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TIMBER

A wood prepared for the engineering purpose

is a timber.

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Introduction.

Wood is a hard and fibrous substance which forms a

major part of the trunk and branches of a tree.

Timber is very widely used in buildings as doors,

windows, frames, temporary partition walls, etc. and inroof trusses and ceilings apart from formwork.

Now a days timber is being rapidly replaced by

composite wood materials like plywood, fibreboards,chipboards, compressed wood, impregnated wood in

which natural wood is just a basic ingredient of a matrix

or a laminate

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Advantages:

It is easily available (this wont be true after some

years) ,easy to transport and handle, thermal

insulation, sound absorption and electrical resistanceas compared to steel and concrete.

Wood is a good absorber of shocks as it is not

completely a dense material.

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Classification of timber

1) On the basis of its position:

Standing timber : implies a living tree.

Rough timber :forms a part of the felled tree.

Converted timber : are logs of timber sawn into planks, posts, etc.

Planks are the long thin flat piece of timber used in buildingA long piece of wood or other material set upright into the ground to serve

as a support for the beam.

PLANK POSTS

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2) On the basis of grading:

Structural grading:Structural grading refers to the principle by which the material is

graded on the basis of visible defects which have known effects on

the strength properties of the material

Commercial grading :

Principle by which the material is graded by consideration of usefulness of

the material and price factors. It is further divided into following grades.

Grade A: This classification is based on dimensions and general

appearance. The dimensions of lengths, widths and thicknesses ofconverted materials are measured.

Grade B: This classification is based on the best ultimate use of the

material.

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Grade C: This classification is based on qualitative evaluation of

defects and rough estimate of out-turn of utilizable material.

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.

3) 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.

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4) 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 yearYCommon, 355 m3 to 1415 m3 per year

ZLess common, below 355 m3 per year

5) ON THE BASIS OF DURABILITY:

Test specimens of size 600 50 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.

High durability :average life of 120 months and over.

Medium durability : average life of less than 120 months but of 60 months or

more.low durability : average life of less than 60 months.

.

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STRUCTURE OF A TIMBER:

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Characteristics of good timber:

1. Narrow annual rings, closer the rings greater is thestrength.

2. Compact medullary rays.

3. Sweet smell and a shining fresh cut surface.

4. When struck sonorous sound is produced.

5. Free from the defects in timber.

6. Heavy weight.

7. No woolliness at fresh cut surface.

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SEASONING OF TIMBER:

Timber cut from freshly felled trees is too wet for normaluse and is dimensionally unsuitable. Seasoning is the process of

reducing the moisture content (drying) of timber in order to

prevent the timber from possible fermentation and making it

suitable for use.

1.Reduce the shrinkage and warping after placement in

structure.Wood warping is a deviation from flatness as a result

of stresses and shrinkage

2. Increase strength, durability and workability.3. Reduce its tendency to split and decay.

4. Make it suitable for painting.

5. Reduce its weight.

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METHODS OF SEASONING

1) NATURAL SEASONING:

The log of wood is sawn into planks of convenient sizes and

stacked under a covered shed in cross-wise direction in

alternate layers The rate of drying is however very slow. Air

seasoning reduces the moisture content of the wood to 1215per cent. It is used very extensively in drying ties and the large

size structural timbers.

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NATURAL SEASONING

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2) Artificial Seasoning:

WATER SEASONING:

The logs of wood are kept completely immersed in running stream of

water, with their larger ends pointing upstream. Consequently the sap,

sugar, and gum are leached out and are replaced by water. The logs

are then kept out in air to dry. It is a quick process but the elastic

properties and strength of the wood are reduced.

Kiln seasoning of timber:

This is adopted for rapid seasoning of timber on large scale to any

moisture content. The scantlings are arranged for free circulation of

heated air with some moisture or superheated steam. The circulating

air takes up moisture required from wood and seasons it.

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Kiln seasoning of timber:

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DEFECTS IN TIMBER:

Defects in timber can occur at various stages, principally during the growing period and

during the conversion and seasoning process.

Due to abnormal growth:

1) Checks:

Finished timber

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In any wood there is always

opportunity for shrinkage in two

directions, along the radial line

following the direction of themedullary rays, and around the

circumference of the log following

the direction of the annual rings. If

the wood shrinks in both

directions at the same rate, the

result will be only a decrease in

the volume of the log, but if it

shrinks more rapidly around the

circumference of the log than

along the radial lines, the log must

develop cracks around the outside

as shown in Fig. 9. Such cracks are

called checks

Checks in Log of wood

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2) Shakes are longitudinal separations in the wood between the annual rings

Heart shake occurs

due to shrinkage of

heart wood, when

tree is over matured.

Cracks start from pith

and run towards sap

wood. These are

wider at centre anddiminish outwards.

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Rindgall is characterized

by swelling caused bythe growth of layers of

sapwood over wounds

after the branch has

been cut off in anirregular manner. The

newly developed layers

do not unite properly

with the old rot,

thereby leaving cavities,

from where decay

starts.

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KNOTS The root of the branch is embedded in the stem, with

the formation of annual rings at right angles to those of the

stem. The knots interrupt the basic grain direction of the wood,

resulting in a reduction of its strength. In addition these affect

the appearance of the wood.

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TWISTED FIBRE are caused by wind constantly turning the trunk of young tree in

one direction.

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Defects due to seasoning:

These defects are directly caused by the movement which occurs

in timber due to changes in moisture content. Excessive or

uneven drying, exposure to wind and rain, and poor stacking

during seasoning can all produce distortions in timber.

checkslongitudinal separation of fibres not extendingthroughout the cross-section of wood;

splittingseparation of fibres extending through a piece of

timber from one face to another;

warpageconsists of cupping, twisting and bowing.