Lesson 12 and 13: Basic Building Construction Materials
Timber
Timber is a natural product and is available in nature. Trees yield timber. Timber is used for fabrication of doors, windows, beams, rafters, posts, shelves, cupboards, furniture etc. Timber used for structural purposes is called as structural timber.
Classifications of Trees: Trees which yield timber are classified as
Exogenous trees or outward growing trees
Endogenous trees or inward growing trees
Exogenous trees: These trees show distinctive annual rings when cross sectioned. Most of the timber that is suitable for engineering purposes is derived from these trees. The timber that is obtained from exogenous trees is further classified as:
Soft wood: Soft wood is obtained from ever green trees. These trees have long and narrow pointed leaves. They show distinctive annual rings and are light in weight and colour. They possess good strength in tension but weak in compression. Ex-Deodar, Fir, Pine etc.
Hard wood:Hard wood is obtained from broad leaf trees. They have indistinct annual rings and are dark in colour. They are strong, hard and weigh heavy. They possess good strength in both compression and tension. Ex. Oak, Salt, Teak etc.
Endogenous trees or inward growing trees: Timber obtained from these trees has limited engineering applications. Trees like Bamboo, Cane, Subabul, Casuarina fall under this category. They are used for temporary structural supporting systems and are unsuitable as permanent structural elements.
Properties of Timber
It has good strength.
It has low heat conductivity.
It is easily workable. It can be sawn and carved to any shape.
It is highly inflammable.
It is likely to warp or bend when exposed to alternate wetting and drying conditions. However this property can be minimized greatly by proper seasoning and by use of good preservatives.
It is likely to be decayed by insects like white ants.
Requirements of Good Timber: Timber to be suitable for use in building construction should posses the following qualities.
Strength: The very basic requirement of any structural element is its strength in compression as well as tension and shear. A good timber should possess good strength and should be able to withstand shocking loads i.e. loads that are applied as a flash.
Durability: Another basic requirement of a good timber is its durability. It should have good resistance against action of weather i.e. it should not warp or bend under alternate wetting and drying conditions.
Defects: A good timber should be free from defects like knots, flaws, shakes, twisted fibers, rind galls etc.
Workability: It should be easily workable. it should be easily sawn and be capable of being planned easily.
Hardness: It should be sufficiently hard. It should not be penetrated easily by another body and should not split easily.
Elasticity: it should have good elastic properties. It should gain its original shape, when a load causing its deformation is withdrawn
Fire resistance: A good timber should not ignite quickly and should offer sufficient resistance to fire.
Toughness: Timber when used for wearing surfaces like floor boarding should not wear out quickly
General: It should weigh heavy and should be dark in colour. It should give clear ringing sound when struck by finger and should smell sweet. Bad smelling indicates decayed limber. The cross section should show close and regular annual rings and fibers should be straight.
