TIMBER as Building Material

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Timber denotes wood, which is suitable for building or carpentry or
various other engineering purposes like for construction of doors,
windows, roofs, partitions, beams, posts, cupboards, shelves etc
Uses of timber:
(i) Used in the form of piles, posts, beams, lintels, door/window
frames and leaves, roof members etc
(ii) Used for flooring, ceiling, paneling and construction of
partition walls
(iii) Used for form work for concrete, for the timbering of
trenches, centring for arch work, scaffolding, transmission
poles and fencing
(iv) Used in wagon and coach building, marine installations and
bridges
(v) Used in making furniture of agriculture implements, sports
goods, musical instruments, well curbs, mortar bodies, carts
and carriages, railway sleeps, packing cases etc

Classification of trees
Depending upon their mode of growth trees may be divided in
the following two categories
(i) Endogeneous 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
(ii) Exogeneous trees: These increases in bulk by growing
outwards and used for engineering purposes.
Exogeneous trees are further sub divided into two groups
a) conifers b) deciduous
a) 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

Structure of tree: From the visibility aspect, the structure of a tree can
be divided into two categories

  1. Macro structure
  2. Micro structure
    I. Macro structure: The structure of wood visible to the naked
    eye or at a small magnification is called macro structure. Fig
    7.1 shows the macro structure of exogenous tree.
TIMBER as Building Material 1

(i) Pith: The innermost central portion or core of the tree
is called pith or medulla
(ii) Heart wood: The inner annual rings surrounding the
pith is known as heart wood. It imparts rigidity to tree

(iii) Sap wood: The cuter annual rings between heart
wood and cambium layer is known as sap wood
(iv) Cambium layer: Thin layer of sap between sap wood
and inner bark is known as cambium layer
(v) Inner bark: The inner skin or layer covering the
cambium layer is known as inner bark
(vi) Outer Bark: The outer skin or cover of the tree is
known as outer bark
(vii) Medullary rays: The thin radial fibres extending
from pith to cambium layer are known as medullary
rays
II. Micro structure: The structure of wood apparent only at
great magnifications is called micro structure under micro
scope, it becomes evident that the wood consists of living and
lead cells of various sizes and shapes.

Defects in Timber:
Defects occurring in timber are grouped into the following
divisions.
a) 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
b) 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 bluesh colour
(ii) Brown rot: Decay or disease of timber by removal of
cellulose compounds from wood and wood assumes the
brown colour
(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 colour 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

c) Defects due to insects:
(i) Beetles: Small insects form holes of size about 2mm diameter
and attack sap wood of all spacies 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
colour and strength
(ii) 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 donot disturb the outershell
or cover
d) Defects due to natural forces:
The main natural forces responsible for causing defects in timber
are abnormal growth and rapture of tissues
(i) Burls: Irregular projections appear on the body of timber
because of shock at younger age
(ii) Callus: Soft tissue or skin which covers the wound of
tree.
(iii) Chemical stain: Discoloured due to the chemical action
caused
(iv) Coarse grain: Annual rings are widened, tree grows
rapidly hence timber possesses less strength

(v) Dead wood: Timber obtained from dead standing tree
(vi) Druxiness: White decayed spots by fungi
(vii) Foxiness: Due to poor ventilation during storage or by
commencement of decay due to over maturity indicated
by red or yellow tinge in wood
(viii) Knots: Bases of branches or limbs which are broken or
cut off from the tree

(ix) Rind galls: Rind means bark and gall indicates abnormal
growth and pecullar curved swellings found on the body
of a tree.
(x) Shakes: These are cracks which partly or completely
separate the fibres of wood as shown in fig. 7.3.

TIMBER as Building Material 2

(xi) Twisted fibres: or Wandering hearts: caused by twisting
of young trees by fast blowing wind as shown in fig 7.4.

TIMBER as Building Material 3

(xii) Upsets or ruptures: Indicate wood fibres which are
injured by crushing or compression as shown in fig 7.5.

TIMBER as Building Material 4

Wood based products:
Timber which is prepared scientifically in a factory is termed as
industrial timber and such timber possesses desired shape,
appearance strength
(a) Veneers: These are thin sheets or slices of 0.40 to
6mm wood of superior quality. Indian timbers, which
are suitable for veneers, are mahagony, oak, rosewood, sissoo, teak etc. The process of preparing a sheet of veners is known as veneering. Veneers are
used to produce plywoods batten boards and lamin boards.
(b) Plywoods: Plywoods are boards, which are prepared
from thin layers of wood or veneers. Three or more
veneers in odd number are pressed using adhesives.
The plywoods are used for various purposes such as
ceilings, doors, furniture, partitions, panelling walls,
packing cases, railway coaches, formwork for
concrete etc. Thickness may vary from 6 to 25mm.
(c) Fibre boards: These are rigid boards and they are
also known as pressed wood or reconstructed wood.
The thickness varies from 3mm to 12mm. These are
available in lengths from 3 to 4.5m and width varying
from 12 to 18m. These are used for
(i) For internal finish of rooms such as wall
panelling; suspended ceilings.
(ii) To construct form work for cement concrete.
(iii) To construct partitions.
(iv) To prepare flush doors, tops of tables etc.
(v) To provide an insulating material of heat and
sound.
(vi) To work as paving or flooring material

(d) Impreg timbers: Timber which is fully or partially
covered with resin is known as impreg timber. The
usual resin employed is phenol formaldehyde which is
soluble in water. Impreg timber is available under
trade names such as formica, sungloss, sunmica etc
and it is used for moulds, furniture, decorative
artuicles etc.
(e) Compeg timbers: The process of preparing compreg
timbers is same as that of impreg timbers except that
curing is carried out under pressure. The strength and
durability of compreg timbers is more as compared to
the impreg timbers.

Characteristics of good timbers:

  1. Appearance: A freshly cut surface of timber should exhibit
    hard and of shining appearance.
  2. Colour: A colour should preferably be dark
  3. Defects: A good timber should be free from series defects
    such as knots, flaws, shakes etc
  4. Durability: A good timber should be durable and capable of
    resisting the action of fungi, insects, chemicals, physical
    agencies, and mechanical agencies.
  5. Elasticity: The timber returns to its original shape when load
    causing its deformation is removed
  6. Fibres: The timber should have straight fibres
  7. Fire resistance: A dense wood offers good resistance to fire
  8. Hardness: A good timber should be hard
  9. Mechanical wear: A good timber should not deteriorate
    easily due to mechanical wear or abrasion
  10. Shape: A good timber should be capable of retaining its
    shape during conversion or seasoning
  11. Smell: A good timber should have sweet smell. Unpleasant
    smell indicates decayed timber
  12. Sound : A good timber should give a clear ringing sound
    when struck
  13. Strength: A good timber should be sufficiently strong for
    working as structural member such as joist, beam, rafter etc.
  14. Structure: The structure should be uniform
  15. Toughness: A good timber should be tough (i.e.) capable of
    offering resistance to shocks due to vibration
  16. Water permeability: A good timber should have low water
    permeability, which is measured by the quantity of water
    filtered through unit surface area of specimen of wood.
  17. Weathering effects: A good timber should be able to stand
    reasonably the weathering effects (dry & wet)
  18. Weight: The timber with heavy weight is considered to be
    sound and strong.
  19. Working conditions: Timber should be easily workable. It
    should not clog the teeth of saw.

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