55152d16bec1e53f7f2bc1aae730e014dd00585c ABOUT TIMBER IN BUILDING CONSTRUCTION - Construction, Final Account & Tender

Monday, April 3, 2017

ABOUT TIMBER IN BUILDING CONSTRUCTION

ABOUT TIMBER IN BUILDING CONSTRUCTION

Timber refers to wood used for construction works. In fact the word timber is derived from an old English word ‘Timbrian’ which means ‘to build’. A tree that yields good wood for construction is called 'Standing Timber'. After felling a tree, its branches are cut and its stem is roughly converted into pieces of suitable length, so that it can be transported to timber yard. This form of timber is known as rough timber. By sawing, rough timber is converted into various commercial sizes like planks, battens, posts, beams etc. Such form of timber is known as converted timber.

Timber was used as building material even by primitive man. Many ancient temples, palaces and bridges built with timber can be seen even today.
ABOUT TIMBER IN BUILDING CONSTRUCTION


Classification of Timber



Various defects which are likely to occur in timber may be grouped into the following three:

1.) Due to natural forces.
2.) Due to defective seasoning and conversions.
3.) Due to attack by fungi and insects.

1.) Defects due to Natural Forces

The following defects are caused by natural forces:

✤ Knots
✤ Wind Cracks
✤ Shakes
✤ Upsets

 Knots
When a tree grows, many of its branches fall and the stump of these branches in the trunk is covered. In the sawn pieces of timber the stump of fallen branches appear as knots. Knots are dark and hard pieces. Grains are distorted in this portion. Figure 1.9 shows some varieties of knots. If the knot is intact with surrounding wood, it is called live knot. If it is not held firmly it is dead knot.

 Wind Cracks
These are the cracks on the outside of a log due to the shrinkage of the exterior surface. They appear as shown in below picture.
wind-cracks-constructionway.blogspot.com
wind-cracks

✤ Shakes
The shakes are cracks in the timber which appear due to excessive heat, frost or twisting due to wind during the growth of a tree. Depending upon the shape and the positions shakes can be classified as star shake, cup shake, ring shakes and heart shakes.

✤ Upsets
Below picture shows a typical upset in a timber. This type of defect is due to excessive compression in the tree when it was young. Upset is an injury by crushing. This is also known as rupture.

upset-constructionway.blogspot.com
upset

2.) Defects due to Defective Seasoning and Conversion

If seasoning is not uniform, the converted timber may warp and twist in various directions. Sometimes honey combining and even cracks appear. This type of defects are more susceptible in case of kiln seasoning.

In the process of converting timber to commercial sizes and shapes the following types of defects are likely to airse: chip marks, torn grain etc.


3.) Defects due to Fungi and Insects Attack

Fungi are minute microscopic plant organism. They grow in wood if moisture content is more than 20°C and exposed to air. Due to fungi attack rotting of wood, takes place. Wood becomes weak and stains appear on it.

Beetles, marine borers and termites (white ants) are the insects which eat wood and weaken the timber. Some woods like teak have chemicals in their compositions and resist such attacks. Other woods are to be protected by chemical treatment.


Preservation of Timber



Preservation of timber means protecting timber from fungi and insects attack so that its life is increased. Timber is to be seasoned well before application of preservatives. The following are the widely used preservatives:

Tar
 Paints
 Chemical salt
 Creosote
 ASCO


 Tar
Hot coal tar is applied to timber with brush. The coating of tar protects the timber from the attack of fungi and insects. It is a cheapest way of protecting timber. Main disadvantage of this method of preservation is that appearance is not good after tar is applied it is not possible to apply other attractive paints. Hence tarring is made only for the unimportant structures like fence poles.

 Paints
Two to three coats of oil paints are applied on clean surface of wood. The paint protects the timber from moisture. The paint is to be applied from time to time. Paint improves the appearance of the timber. Solignum paint is a special paint which protects the timber from the attack of termites.


 Chemical salt
These are the preservatives made by dissolving salts in water. The salts used are copper sulphate, masonry chloride, zinc chloride and sodium fluoride. After treating the timber with these chemical salt paints and varnishes can be applied to get good appearance.


✢ Creosote
Creosote oil is obtained by distillation of coal tar. The seasoned timber is kept in an air tight chamber and air is exhausted. Then creosote oil is pumped into the chamber at a pressure of
0.8 to 1.0 N/mm² at a temperature of 50°C. After 1 to 2 hours timber is taken
out of the chamber.


✢ ASCO
This preservative is developed by the Forest Research Institute, Dehradun. It consists of 1 part by weight of hydrated arsenic pentoxide (As₂O₅, 2 H₂O), 3 parts by weight of copper sulphate (CuSO₄⋅5 H₂O) and 4 parts by weight of potassium dichromate (K₂Cr₂O₇) or sodium dichromate (Na₂Cr₂O₇⋅2 H₂O). This preservative is available in powder form. By mixing six parts of this powder with 100 parts of water, the solution is prepared. The solution is then sprayed over the surface of timber. This treatment prevents attack from termites. The surface may be painted to get desired appearance.

Uses of Timber


 For heavy construction works like columns, trusses, piles

 For light construction works like doors, windows, flooring and roofing.

 For other permanent works like for railway sleepers, fencing poles, electric poles and gates.

 For temporary works in construction like scaffolding, centering, shoring and strutting, packing of materials.

 For decorative works like showcases and furnitures.

 For body works of buses, lorries, trains and boats

 For industrial uses like pulps (used in making papers), card boards, wall papers

For making sports goods and musical instruments.

ABOUT TIMBER IN BUILDING CONSTRUCTION

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