Timber is a naturally occurring material and is used for several engineering purposes. From tall tower blocks and bridges to doors and windows and interior decoration- timber structures find their application everywhere. What becomes extremely essential is the choice of good quality timber for construction. Here in this article, we discuss some factors based on which good timber is characterized.
Characteristics of Good Timber
A timber is said to be good based on the following characteristics :
- Fire resistance
- Mechanical wear
Durability is the first priority for any kind of engineering material. A good timber should be durable enough to resist the actions of chemical agents, biological agents, physical agencies, etc. However, wood is weak against the actions of strong acids and alkalis but it can withstand weak acids and alkali solution.
Strength is the resistance against failure. A good timber should not fail easily and it should be strong enough to take loads acting on it. The strength of timber varies depending on the direction (transverse or direct) it is loaded. Strength is considered as an important property while designing structural wood members such as wooden beams, joists, rafters, etc.
The permeability of timber should be less. A good timber should not absorb more than 8 to 12% of water by its weight when placed in water. The water permeability of timber depends upon various factors such as moisture content of timber, age of timber, type of timber, type of cut, etc.
Hardness is the resistance against penetration. Good timber should be hard and workable. The hardness of timber depends upon its density and heartwood properties. Stronger is the heartwood higher is the hardness of timber.
Toughness is the resistance against sudden shocks and vibrations. A good timber should be tough enough to resist sudden impact loads and vibrations. The toughness of timber is improved by the good interlocking of grains in its structure. A good tough timber is used to make handles of different tools, parts of automobiles, etc.
A good timber should be elastic. Elasticity is the property of regaining its original shape after deformation causing loads are removed. A good elastic timber is used to make sports items, shafts, bows, etc.
Workability of timber is the ease with which it can cut into the required shape. A good timber is easily workable. It should not damage or block the teeth of the saw during cutting.
The timber should be heavy as much as it looks. A timber is said to be good if it weighs heavy. Lightweight timbers are less in strength and unsound.
A timber with uniform structure is said to be a good timber. The fibers of timber should be straight and firm. The annual rings of timber should be closely located indicating the fact that the tree is mature enough to use it for making timber products. The medullary rays should be hard and compact. In general, an age of 50 to 100 years is considered a good age for felling of trees.
A timber is said to be good when it is free from defects caused by natural forces such as burls, knots, shakes, etc. and also it should be free from various fungal defects such as blue stain, dry rot, wet rot. etc.
11. Fire resistance
Fire-resistance of timber is very low. Of all the timber types, dense wood offers the most resistance against fire but up to certain limit only. The thermal conductivity of timber depends upon various factors such as moisture content, density, porosity, etc.
12. Mechanical Wear
Wooden floors, pavements, etc. made of timber are continuously subjected to traffic loads. Hence, good timber should not deteriorate easily against mechanical wear or abrasion.
The appearance of a good timber should be shiny when it is freshly sawed. This shining appearance is due to resinous matter present in the timber. The appearance of timber depends upon the arrangement of cells in its structure. If they are arranged in a uniform manner, the uniform appearance will be obtained after cutting. Fungal defects such as blue stain, sap stain, etc. damage the appearance of timber.
The strength of timber can be estimated by looking at its color. Light color designates low strength timber while dark color indicates high strength timber. Hence, dark-colored timber is preferable for engineering works.
The shape of timber should not be disturbed while cutting and seasoning. A bad timber may bow or warp or split during conversion.
The smell of a good timber is sweet. Bad or unpleasant smell indicates decayed timber.
The velocity of sound in timber (hardwood) is approximately 12 times more than that in the air. Hence, a good timber should produce clear ringing sound when struck. This is the reason why many musical instruments are made of wood. A dull heavy sound designates decayed timber.