Timber
Timber or Lumber is nothing but wood used for building and other
engineering purposes. Timber, being a natural product available in abundance in
India, is used for the construction of doors. windows, roofs, partitions,
beams, posts, cupboards, shelves etc.
Timber is a type of wood which has been processed into beams and
planks. It is also known as “lumber” in US and Canada. Basically, timber or
Lumber is a wood or firewood of growing trees. Any wood capable of yielding a
minimum dimensional size can be termed as a timber or lumber. It is a stage in
the process of wood production. Timbers are used for the structural purpose.
Those woods which are adapted for building purposes are timbers. Finished
timber is supplied in standard sizes for the industry. Timber is used for
building houses and making furniture.
Timber is obtained from
the trunk of trees. Trees give different types of timber which can be used for
different kinds of works.
Classification of Trees -
According to the manner of growth, trees may be broadly
classified as Endogenous trees and Exogenous trees.
1. Endogenous Trees -
These trees grow inwards and fibrous mass can be seen in the
longitudinal sections. These trees grow by the formation of layers. Eg: Bamboo,
Cane, Palm, palmyrah, coconut etc. These trees have limited engineering
applications.
2. Exogenous Trees -
These trees grow outwards by the addition of rings of young
wood. A cross-section of these trees shows distinct approximate concentric
rings called annular rings.
They can be further
divided into two types:
a) Soft wood -
Softwoods have needle-like leaves and are also known as conifers
as they bear cone-shaped fruits. The conifers are also known as ever-green
trees and leaves of these trees do not fall till new ones are grown. Trees
under this group are pines, Chir, deodar, spruces, etc.
The qualities of softwood are:
(a) Light in weight and colour
(b) Has straight fibres and is resinous
(c) Has distinct annular rings
(d) Is weak and can split easily
b) Hard Wood -
Hardwoods are mostly of broad-leafed trees or deciduous trees.
Leaves of these trees fall in autumn and new ones appear in spring season. They
are deciduous trees like Babul, Mahogany, oak, sat, teak, etc.
Qualities of hardwood are:
(a) Heavy in weight and dark in colour
(b) Close-grained and non-resinous
(c) No distinct annular rings
(d) Strong and durable
Lumber mostly used for engineering purposes comes from Exogenous Trees.
Structure of Tree -
The horizontal section of a typical exogenous tree has following
parts:
Annual rings: A cross-section of an exogenous tree shows almost distinct concentric rings called annual rings.
Pith: This is the inner most part or core having soft tissues. It is found near the center of log or a Lumber. Its size varies from 1.5 mm to 10 mm in diameter.
Heart Wood: The inner part of the tree surrounding
the pith is called Heart Wood. It imparts rigidity to tree and provides strong
and durable timber for various engineering purposes.
Sap Wood: It is the area between the heart wood and
cambium layer. Sap wood contains living cells and takes active pan in growth of
tree. Usually, it is light in colour and weight.
Cambium Layer: If is the thin layer between sap wood and inner bark. It indicates sap which has yet to be converted into sap wood.
Inner Bark: It is the inner skin or layer covering
the cambium layer. It gives protection to the cambium layer.
Outer Bark: It is the outer most protective layer
of the tree. It consists of cells of wood fiber and it is known as cortex.
Medullary Rays: These are thin radial fibers extending from pith to cambium layer. It holds together the annual rings of heart wood and sapwood. One ring is added every year which decides the age of the tree.
Classification of Timber -
The terms timber and wood are often used synonymously, but they
have distinct meanings in the building industry. Wood is the hard, fibrous
material that makes up the tree under the bark, whereas timber may be defined
as a wood which retains its natural physical structure and chemical composition
and is suitable for various engineering works.
Timber can be classified as :
On the Basis of its Position -
Standing Timber - Living tree yielding
good timber is called standing timber.
Rough Timber - After felling and separating branches
of tree, it is known as rough timber.
Converted Timber or Lumber - when bark its
removed and stem is roughly converted into pieces of suitable length for
transportation is known as log. After the log is seasoned and converted into
commercial sizes like planks, battens, posts and beams, it is known as converted
timber.
On the Basis of Grading -
Commercial grading also known as yard grading or utility grading
refers to the principle by which the material is graded by consideration of
usefulness of the material and price factors. Commercial grading is further
divided in the following classes:
Grade A: This classification is based on dimensions and
general appearance. The dimensions of lengths, widths and thicknesses of
converted materials are measured. This system is prevalent is Kerala and
Mysore.
Grade B: This classification is based on the best
ultimate use of the material. Such a system is mostly in Andhra Pradesh and
some parts of Tamil Nadu.
Here, each grade is further divided into A, B and C classes to
indicate occurrence of defects. Only two lengths are recognized, long (L) which
is 5m and above, and short(S) that is under 5m. Each log is stamped such as BAL
(Beam, A-class, long), PBS (Plank, B-class, short), etc.
Some times another letter is also added indicating the species,
e.g. T for teak.
Grade C: This classification is based on qualitative
evaluation of defects and rough estimate of out-turn of utilizable material. It
is prevalent in Madhya Pradesh.
Grade D: This classification is based on evaluation of
units of defects and fixing the permissible number of standard volume of area
or the material in each grade. This system is prevalent in Bombay region and is
increasingly adopted in Indian Standards and is recognized internationally.
On the Basis of Modulus of Elasticity -
The species of timber recommended for constructional
purpose are classified as -
Group A: Modulus of elasticity in bending above 12.5
kN/mm².
Group B: Modulus of elasticity in bending above 9.8
kN/mm² and below 12.5 kN/mm².
Group C: Modulus of elasticity in bending above 5.6
kN/mm² and below 9.8 kN/mm².
On the Basis of Availability -
According to availability, timber can be of three grades, namely
X, Y and Z.
X - Most common, 1415 m³ or more per year.
Y - Common, 355 m³ to 1415 m² per year.
Z - Less common, below 355 m² per year.
This is based upon the figures supplied by the forest departments.
On the Basis of Durability -
Test specimens of size 600 x 50 x 50 mm are buried in the ground
to half their lengths. The condition of the specimen at various intervals of
time are noted and from these observations their average life is calculated.
Timbers are classified based upon such observations as of:
High durability average life of 120
months and over.
Moderate durability average life of less
than 120 months but of 60 months or more.
Low durability average life of less than 60 months.
On the Basis of Seasoning Characteristics -
Timbers are classified depending upon their behaviour to
cracking and splitting during normal air- seasoning practical under three
categories.
Highly refractory
(class A) are slow and difficult to season-free from defects.
Moderately refractory
(class B) may be seasoned free from surface defects, etc. If some
protection is given against rapid dyeing.
Non-refractory (class
C) these can be rapidly seasoned free from defects.
On the Basis of Treatability -
This classification is based upon the resistance offered by the
heartwood of a species to preservation under a working pressure of 1.05 kN/mm²
as
(a) Easily treatable,
(b) Treatable but complete preservation not easily obtained.
(c) Only partially treatable,
(d) Refractory to treatment.
(e) Very refractory to treatment, penetration of preservative
being practically nil from the sides and ends.
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