Classification of Timber | Hardwood vs softwood pellets
Classification of Timber | Hardwood vs softwood pellets
Timber refers to wood used for construction works. In fact, the word timber is derived from an old English word ‘Tim Brian’ which means ‘to build’. A good tree that yields good wood for the construction is called ‘Standard Timber.’ After feeling a tree, it is branches are cut and it is stem is roughly converted from pieces of suitable length so that it can be transported to the timber cutting yards. This form of timber is known as called rough timber. By sawing(cutting), The rough timber is converted from various commercial sizes like planks, battens, posts, construction beams, etc. Such a form of timber is known as converted timber.
Timber was used in building material even by primitive man (the doors, windows). Many temples, Big palaces, and bridges built with timber can be seen even today.
Various types of bases are considered for the classification of timbers. The following are the important basics:
- Mode of growth
- Modulus of elasticity
Classification Based on Mode of Growth:
There are many Based on the mode of growth the trees are classified as;
(a) Exogeneous and
- Exogeneous Trees: These trees grow outward by adding distinct consecutive rings every year. These rings are known as annual rings. Here it is possible to find out the age of timber by counting these annual rings.
These trees may be further divided into (1) coniferous and (2) deciduous.
Coniferous trees are having cone-shaped leaves and fruits. The tree leaves do not fall in till new ones are grown. They yield softwood.
Deciduous trees are having broad leaves. These leaves fall in autumn and new ones appear in springs. They yield strength of the wood and hence they are commonly used in building construction works.
The classification of softwood and hardwood have commercial importance. The difference between softwood and hardwood is given below:
- In softwood annual rings are seen distinctly whereas in hardwood they are indistinct.
- The color of softwood is light whereas the color of hardwood is dark.
- Softwoods have lesser strength in compression and shear compared to hardwoods.
- Softwoods are light and hardwoods are heavy.
- The fire resistance of softwood is poor compared to that of hardwood.
- The structure of softwood is resinous while the structure of hardwood is close-grained.
Pith: It’s the innermost part of the tree and hence the oldest part of the exogenous tree center part when the plant becomes old, the pith dies, and becomes fibrous and dark. It varied in size and shape.
Heart Wood: This is the portion surrounding the pith. It is dark in color and strength. This portion is useful for various engineering purposes. This is the dead part of the wood. It consists of several annular rings.
Sap Wood: It is the layer next to heartwood. It denotes recent growth and contains sap. It takes an active part in the growth of trees by allowing the sap to move in an upward direction. The annual rings of sapwood are less sharply divided and are light in color. The sapwood is also known as laburnum.
Cambium Layer: It’s a thin layer of fresh sap lying between sapwood and the inner layer. They contained sap which is not yet converted into sapwood. If they’re the bark is removed and the cambium layer is exposed to the atmosphere, cells cease to be active and the tree dies.
Inner Bark: It is the inner skin of a tree protecting the cambium layer. It gives protection to the cambium layer.
Outer Bark: It is the outer skin of the tree and consists of wood fibers. Sometimes it contains fissures and cracks.
Medullary Rags: These are thin radial fibers extending layer from pith to cambium layer. They hold annular rings together. In some trees, they are broken and in some others, they may not be prominent.
These trees grow inwards. Fresh fibrous mass is in the innermost portion. Examples of endogenous trees are bamboo and cane. They are not useful for structural works.
Classification Based on Modulus of Elasticity:
Young’s modulus is determined by conducting a bending test. On this basis, timber is classified as:
Group A: E = 12.5 kN/mm2
Group B: E = 9.8 kN/mm2 to 12.5 kN/mm2
Group C: E = 5.6 kN/mm2 to 9.8 kN/mm2.
The Durability tests are conducted from the forest research establishment. They contacted bury test specimens of size 600 × 50 × 50 millimeters in the ground to half their length and observe their conditions regularly over several years. Then timbers are classified as:
High durability: If their average life is more than 10 years.
Moderate durability: The Average life between 5 – 10 years.
Low durability: Average life for less than 5 years.
Classification Based on Grading:
IS 883-1970 classifies the structural timber into three grades-select grades, grade I, and grade II. This classification is based on permissible stress value, defects, etc.
Classification Based on Availability:
Forest departments classify timbers based on availability as X—Most common. 1415 m3 or more per year Y—Common. 355 meters3 to 1415 meters3 per year Z—Less is common. Less than 355 m3 per year.
Properties of good timbers are:
Colour: It should be uniform.
Odor: It should be pleasant when to cut freshly.
Soundness: Timber is A clear ringing sound when struck indicates the timber is of good quality.
Texture: The texture (roughness) of good timber is fine and even.
Grains: In good timber grains are close.
Density: Higher the density stronger is the timber.
Hardness: Harder timbers are strong and durable.
Warping: Good timber does not warp under changing environmental conditions.
Toughness: The Timber should be capable of resisting shock loads (Electricity shock).
Abrasion: Good timber does not deteriorate due to wear. This property should be investigated if timber is to be used for flooring.
Strength: The good Timber should have high strength in bending moment, shear, and direct compression.
Modulus of Elasticity: Timber with a higher modulus of elasticity is preferred in construction.
Fire resistance: A good timber should have high resistance to fire (for safety purposes).
Permeability: Good timber has low water permeability.
Workability: Timber should be easily workable. It should not clog the saw.
Durability: Good timber is one which can resist the action of fungi and insects’ attack
Defects: Good timber is free from defects like dead knots, shakes, and cracks.