Wood as Construction Material – Types, Structure, Processing

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Wood is one of the most used natural building materials in the world. A number of valuable properties such as low heat conductivity, small bulk density, relatively high strength, amenability to mechanical working etc. makes wood as famous building material. Timber can be used in most economical way without wasting any of the derivative of it. Even the saw dust obtained during wood sawing can also be used to make fiber boards, paper etc. In this article, we are going to discuss about the Classification and structure of tree and also about the Processing of timber from felling of tree to preservation of timber.

Types of Trees for Timber Production

The trees are classified into following types based on their mode of growth.

1. Exogenous
2. Endogenous

1. Exogenous 

Exogenous trees are outward growing trees. Horizontal section of such tree contains several rings which are nothing but annual rings. These rings can be used to predict the age of tree. Most of the exogenous trees are useful for many engineering purposes. Exogenous trees are sub classified into following types.

• Conifers
• Deciduous

Conifers

Conifers are nothing but soft wood producing trees which are also called as ever green trees. The timber of these trees is light colored, light in weight, low dense and poor against fire. Examples: Pine, Fir, redwood, spruce, deodar, cedar etc.

Deciduous

Deciduous trees are hard wood producing trees. The leaves of this type of trees are generally broad in size and they fall in autumn and grow in spring. Deciduous trees are most suitable for constructional purposes. Timber of deciduous trees is dark colored, dense, heaviest and good against fire. Examples: Maple, Mahogany, Oak, teak, walnut, babul etc.

2. Endogenous

Endogenous trees are inward growing trees which contains fibrous mass in their longitudinal section. The timber from these trees is useful in some limited engineering purposes. Examples: bamboo, palm, cane etc.

Structure of a Tree

The structure of tree can be divided in to two categories as follows

1. Macrostructure
2. Microstructure

1. Macrostructure

The structure of a tree which is visible to the naked eye is called macro structure of tree. Macrostructure of tree contains following components

• Pith
• Heart wood
• Sap wood
• Cambium Layer
• Inner bark
• Outer bark
• Medullary rays

Pith

The core part or innermost part of the tree is called as pith. It contains cellulose tissues which are helpful for the growth of plant during its young age.

Heart wood

Heart wood is the portion around the pith which is dark in color and contain some annual rings in it. It is very hard and provides rigidity to the tree. Heart wood is used for several engineering purposes because of its strength and durability.

Sap wood

Sap wood contain outer annual rings. This indicates the recent growth of tree and is light in color. It contains sap which helps in the growth of tress.

Cambium Layer

Cambium layer contains sap which will turn into sap wood after some time. It should not exposed to atmosphere otherwise the tree may dead.

Inner bark

The protecting layer of cambium layer is known as inner bark.

Outer bark

The outermost layer of the tree section is called outer bark or cortex. It contains cells of wood fiber.

Medullary rays

The rays extending from pith to cambium layer are known as medullary rays. These rays hold the annual rings of sap wood and heart wood together.

2. Microstructure

Micro structure of a tree can only be visible with great magnifications. It contains cells of different shapes and sizes. These cells are responsible for many actions like nutrients transport to branches from stem, strength of tree etc.

Processing of Timber

Processing of Timber contains following steps

1. Felling of Trees
2. Seasoning of Timber
3. Conversion of Timber
4. Preservation of Timber

1. Felling of Trees

Felling of trees is nothing but cutting of trees which are suitable for engineering purposes. Felling should be done when the tree is matured. Then only it contains more amount of heart wood than sap wood. The perfect age of trees for felling varies from 50 to 100 years. The best season for felling of trees is Mid-winter for plain areas and mid-summer for hilly areas. Firstly a cut is made at the lower most part of the trunk on a side where tree is expected to fell. The cut should be beyond center of gravity of tree. Then provide parallel cut which is exactly opposite to the first cut. Then tie up the tree top with 4 ropes on 4 sides. Now pull the rope of first cut side and loosen the rope on the opposite side. Using other two ropes swing the tree slowly. Then the tree starts breaking along the cuts and gently fall on the ground. The branches are chopped off, bark is removed and is cut into required sizes.

2. Seasoning of Timber

Seasoning of timber means removal of moisture content from timber. A newly fell tree contains water up to 50% of its dry weight. The timber contains free moisture and bound moisture. Free moisture is present in timber as water vapor while bound moisture is present in cell walls. When it is allowed to seasoning, free moisture evaporated first and this point is called as fiber saturation point. After Fiber saturation point, the timber will shrink on drying which is nothing but evaporation of bound moisture. There are two methods of seasoning are there namely

• Natural seasoning
• Artificial Seasoning

Read also: Various Methods of Seasoning of Timber

3. Conversion of Timber

Conversion of timber is the process of cutting of timber into required sections. This can be done by using power machines. Skilled persons should be required for economic conversion of timber. The conversion can be done by four types as follows

• Ordinary sawing
• Tangential sawing
• Quarter awing
• Radial sawing

Ordinary sawing

It is the most used and easy method of sawing. The cutting is done through the section of timber piece at perpendicular to the annual rings. Wastage of timber is minimum in this case. The obtained planks are not of equal strength. The outer planks contain sap wood and shrinks more while the inner portion planks contain heart wood which shrinks less.

Tangential sawing

In this type of sawing the cuts are tangential to annual rings and they meet each other at right angles. This method is suitable when the annual rings are very distinct with each other.

Quarter sawing

In quarter sawing, the cuts are made right angles to each other. This is suitable when the timber have no distinct medullary rays.

Radial sawing

In this type of sawing, the cuts are made parallel to the medullary rays radially. In this method wastage of timber is maximum.

4. Preservation of Timber

The final stage of timber processing is preservation which is carried out to increase the durability of timber and also to resist the attacks of fungi, insects etc. on timber. In general ASCU, Coal tar, Oil paints, Solignum paints etc. are used as preservatives. Read also: Preservation of Timber – Methods and Materials Used