The document provides information on wood as a building material, including its structure, properties, seasoning, defects, and grading. It discusses the following key points in 3 sentences: Wood comes from two types of trees - endogenous trees which grow inwards and exogenous trees which grow outwards. Exogenous trees are further divided into conifers like pine and fir, and deciduous trees like oak and teak that are most commonly used for engineering purposes. The document outlines various methods for seasoning wood including natural air drying, kiln drying, and chemical treatments to reduce moisture content and prevent defects before use in construction.

1. Building Materials
& Construction II
Lecture 1
Natnael Nemomsa

2. Part I (Wood, Steel & Glass)
1. WOOD: Wood comes from trees and is produced through natural
growth processes. Understanding tree physiology is essential to
knowing how to build with wood. Depending upon their mode of
growth trees may be divided in the following two categories:
o Endogenous trees – These trees grow inwards and fibrous
mass is seen in their longitudinal sections. Timber from these
trees has very limited engineering applications Ex: bamboo,
cane, palm etc.
o Exogenous trees – These increases in bulk by growing
outwards and used for engineering purposes. Exogenous trees
are further sub divided into two groups
a) conifers b) deciduous

3. a) Conifers: Conifers or evergreen trees: These trees having pointed,
needle like or scale like leaves and yield soft wood
b) Deciduous trees: The trees having flat broad leaves and leaves of
those trees fall in autumn and new ones appear in spring season.
Timber for engineering purpose is mostly derived from deciduous
trees. These trees yield hard wood.
Ex: ash, beach, oak, sal, teak, shishum and wallnut

4. • Comparison

5. Structure of tree
• The structure of timber visible to naked eye or at a small
magnification is called macro structure, and that apparent only at
great magnifications, the micro structure.

6. MECHANICAL PROPERTIES OF WOOD
• Because of its structure, wood has different strength properties
parallel and perpendicular to the grain.
◊ Tensile, bending, and compressive strengths are greatest
parallel to the grain and least across the grain,
◊ Shear strength is least parallel to the grain and greatest across
the grain. Except in plywood, the shearing strength of wood is
usually governed by the parallel-to-grain direction.
◊ Increasing moisture content reduces all strength properties
except impact bending, in which green wood is stronger than
dry wood.

7. Seasoning of Timber
• Seasoning is the process of reducing the moisture content (drying) of
timber in order to prevent the timber from possible fermentation and
making it suitable for use.
• It can also be defined as the process of drying the wood to a moisture
content approximately equal to the average humidity of the
surroundings, where it is to be permanently fixed.
• Objectives of Seasoning
1. Reduce the shrinkage and warping after placement in structure.
2. Increase strength, durability and workability.
3. Reduce its tendency to split and decay.
4. Make it suitable for painting.
5. Reduce its weight.

8. Methods of Seasoning
Natural Air: The log of wood is sawn into planks of convenient sizes
and stacked under a covered shed in cross-wise direction in alternate
layers so as to permit free circulation of air. The duration for drying
depends upon the type of wood and the size of planks. The rate of
drying is however very slow. Air seasoning reduces the moisture
content of the wood to 12–15 per cent. It is used very extensively in
drying ties and the large size structural timbers.
Artificial Seasoning: The prevalent methods of artificial seasoning are
as follows:

9. i. Water Seasoning: The logs of wood are kept completely immersed in running stream of water, with their larger ends
pointing upstream. Consequently the sap, sugar, and gum are leached out and are replaced by water. The logs are then
kept out in air to dry. It is a quick process but the elastic properties and strength of the wood are reduced.
ii. Boiling in water or exposing the wood to the action of steam spray is a very quick but expensive process of seasoning.
iii. Kiln Seasoning is adopted for rapid seasoning of timber on large scale to any moisture content. The scantlings are arranged
for free circulation of heated air with some moisture or superheated steam. The circulating air takes up moisture required
from wood and seasons it. Two types of kilns, the progressive and the compartment are in use.
iv. Chemical Seasoning: An aqueous solution of certain chemicals have lower vapor pressures than that of pure water. If the
outer layers of timber are treated with such chemicals the vapor pressure will reduce and a vapor pressure gradient is
setup. The interior of timber, containing no salts, retains its original vapor pressure and, therefore, tends to dry as rapidly as
if there had been no treatment. The result is to flatten the moisture gradient curves, to reduce the slope of the curves, and
consequently to reduce the internal stresses induced during drying. Since it is these stresses which are responsible for
defects such as checks, etc. a chemically treated timber will exhibit fewer defects. Common salt or urea are generally used;
the latter is preferred as the corrosive action of common salt is a drawback.
v. The logs are placed in such a way that their two ends touch the electrodes. Current is passed through the setup, being a
bad conductor, wood resists the flow of current, generating heat in the process, which results in its drying. The drawback is
that the wood may split.

10. Defects in Timber
Defects due to conversion: During the process of converting
timber to commercial form, the following defects may occur.
(i) Chip mark: mark or sign placed by chip on finished surface of
timber
(ii) Diagonal grain: Due to improper sawing of timber
(iii) Torn grain: Due to falling of tool small impression is formed
(iv) Wane: Presence of original rounded surface on the
manufactured piece of timber

11. Defects in Timber
Defects due to fungi: The attack of timber by fungi when moisture content of timber is above 20%
and presence of air and warmth for the growth of fungi the following defects are caused
(i) Blue stain: Sap of wood is stained to bluish color
(ii) Brown rot: Decay or disease of timber by removal of cellulose compounds from wood
and wood assumes the brown color
(iii) Dry rot: Convert the wood into dry powder form
(iv) Heart rot: This is formed when branch has come out of a tree and the tree becomes
weak and gives out hallow sound when struck with a hammer
(v) Sap stain: The sap wood looses its color because of feed on cell contents of sap wood.
(vi) Wet rot: Caused chemical decomposition of wood of the timber and timber converts to
grayish brown powder known as wet rot.
(vii) White rot: Attack lignin of wood and wood assumes the appearance of white mass

12. Defects due to insects:
(i) Beetles: Small insects form holes of size about 2mm diameter and
attack sap wood of all species of hard woods. Tunnels are formed in all
directions in sapwood by the larvae of these beetles and converted
into fine flour like powder. They do not disturb outer cover and looks
sound.
(ii) Marine borers: These make holes or bore tunnels in wood for taking
shelter. The wood attacked by marine borers loses color and strength
(iii) Termites: White ants are very fast in eating away the wood from the
core of the cross section. They make tunnels inside in different
directions and usually do not disturb the outer shell or cover

13. Defects due to natural forces:

16. Characteristics of Good Timber
• The principal characteristics of timber of concern are strength, durability
and finished appearance.
1. Narrow annual rings, closer the rings greater is the strength.
2. Compact medullary rays.
3. Dark color.
4. Uniform texture.
5. Sweet smell and a shining fresh cut surface.
6. When struck sonorous sound is produced.
7. Free from the defects in timber.
8. Heavy weight.
9. No woolliness at fresh cut surface.

17. Grading