showcases.

6. Body works of buses, trucks, trains, boats, etc.

7. Bridges.

8. Industrial purposes like manufacturing pulp (for making papers), cardboards, wallpapers, plywood and boxes.

9. Making sports goods and musical instruments.

 

2. GENERAL PROPERTIES OF TIMBER

1. Strength: IS: 883-1970 gives bending strength, shear strength and compressive strength for commonly used Indian Timber. Sal and teak have good strength.

2. Modulus of Elasticity: It is defined as the force per unit area (stress) required for unit extension in unit length (strain). Timber is classified as Group A, Group B and Group C based on the above property.

3. Density: Denser wood in stronger.

4. Hardness: It is measured by resistance to scratching. Based on hardness, timber is classified as follows:

a. Soft to very soft - can be marked with a fingernail.

b. Moderately hard - cannot be marked with a fingernail, but markings are easily made with a knife.

c. Hard to very hard - knife markings are made with great difficulty.

5. Alignment of Grains: It refers to the direction of cells. Close grain refers to a type of wood in which fiber bundles are very closely packed together. If there are open pores on the face, the wood is said to have grains. If it has wavy grains, the wood is weak. Timber has good strength along the grain, when compared to its strength across the grains.

6. Texture: It refers to the structure of cells, i.e., size and distribution of cells. Most types of wood have an even texture. However, woods like teak exhibit uneven texture due to the difference in size and distribution of cells. Based on its structure, timber is classified as:

a. Fine textured b. Medium textured c. Coarse textured

7. Colour: It varies from creamy white to jet black through varying shades like yellow, grey, brown and purple. In general, the colour varies between the outer and inner layers of wood. Darker wood is stronger and preferred for construction.

8. Odour: It has a characteristic smell when freshly cut. Smell reduces gradually after exposure to atmosphere. Teak has a characteristic smell of old leather.

 

3. SEASONING

Seasoning is the controlled process of reducing the moisture content (MC) of the timber. It is done to reduce shrinkage due to MC and avoid warping. Construction grade timber must have MC below 20% to reduce the chances of dry rot and other fungi infestations. Seasoned timber although lighter will be stronger and more reliable. It is easier to work and apply paint, varnish, etc.

The methods of seasoning are classified as Natural seasoning and Artificial seasoning. Natural seasoning is done in the form of Air seasoning or Water seasoning. Artificial seasoning is done in the form of Boiling, Kiln seasoning, Chemical seasoning or Electric seasoning.

 

4. PROPERTIES OF GOOD TIMBER

Good quality timber has the following properties:

1. Strength: It should have high strength in bending, shear, direct compression and tension.

2. Modulus of elasticity: High modulus of elasticity is preferred in building construction.

3. Density: Higher the density, stronger is the timber.

4. Hardness: Harder timber is strong and durable.

5. Alignment of Grains: It should have close grains.

6. Texture: It should be fine and even.

7. Colour: It should be uniform and dark. Darker wood is stronger.

8. Odour: It should be pleasant when freshly cut.

9. Sound: It should have a clear ringing sound when struck.

10. Abrasion: It should be capable of resisting wear. This property is especially required if the timber is used for flooring.

11. Toughness: It should be capable of resisting shock loads.

12. Warpage: It retains its shape under changing environmental conditions.

13. Fire resistance: It has high resistance to fire. 14. Permeability: It has low water permeability.

15. Workability: It should be easily workable. It should not clog the saw.

16. Durability: It should be capable of resisting the action of fungi, insects, chemicals and changing weather conditions.

17. Defects: It should be free from defects like knots, cracks, etc.

 

5. ADVANTAGES OF TIMBER CONSTRUCTION

The advantages of timber construction are given below:

1. It can be worked easily with tools.

2. Connections can be made easily and neatly.

3. Additions, alterations and repairs can be carried out easily.

4. Its appearance can be enhanced by polishing or painting.

5. It has good thermal, sound and electrical resistance.

6. Its strength is proportional to its weight.

7. It can be used for load-bearing as well as for non load-bearing work.

8. It is cheap and economical.

9. It has good salvage value.