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CLASSIFICATION OF TIMBER:
When it forms a part of a living tree, it is called standing
timber
When the treehas been felled,it is called rough timber
When it has been sawn to various market forms such as beams,
battens, planks etc, it is called converted timber
ENDOGENOUS:
Trees which grows inward in a longitudinal fibrous mass,
such
as canes, bamboo, palms etc
EXOGENOUS:
Trees which grows outward from the centre in approximate
concentric
rings across the longitudinalsection of the stem. Each ring
representing a layer deposited every year. Extensively usedin
engineering works. Exogenous trees are further divided into
CONIFER:
Evergreen tree yielding softwood
Trees with pointed leaves
Pine, Chirr
DECIDUOUS:
Trees with broad leaves yielding hardwood
Shed their leaves in winter (Sal, Sheesham etc)
Wood suitable for building or
other engineering works is called timber
TIMBER CLASSIFICATION OF TREES
Trees can be divided into the following groups:
standing timber
rough timber
converted timber
Palm tree (endogenous)
Exogenous tree
METHODS OF SAWING TIMBER
ORDINARY SAWN OR FLAT SAWN
Parallel cuts made throughout the length of the log
Cutting parallel slices of planks
Easiest and economical method
Shrinkage of sapwood more than the heartwood
Causing warp and twisting of planks
QUARTER SAWING
Tendency to cup i.e. to curve in a transverse direction
When applied to wood, not having distinct medullar
rays
this method produces very fine wood
RIFT OR RADIAL SAWING
Timber cut parallel to medullar rays and perpendicular to annual
rings
least shrinkage but most wasted
limited rift is adopted
Greater decorative effectmedullar rays pronounced
TANGENTIAL SAWING
Boards or planks sawn tangentially to annual rings
Not suitable for flooringPlanks cut by this method warp too
much
Decidous tree
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PROPERTIES OF WOOD & TIMBER
Wood is the oldest material used by humans for construction
after stone.
It is readily and economically available; easily machinable;
amenable
to fabrication into an infinite variety of sizes and shapes
using simple
on-sitebuilding techniques;
Specific Gravity (SG):Generally, specific gravity (SG) and the
major strength properties
of wood are directly related. SG for the major , usually
used
structural species ranges from roughly 0.30 to 0.90.
Moisture Content (MC) and Shrinkage:Wood is hygroscopic ; that
is, it picks up or gives off moisture to
equalize with the relative humidity and temperature in the
atmosphere.
Wood decay or fungal stain do not occur when the MC is below
20%. There is no practical way to prevent moisture change in
wood;
most wood finishes and coatings only slow the process down..
Thermal Properties/Temperature Effects:Although wood is an
excellent heat insulator, its strength and other
properties are affected adversely by exposure for extended
periods to
temperatures above about 100 F.At temperatures above 220 F, wood
takes on a thermoplastic
behavior.
Environmentally friendlyTimber is the most environmentally
responsible building material.
Timber has low production energy requirements and is a net
carbon
absorber.
Timber is a renewable resource.
In plentiful
Strong and lightweightTimber is strong, light and reliable
making timber construction simpler
and safer than steel or concrete construction.
Easy to install
Cost effective
Durable
Comfortable
Flexible
DECAY & PRESERVATION OF WOOD & TIMBER
The principal destroyers of wood are decay, caused by fungus,
and attack by a number of animal organisms of
which termites, carpenter ants, grubs of a wide variety of
beetles, teredo, and limnoria are the principal
offenders.
Decay will not occur if wood is kept well ventilated and air-dry
or, conversely, if it is kept continuously
submerged so that air is excluded.
The sapwood of all species is relatively vulnerable; only the
heart wood can be considered to be resistant
PRESERVATION & PRESERVATIVES:
Wood preservation protecting wood, wood products or timbers from
deterioration, decomposition or damage
dueto pest attacks throughapplication of chemical
substances.
Wood is usually deteriorated by fungi (white rot, brown rot, dry
rot, soft rot fungi), insects (termites, beetles,
carpenter ants), marine borers (molluscans, crustaceans), and a
host of other agents. No single preservative is
available which can meet all desirable requirements.
Wood preservatives may be oil-borne, water-based or a mixture of
different substances. Creosote and
pentachlorophenol (PCP) are the organic oil-based preservatives,
earlier used for treating of wood poles, piles
and railway sleepers.
The third type is a preservative paste, formulated by mixing the
oil-borne and water-borne types, such as
heavy creosoted boron (HCB).
The penetration and retention of a preservative, however,
depends on the preservation method. The different
methods being used include washing and coating; brushing,
spraying and dipping; soaking; Boucherie process;
hot and cold bath process; diffusion process; and pressure
process.
DRYING AND SEASONING OF WOOD:The process of removal of moisture
content from wood so as to make it useful for construction and
other uses is cal led drying of wood or seasoning of wood. This
reduces the chances of decay,
improves load bearing properties, reduces weight, and exhibits
more favorable properties like
thermal & electrical insulation,glue adhesive capacity &
easy preservativetreatment etc.
NATURAL SEASONING ARTIFICIAL KILN SEASONING
In this method wood is dryed usually by the
keeping the wood exposed to air, so that the
moisture evaporates and wood is seasoned.
This method is very economical is a sense that no
operational charges exists but the process is tooslow
In this method the wooden pieces are kept on huge
trolleys and then it is placed inside the hot chamber
of a furnaceknown as kiln.
In this way wooden pieces are kept in a controlled
atmospheric condit ions for two weeks. The timedepends upon the
type of wood and initial moisture
content. Blowers are also used. rates in a kiln can be
carefully controlled and defect losses reduced to a
minimum.
WATER SEASONINGIn this method the wooden pieces are immersed
in flowing waterfor about one month.
During this period the flowing water removes the
excess of sap in the wood. Then the wooden
pieces are taken for complete seasoning is lesser
but by this method strength of wood gets reduced
afterseasoning.
Afterit air seasoning is done.
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DEFFECTS IN WOOD:
The common defects occurring in the wood and it can be divided
as:Defects which are caused in the tree due to abnormality in the
growth.
Defects are also caused during seasoning operation.Some defects
are also there due to termites or insects
NATURALDEFECTS:
1. Knots:It is the centre of the irregular part of the tree.
The
fibre cells of tree are wrapping around it and thenormal shape
of the internal structure is disturbed.
In presence of knots processing a timber is not so
easy and due to this timber also becomes weaker
in section. Types of knots are:
LIVE KNOTS (these occur after felling of tree)
DEAD KNOTS (these occurs before felling of the tree)
Knots are also classified according to their shapes.
Circular knot
Oval knot
Irregular knot
Line knot
Symmetrical knotGroup knot
2. Shakes:It is produced by internal and external diseases of
the tree.
When there is delay in cutting of a tree even after complete
maturity at that time bonding materials between the grains
of
the tree evaporates. Sometimes the growth of tree cells from
one or another portion remains dead. Shakes appears at such
places. Types are:
a. HEART SHAKES (these defects occur generally in the oldtrees.
A small cavity is formed at the centre of the tree i.e. in
heart wood)
b. RING SHAKES (these shakes are formed as cavity on
thestructural rings of the tree. These are generally caused by
winds)
RING SHAKES
c. TWISTED SHAKES (these are continuous shakes in twisted
form)
d. PERIPHERAL SHAKES (These appears onthe outer
part of the tree i.e. bark. It grows
towards to centre of the trunk)
TWISTED SHAKES
PERIPHERAL SHAKES
3. Twisted Grains:When the trees are affected by fast winds
their grains becomes twisted.
The grains of the wood are not straight but are at an
inclination with
respect to the axis. It is very difficult to work with the tools
due to twisted
grains.
4. Ring gall:It is an uneven broken part of tree. It is
generally appears
where branches are cut or irregularly broken. From the
broken portion there is growth of new sap wood that donot unite
with the parent wood, as such it creates a cavity
between two where decay takes place.5. Distortion:Due to uneven
moisture
inside the wood, the
shape of wood is
distorted. It is caused by
improper seasoning.
Even cracks may occur
inside the wood due to
distortion. Twisting and
bowing occurs generally
DISTORTION TWISTED SHAKES
