Timber

Introduction
• Timber Is one of the oldest material of
Construction. Parts of the trees were the first
to be used by man for shelter even before he
entered the age of civilization.
• From the prehistoric times till date, timber
has been used in a variety of ways in
building construction, bridge construction,
in tunnels and many other engineering
activities.

Introduction
Among many Qualities of timber the following may be mentioned.
• It is a natural material and a renewable resource, with proper
planning, trees can form in-exhaustable source of construction
materials.
• It has quite satisfactory engineering properties Thus, it has
better strength than concrete and steel.
• Interestingly it is better fire resistant than steel or reinforced
concrete. Steel on heating softens, whereas in timber, as a structural
member heat travels from outside to inside at a slow rate and an
inner unburnt part of timber beam may stay intact without collapsing
if designed with a good factor of safety.
• It is easily workable, any convenient shape can be given to
timber
• It is durable. If used with care and caution, timber parts can stay
faithfully for centuries.

Growth of Wood
• There are two ways in which trees grow up the
endogenous growth and the exogenous growth.
• In the endogenous growth, a plant grows by
addition of new cells only at the end of the
previous year’s plant height. Such a tree
obviously grow upward without becoming thicker
and broader. They remain, thin, slender, and
knotted, Examples are Bamboo, Palms, and
Yuccas. Endogenous trees are practically useless
as a structural timber.

Growth of Wood
• In the exogenous growth, the plant grows by
the addition of a new layer of cell all around
those grown in the previous year. This process
naturally result in increasing in the height as well
as diameter of the plant. Result is a big, tall, fat
type of tree.
• Almost all the structural timber is derived
from the tree having exogenous growth, among
them we can mention, pines, deodar, walnut ,
teak, mango, shishum, spruce, etc.

Structure Of Exogenous Wood
• The structure of exogenous wood may be
conveniently studied under three headings, the Wood
element and the arrangement of these elements.
• (a) Wood Element
• The fundamental element in all types of wood tissue
is a cell. It is microscopic to sub-microscopic structure
of different shapes and sizes.
• An outer wall called the cell wall is made up of
lignin, starch and glucose. It is the cell wall that is
most important from engineering point of view.

Structure of Exogenous wood
• Within the (inner) cell wall is enclosed the cell
cavity (also called lumen) it is filled with sap in
the living cell.
• Since the wood tissue consist of thousands of
such cells the adjacent cell walls are separated
by thin layers of tissue called middle lamella.
• The primary type of cell of wood tissue are:
• Tracheids: These consist of very thin,
submicroscopic, human hair like cell which
run vertically in the tree the trachids form the
principal cell of the soft wood group.

• Vessels These are also longitudinal cell similar to trachids,
but they differ in two main aspect their wall are
comparatively thick and strong; and are open at ends when
they make up the tissue of a tree the wood is called a
hardwood.
• The Storage cell
• These form the so called medullar rays that are developed
radially and connect the outermost layer of the
longitudinal cell with the inner layers they are also referred
as Parenchyma.
• In a living tree, growth is achieved by the natural process
of cell division in a special layer of cell called cambium
which is located just under the skin of tree

• Arrangement of wood element
• The wood element namely the trachids, vessel
and rays are arranged in a definite manner to
give rise to a typical structure of wood.
• 1) Bark
• It is the outer most zone of wood structure
which can be described as skin of the tree. It
is hard, rough and thick in some trees. In some
trees it may be soft and smooth.

The Cambium
• The Cambium
• In the living tree, there is always present
just inside, the inner bark a delicate layer of
special cells, this layer is called cambium. It
is in this layer the process of growth takes
place by cell division.
• In old and cut trees, the cambium is no
longer distinguished from the rest of wood.

Wood
Wood
• It is main zone of tree tissue, It starts from cambium
and extend right up to the centre of the tree trunk. It
is made up of concentric rings of cells.
• The wood zone is further distinguished by
heartwood and Sapwood.
• The heartwood is made of wood cell lying close to
the core of the tree they are older in age and darker
in appearance. In heartwood, cell almost become
inactive and rigid. They provide strength and support to
the tree they contain little or no sap.

Wood
• The sapwood is made up of new and light tissue that lies close to
the skin or bark. In this zone cells are yet active and alive and full
of sap In fact the name sapwood is because of their higher sap
content.
• The Pit
• The innermost cell of heartwood becomes practically inactive
and dead with the passage of time. These cells being in centre of
the tree, receives little or no sap food in the later years. Hence, they
start deteriorating and decay. The older the tree grows, bigger the pit
zone becomes in size, this is the reason that older trees do not give
sufficient yield of good timber.
• The Medullar Rays These are made up of rectangular cells
which arise from cambium and spread towards the inner part of
the trees.

Classification of Woods
• Botanically trees are divided into two main
divisions the angiosperms and
gymnosperms. In the former, the seed is
enclosed within a fruit (mango) or a nut. In the
gymnosperms, however the seeds have no such
cover, they are naked, Pines are the best
example of this group of trees.

• In another classification trees are classified
as :
• The Deciduous Trees
• These are characterized with broad, well
defined leaves which grow in spring and fall
in autumn. Walnut, mango, teak, and oak
belong to this group.

• The Conifers
• These are characterized with needle shaped,
very thin leaves which do not fall every year.
They fall only when they become old. Such
trees are also Commonly called ‘evergreen’
Pines, deodar, kail and fir are some of common
conifers

• Trees are sometimes classified as Softwood and
Hardwoods.
• The Softwoods
• Comprise of such trees which yield lightweight
timber. They are lighter in shades and show
distinct annual rings. They are soft to work and
comparatively weaker in strength. But these
yields the most commonly used timber for
construction purpose. All the conifers yield
softwood.

• The Hardwoods
• Trees which yield dark colored, denser and
stronger woods are classed as hardwoods.
Most of the deciduous trees are teak, ash,
and oak yield hardwoods. In these trees
annual ring are faint, The individual cells are
thick walled and hence give additional strength
and density to the woods.

Properties of Wood
• Most important properties of wood may be discussed
under following headings;
• Color and Odor
• Most trees are characterized with a typical color and
odor. Thus Walnut wood is distinguished by its
typical brown color.
• Similarly, a fresh cut teak wood has golden yellow
shade. The softwood like deodar and pine show light
color. As regards odor smell quite a few woods are
immediately identified by their characteristic smell.
Teak wood for example has an aromatic smell. The pine
smells of resins. The color and odor, however, may
show variations.

Properties of Wood
• Specific Gravity
• Wood is a very light material, its specific
gravity being always less than. Hence wood
float on water. Wood show great deal of
variation in their specific gravity, Some
variety may be as light as 0.3 whereas in
others the specific gravity may approach 0.9
this depends on their structure and presence
of pores in them.

Properties of Wood
Moisture Content
• All woods are porous to some extent. Further all woods are
hygroscopic as nature. They gain moisture from the atmosphere
depending on moisture content of their cell the humidity in the
atmosphere. The natural moisture content can be easily determined
by following relation :
• Mc= W 1- W 2 x 100
W 2
Where, W1= Weight of the natural wood sample.
W 2 = Weight of the same sample when oven dried.
Hence Mc values of natural green wood may sometimes be as high as
250 percent.
A moisture content of 12 to 15 percent of air seasoned woods is
considered quite safe for timber in any construction.

Properties of Wood
Grain
• By grain is understood the arrangement and
direction of growth of the wood element in the
wood. In the normal wood fibers, grow parallel
grow parallel to the length of the tree trunk. This
type of structure is called a straight grain.
• The fiber may be very tightly and closely
packed giving rise to a fine-grained texture in
wood. In other cases they may be
comparatively broad and quite wider. The
structure is then termed as coarse grained.

Properties of Wood
Shrinkage and Swelling
• The newly cut wood looses moisture when subjected to
drying naturally or artificially. On drying, the wood
undergoes shrinkage. Similarly, dry wood on getting rain
soaked or wetted may undergo considerable swelling.
• It is known that in the drying process, moisture from the
wood is lost first from the cell cavity and then from the
cell walls. It is only when the water is lost from the cell
wall that the wood start shrinking.
• Conversely when dry wood is wetted, the water is first
received by cell walls. Only when the wall becomes
saturated, water goes to the cell cavities Hence on wetting
the swelling starts quickly.

Strength
• The most important fact about strength of timber is that
it is not the same in all the directions. This is because
wood is anisotropic material (having different structure
in different direction) Hence the strength of wood is
determined with reference to the direction of grain of wood
under load. Hence the strength of wood is determined with
reference to direction of grain of the wood under load.
Besides grain, many other factors also influence the strength
of timber, these are,
• Density: Higher the density of timber, greater will be its
strength, this is because high density of timber is a result
of thicker cell walls. i.e.. greater amount of wood
substance per unit volume.

Strength
Moisture Content
• Higher the moisture Content, lower is the strength
of the timber. This is because water in itself has no
load bearing capacity. Its increased volume in the cell
simply decrease the volume of wood tissue They tend
to reduce the strength in an indirect manner.
Presence of defects
• There may be a number of natural and artificial
defects in timber such as cross-grain, knots, and
shakes etc. All of them cause a decrease in the strength
of the timber

Tensile Strength
• Wood is very strong to tensile force acting
parallel to grain but very weak when these
forces are made to act perpendicular to the
grain, thus the tensile strength of some wood
ranges from 500 – 2000 kg/cm2 parallel to
grain whereas same value lie between 10 – 100
kg/cm2 for the same varieties when tested
perpendicular to the grain.

Transverse or Bending Strength
• The most important use of timber as beams
is based on the fact that wood has very high
bending strength, it may vary from 300 to 900
kN/cm2 or more.

Defects in Wood
• As defect in timber may be defined as any
undesirable character natural or artificial
that lowers its strength, durability or
quality. Defect in timber are generally
distinguished as natural defects and artificial
defects.

Natural Defects
• This group includes all those defects that
are developed in a tree during its growth.
Their development.
• Therefore, Cannot be generally controlled by
man. Most common natural defects are knots,
shakes, cross grain, crookedness, rind galls,
burr and curl.

Natural Defects
• Knots
• A knot marks the position of growth of a
branch of tree. On the cut boards, it is most
easily seen by its conspicuous darker
appearance and harder character. A knot is
essentially a discontinuity in the timber and
hence a place of weakness.

Live Knot
• Knots show great variation in their size, shape,
and stage of development.
• Live Knot
• It is the part of the branch that becomes
completely enclosed within the growing
trunk of a tree. In such a knot, there is
complete structural continuity between fibers
of branch and the main tree.

Dead Knot
• Dead Knot
• In such a knot, the continuity of the
structure is almost lost or is only up to 25 %
• An Intergroup Knot
• This is a term used when almost three-
fourth of the fibers of the knot show
continuation with the main tree.

Knots
• Encased Knot
• It is a knot surrounded entirely by bark.
• Edge Knot
• It is seen only in sawn timber, on one edge of cut
portion.
• Face Knot
• Is also seen on cut timber on the face of the board.
• Knot of whatever type and shape are always
considered to be defect.

Shakes
• After knots shakes are more serious and
common defect. In simple language they are
also called cracks or fissures.
• Shrinkage on aging of the tree
• Due to movement caused by wind action in the
growing.
• Freezing of sap in the cell during its ascent.

Shakes
• Heart Shakes
• These are cracks or clefts occurring in the heart wood
i.e.. towards the inner regions they indicate beginning of
decay of the tree. They become thinner as they extend
outward.
• Star Shakes
• These are also called radial shakes they generally arise
in sapwood and extend toward inner region.
• Hence, these are wider in the outer regions and become
narrows as they pass inwards. Severe temperature
difference during different seasons is generally the cause of
their development.

Shakes
• Cup Shakes
• These are also called ring shakes these are
cracks that develop parallel to the annual
rings. As such when fully developed they may
separate portion of timber longitudinally along
the rings. Reasons for development of cup or ring
shakes are to be sought in the unequal growth of
the timber. The most important effect of shakes on
the timber is that the shakes reduces resistance to
shear.

Cross Grain
• Cross Grain
• As said early, fibers of the wood in a normal
tree are usually parallel to the axis of growth,
the structure is called straight grained. In some
cases however, these fibers are not straight they
make angle or slope with the axis of tree, the
structure is then called cross- grained. The slope
of the grain becomes harmful when it exceeds
specific limits.

Rind Gall
• Rind gall i.e. Overgrowth of timber in some
parts of a tree may result in some typical
defects. Rind galls are quite common. A rind
gall is a simple a highly thickened, enlarged wood
cover developed over injured part of the tree. It is
a defect because it will be removed first of all
conversion of tree.
• Burr and Curl are generally developed as
thickened overgrowth. Such overgrowths
commonly result in severe twisting of fibers in the
board cut from this portion.

Reaction Wood
Reaction Wood
• In the growing trees strong winds commonly
cause “Shaking” of the standing tree again &
again. As a reaction to such shaking action,
wood in these trees develops additional
strength in those parts where bending action is
most severe and often repeated. Within a tree
there may be zones of normal wood and also
zones of exceptional strong wood.

Secondary Defects
• These include such defects that develop in the timber after its
felling and conversion to different sizes.
• Checks
• These are cracks or clefts developing longitudinally in the logs
after their felling. These cracks or checks are often cutting
across the annular ring of the log. In most of the cases their origin
is due to simple cause of unequal drying. Thus, when a log of wood
remain exposed to atmosphere only on one side, the upper side will
loose more moisture than the under side.
• This will result in shrinkage in the exposed region to such an
extent that tensile force sufficient to cause crack or cracks in the
wood will be easily developed. These cracks can be avoided by
changing the position of the timber log at regular intervals. This
will insure Uniform Shrinkage, which will not allow checks to
develop.

Warps
• Warping is defined as a deformation in the original plane surface of a
board cut from the timber, this is attributed to unequal shrinkage. It is
most commonly developed in those boards that are cut from green or
“unseasoned” timber.
• Warping timber can take many shapes
• Box It is a Curvature of the board developed in longitudinal direction.
The thin board takes the position of a bow.
• Cupping It is a distortion developed transversely, Length of the board
remains un affected but the width suffers distortion.
• Twisting
• It is a complex type of deformation in which one end of the board is so
much distorted that it comes to lie over a part of the other edge.

References
• Building Construction : Dr B.C. Punmia
• Civil Engineering Material : Prof. Singh
• Internet Web Sites

Timber

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