INTRODUCTION
Timber is one of the important construction materials. Wood is used as structural elements in buildings, widely for doors, windows and partitions and find large I use through secondary wood products like plywood, particle boards and laminated boards, etc. Both hard and soft woods are in use. As such defects in this construction material for buildings will affect the structural safety. aesthetic aspects and even functional use including fire protection.
Objectives
At the end of this unit, you should be able to:
-I understand the classification of timber, and
- how their properties and the defects in timber along
with extensive details concerning the causes for the defects.
DEFINITIONS
Let us understand the term timber. The word 'timber'
is &rived from an old English word. 'timnbrian' which means to build.
Timber, thus denotes wood which is suitable for building or carpentry or
various other Engineering purposes. The Timber is further termed as:
1) Standing
Timber, this indicates timber contained in a living tree.
2) Round Timber, indicates timber which is obtained
after felling a tree.
3) Converted
Timber This indicates timber which is sawn and cut hto suitable commercial
sizes.
Properties
Now,
let us
understand the properties of timber: Defects in Building Timber or wood, as a
building material, possesses a number of valuable properties such as :
i)
low heat conductivity,
ii)
amenability to mechanical working,
iii)
small bulk density, and
iv)
relatively high strength, etc.
Drawbacks Timber,
has its own drawbacks such as:
i)
susceptibility to decay and inflammability,
ii)
fluctuations in properties due to changes
in moisture content,
iii)
variations in strength in length and
across fibres, etc. These shortcomings of timber require careful consideration
while making use of it.
CLASSIFICATION OF TIMBER
Classification of Trees trees,
let us classify the trees for the Engineering purposes. Trees are classified
according to their mode of growth as below:
1. Exogenous Trees - These trees increase in bulk by growing outwards and distinct consecutive rings are formed in the horizontal section of such a tree. These rings are known as annular rings because one such ring is added every year and these rings are useful in predicting the age of tree. Timber which is mostly used for Engineering purposes belongs to this categories.
i) Conifers :
These are also known as ever-green trees and leaves of these trees I do no fall
till new ones are grown. As these trees bear cone-shaped fruits, Uley are given
the name conifers. These trees yield soft woods.
ii) Deciduous : The trees are also known as broad leaf
trees and leaves of these trees fall in autumn and new ones appear in spring
season. Timber for Engineering purposes is mostly derived from deciduous trees.
These trees yield hard woods.
Soft woods and hard woods:
soft woods form a group of ever-green trees. Hard woods form a group of broad
leaf trees. It is quite likely that some variety of soft wood may prove to be
stronger than some variety of hard wood. Examples of soft woods are chir,
deodar, fir, kail, pine, spuce, etc and those of hard woods are babul,
mahogany, oak, sal, teak, etc. iv) Generally hard wood is preferable for
structural components. Soft woods are also used extensively in the building
industry for structural components joinery and less sophisticated purposes like
partitions, pelmets, etc. Many products of secondary manufacture like plywood,
particle boards, roofing sheets and laminated boiud. have also come into wide
use.
2. Endogenous Trees -
these trees grow inwards and fibrous mass is seen in their longitudinal
sections. Timber from these trees has very limited Engineering applications.
Examples of endogenous trees are bamboo, cane, palm, etc.
STRUCTURE OF A TREE
Now, after the classifications, we should know the
structure of a tree how it has formed and we term. in its formation. '( A tree
basically consists of three parts, viz., trunk, crown and roots. From the
visibility aspect, the structure of a tree can be divided into three
categories:
Macrostructure -The
structure of wood visible to the naked eye or at a small magnification is
called macrostructure.
i) Pith: The
inner most central portion or core of the tree is called the pith or medulla.
It varies in size and shape for different types of trees. It consists entirely
of cellular tissue and it nourishes the plant in its young age. When the plant
becomes old, the pith dies up and decays and sap is then transmitted by the
woody fibres deposited round the pith.
ii) Heart Wood: The inner annual rings surrounding the
pith is known as heart wood. It is usually dark in colour. As a matter of fact,
it indicates dead portion of tree and as such, it does not take active part in
the growth of the tree. But it imparts rigidity to tree and hence, it provides
strong and durable timber for various engineering purposes. ‘
iii) Sap Wood: The outer annual rings between
heartwood and cambium layer is known as sapwood. It is usually light in colour
and weight. It indicates recent growth and it contains sap. The annual rings of
sap wood are less sharply defamed than those of heart wood. It takes active
part in the growth of tree and sap moves in upward direction through it. Sap
wood is also known as a1 Burnum.
iv) Cambium
Layer: The thin layer of sap between sap wood and inner bark is known as
cambium layer. It indicates sap which has yet not been converted into sapwood.
If the bark is removed for any reason, the cambium layer gets exposed and the
cells cease to be active resulting in the death of the tree.
v) Inner Bark: The inner skin or layer covering the
cambium layer is known as inner bark. It gives protection to cambium layer from
any injury.
vi) Outer Bark: outer skin or cover of the tree is
known as outer bark. It is the outermost protective layer and it sometimes
contains cracks and fissures. It consists of cells of wood fibre and is also ban
as cortex. vii) Medullary Rays: The thin radial fibres extending from pith to
cambium layer are known as medullary rays. The function of these rays is to
hold together the annual rays of heart wood and sap wood. These rays are
sometimes broken and in some varieties of trees, they are not very prominent.
Microstructure
-The structure of wood apparent only at great magnifications is called
microstructure. When studied under a microscope, it becomes evident that wood
consists of living and dead cells of various sizes and shapes. The moisture
content plays an important part in the use of timber. For example, the moisture
content of a door frame or door shutter should be appropriate for the service
conditions. There is an average of 15% for exterior joinery and an average
varying from 10% to 12% for interior joinery. Each species of timber has a
certain natural durability which is normally greater and never less for the
heartwood than the sap wood. Durability in this context is a measure of the natural
resistance of fungal decay and not to insect attack. It is seen that the heart
wood of Defects in Timber timber classified as very durable can be expected to
have an approximate life in excess of 25 years even if in contact with ground.
In general, larger pieces would have longer life. It is important to realise
that even species with low natural durability will not decay if the moisture
content in service is kept below 20%. Even a piece of timber given the descripting
of perishable will not decay if the details and service conditions cause it to
have a moisture content in service below 20%. The sap wood of almost all
timbers not just soft woods is either perishable or non-durable.
