Timber
Building material Civil Engineering
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Timber
- It is wood prepared for use in building and carpentry and other construction works as a building material
Wood
- Wood is a hard and fibrous natural substance which forms a major part of the the trunk and branches of a tree.
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Types of timber
- Timber is used for Engineering Activities and is obtained from trees that are generally two types.
1. Exogenous
- Conifer Trees or Soft wood tree
- Deciduous Trees or Hard wood tree
2. Endogenous
| Soft Wood or Conifer tree | Hard Wood or Deciduous Tree |
|---|---|
1. Deodar, ChirPine ,spruce,fir,kail and larch. 2. Posses distinct Annular ring and Indistinct Medullary rays. 3. These tree have pointed Needle like leaves. 4. light in weight,colour. | 1. Sal, Shishum, Oak,Teak. Babul. 2. Thes tree have flat Broad leaves 3.They shed their leaves in autumn and which grow back again in spring. 4. Posses indistinct annular rings and distinct medullary rays |
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| Type | Description |
|---|---|
Example | Bamboo Cane Palm PCB |
Standing Timber | Timber containing a Living Tree. |
Rough Timber | Timber which is obtain after falling of tree. |
Log Timber | It is trunk of tree felt after cutting leaf of all branches of tree. |
Converted Timber | Timber which is cut into commercial tree. |
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| No | Property | Soft wood | Hard wood |
|---|---|---|---|
1. | Color | Lighter | Darker |
2. | Growth | Faster | Slower |
3. | Density | Low | High |
4. | Weight | Less | More |
5. | Annular Ring | Distinct | Indistinct |
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With respect to fire resistance, timber is classified as refractory timber and non-refractory timber.
1. The non-refractory timber
- it is resinous and it catches fire easily. Examples of non-refractory timbers are chir, deodar, fir, etc.
2. The refractory timber
- it is non-resinous and does not catch fire easily. Examples of refractory timbers are sal, teak, etc.
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| Durability | Description on basis of its average life |
|---|---|
| 1 High 2. Moderate 3.Low | More than 10 years 5 to 10 years less than 5 years |
Annular Ring
These are also called Growth Ring
These are the concentric (same center) layer of wood.
these are helpful in finding the age of tree
| Sn | property | description |
|---|---|---|
| 1. | Specific Gravity | generally SG. of wood is 1.54 IS code tells that wt of timber should be measured at 12% water content. |
| 2. | Strength | The modular ratio of Elasticity of timber is .5 to 1 × 10^4 Compressive strength of timber is maximum when load is applied parallel to grain On Application of external stress on time It behaves like as elastic material |
| 3. | Moisture content | Timber by nature is hygroscopic, Anisotropic And Orthographic ( hetrogenous) Recommended moisture content for structural element is 12 to 20% for door 10 to 16% for windows not be more than 15 % for frame work If a dry piece is kept in a Saturated atmosphere it can absorb 15% of the weight. |
| 4. | Shrinkage | Shrinkage of timber is about 0.1% along the length. 3-5% along the radius. 10-15% along the circumference. |
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Defects in Timber
1. Defect due to seasoning of Timber
- Bow defect.
- Cup defect.
- End spring.
- Twist defect.
2. Defects due to Conversion
- Chip mark - Rough surface
- Diagonal Grain
- Chip mark,
- Torn grain,
- wane
- (i) Chip mark:These are the marks or signs placed by chips on finished surface of timber.
- (ii) Wane:It is the presence of original rounded surface on the manufactured timber.
- (iii) Diagonal grain: Improper sawing, Indicated by diagonal mark or straight grained surface of timber.
- (iv) Torn grain: It is the small depression on the finished surface due to falling of tool
3. Defect due to Fungi
Fungi attack the timber only when Following two condition are satisfied together:-
- a. Moisture content of timber is above 20%
- b. Presence of air for the growth of fungi.
Note:- A wood submerged in water will not be attack by fungi because of absence of air.
Following are the defects caused by fungi on the timber
- Dry Rot,
- wet rot,
- sap stain,
- blue stain,
- Heart Rot,
- Brown Rot,
- White Rot,
Dry Rot
- It take place due to improper ventilation,
- no circulation of air ,presence of dampness on place
- It can be prevented by the proper seasoning of timber
following two condition is responsible for dry rot
- 1. greater than 20% moisture on timber
- 2. Presence of air for fungi growth
Note:- A wood submerged in water will not be attack by fungi because of absence of air.
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Wet Rot
- Some special type of fungi convert the timber into grayish brown powder
- Wet rot is caused by moisture being absorbed into the timber and make it soft and weak.
4. Defects due to Natural Forces
- a. Knots.
- b. Rind gall
- c. Shakes - heart shakes, star shakes, radial shakes, cup shakes.
- d.Cup shakes
shakes is the Longitudinal separation between annular ring
Heart shakes
- 1 It starts from pith and move toward the sap wood
- 2 it is wider at pith and diminish towards outward
- 3 it is opposite to star shakes
Star shakes
- Cracks are wider at the circumference and diminish toward the pith or center of tree
5. Defects due to Insect
Is code related to Timber
- IS: 1141-1958- Classification of timber for seasoning purposes
- IS: 399-1963- Classification of commercial timbers and their zonal distribution
- IS: 1708-1969-Methods of testing of small specimen of timber
- IS: 4970-1973-Key for Identification of commercial timbers
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Timber board
- Block board: Block board consists of smaller timber block up to 25 mm in width. These blocks are cemented edge to edge and on each face plies up to 3 mm thickness are glued. Block board is extensively used for construction of railway carriages, bus bodies, marine and river crafts and for furniture making, partitions, panelling, prefabricated houses, etc.
- Batten board: The batten board is a board having a core made up of strips of wood usually 80 mm wide, each laid separately or glued or otherwise joined to form a slat which is glued between two or more outer veneers with the direction of the grain of the core running at right angles to that of the adjacent outer veneers. These are used for door panels, table tops etc.
- Fibre board are used for internal finish, wall panelling, floor, flush doors, table tops. These are used for fire and sound insulation in large commercial buildings and cinema houses. Hard board imparts internal appearance and finish to the structure.
Processing of Timber
Timber processing from trees includes the following stages:
- 1. Felling of trees
- 2. Seasoning of timber
- 3. Conversion of timber
- 4. Preservation of timber
Felling of trees:
- The trees are knocked down (or) cut down to the ground. This is known as falling of trees.
Preservation of timber
Preservation of timber is carried to achieve the following three objectives:
- 1. Increase the life of timber
- 2. Make the timber durable
- 3. Protect the timber structures from the attack of destroying agencies such as fungi insects.
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Seasoning and Preservation of Timber
Make timber Fire Resistance
- Sir Abel’s Process: The surface of the timber is first painted with a weak solution of sodium silicate. Thereafter slaked fat lime solution of followed by a concentrated solution of silicate of soda. This makes timber fire-resistant.
- The fire resistance of timber can be enhanced by phosphates of ammonia, a mixture of ammonium phosphates and ammonium sulphate, borax and boric acid, sodium arsenate etc.
Types of seasoning and Preservation
Seasoning of timber:
The process of drying of timber is known as seasoning of timber.
Under controlled environment, as early as possible, moisture is extracted at uniform rate from all parts of timber during seasoning.Seasoning is done in two ways:
- 1. Natural seasoning
- 2. Artificial seasoning
Natural seasoning: In this method, the seasoning of timber is carried out by natural air and hence, it is sometimes referred as air seasoning.
Artificial seasoning: This seasoning can be done in various methods like boiling, kiln seasoning, chemical seasoning,water seasoning and electrical seasoning.
Water seasoning
- Water seasoning means a sap of wood section is removed by the weathering action of water after that it is subjected to natural drying. This results in the reduction of twist and warping
Electrical seasoning
- Electrical seasoning uses high-frequency current which produces heat to dry out the timber. It is very fast method of seasoning. The only disadvantage is that it may cause the splitting of timber from the ends.
ASCU treatment
- ASCU treatment is a method of preservation of timber, which results in the high durability of wood.
- In this method, one part of Arsenic penta-oxide, 3 parts of Copper Sulphate, and 4 parts of Potassium di-chromate are mixed and applied over timber which increases its resistance against white ants and hence, durability increases.
- This treatment increases resistance against attacks of white ants.
Creosote Oil:
- Produced by distillation of tar wood or coal
- Creosote is obtained from distillation of tar. For creosoting the timber is dried and then creosote oil is pumped under high pressure (1N/mm2) and high temperature.
- It is one of the best antiseptic against fungi and other agents.
- I almost doubles the life of timber.
- Process is known as creosotic.
- Oil temperature of 50 degree cetigrade.
- Applied Pressure 0.7 to 1 N/mm2 uniformly over the timber
Sir Abel’s Process:
- The surface of the timber is first painted with a weak solution of sodium silicate. Thereafter slaked fat lime solution of followed by a concentrated solution of silicate of soda. This makes timber fire-resistant.
Chemical Salts:
- Copper sulphate, Zinc Chloride, Mercury Chloride, Sodium Fluoride increase the durability of Structure.
Preservation with Coal Tar
- Hot tar is applied over the surface.
- It increase resistance against fire.
Oil paints or Solignum Paints:
- It increases the resistance to water.
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Some Important wood
- Mulberry wood is brown in colour and is so famous because of its elastic nature. It is strong, tough and elastic also. It can be easily turned and carved. It is used for making sports goods like cricket bats, hockey sticks etc.
- Mahogany is reddish Brown in colour and is durable in under-water conditions. It is used in furniture, fancy goods, ornamental panelling etc.
- Sal wood is hard, brown and close grained. It is fibrous and good polishing is difficult to achieve but it is good against moist conditions. It is used in railway sleepers, Wagons, Bridges etc.
- Deodar tree produces soft timber which is used for several purposes. It is used in cheap furniture, railway carriages, packing boxes etc.
Depending upon the ease with which Indian timbers can be seasoned, they are divided into three groups, namely, non- refractory timbers, moderately refractory timbers and highly refractory timbers.
- 1. Non-refractory timbers can be rapidly seasoned without any trouble. They can be seasoned even in the open air and sun. Examples are deodar, simul, etc.
- 2. Moderately refractory timbers have tendency to split and to crack during seasoning. They are therefore to be protected against rapid drying conditions. Examples are mango, rosewood, sissoo, teak, etc
- 3. Highly refractory timbers are likely to be damaged severaly during seasoning. They are difficult to season. Examples are axle wood, hopes, laurel, sal, etc.
Note:
- The cost of seasoning of timber will naturally depend on the thickness of timber and type of timber with respect to seasoning. It will be more for highly refractory timbers and less for non- refractory timbers.
The timber shows high strength in the direction parallel to grains.
- Compressive strength along grain > Compressive strength across grain
- Tensile strength along grain > Tensile strength across grain
- Note:
- Tensile strength along grain is 2 to 4 times Compressive strength along grain.
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The various methods of sawing timber are as follows:
1. Ordinary sawn or flat sawn:
- Parallel cuts made throughout the length of the log, cutting parallel slices of planks, etc.
- Easiest and economical method
- Shrinkage of sapwood more than the heartwood
- Causing warp and twisting of planks
2. 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.
3. Rift or radial sawing
- Timber cut parallel to medullar rays and perpendicular to annual rings.
4. Tangential sawing
- Boards or planks sawn tangentially to annual rings. It is not suitable for flooring
- The different types sawing is as depicted below:
Strength of Timber
Wood has three principal axes namely longitudinal, tangential and radial axes. Since it is orthogonal material m it has three values of modulus of elasticity varying by as much as 150 to 1, three shear moduli varying from 20 to 1, and six Poisson’s ratios varying by 40 to 1.
For different strength:
a) Compressive strength:
- The compressive strength is found to be highest when acting parallel to the axis of the growth. However compressive strength perpendicular to fibres of wood is much lower than that parallel to fibres of wood. Compressive strength parallel to fibre varies from 30.0 to 77.5 N/cm2.
b) Tensile strength:
- Tensile strength along direction parallel to the grain is found to have greatest strength that can be developed under any other kind of stress. Tensile strength parallel to fibres is of the order 80.0 to 190.0 N/cm2.
c) Shearing strength:
- Resistance to shear in across direction is found 3 to 4 times greater than that along fibres. The shear strength along fibre is found of the order 6.5 to 14.5 N/mm2.
- ∴ The strength of timber is highest in the direction of an angle of 0° to the grains.
Defect on timber
Checks:
- These are also called as cracks which occur on timber.
Shakes:
- Shakes are nothing but cracks which separates the wood fibres partly or completely. There is a longitudinal separation in the wood between the annual rings.
Knots:
- These are bases of branches or limbs which are cut-off from tree.
- The portion from which branch is removed receives nourishment from the stem for the long time and ultimately results in the formation of dark hard rings which are known as knots.
Rind galls:
- Overgrowth of Timber in some parts of a tree may result in some typical defects.
- Rind gall are quite common. A rind gall is simply a highly thickened, enlarged wood cover developed over an injured part of the tree.
- The meaning of rind is bark and gall is abnormal growth.
- So abnormal growth of the bark of the trees is called Rind galls. Improper cutting of branches causes this abnormal growth
Shakes
- Shakes are defects that occur around the annual ring or growth ring of timber.
Warehouse
- A warehouse is a large building where raw materials or manufactured goods are stored until they are exported to other stores, sites.
Characteristics of warehouse are as follows :
- Building should have minimum number of windows.
- Building should be leak proof as well as moisture proof.
- Roof should be waterproof.
Wood Products
1. Veneer
- Veneering is the process of manufacture of wood based products in which thin sheets of woods are produced which are known as veneers.
- The thickness of veneers varies from 0.4 to 0.6mm
- In no case it should be exceed 1mm
- Walnut is most preferable wood
2. Plywood
- A wood panel glued under pressure from an odd number usually 3 to 13 of layers/piles of veneers is known as Plywood.
- Used for Partition, ceilings, doors, concrete form work, plywood board, making furniture top sheet.
