Timber

Wood suitable for building or other engineering purposes is called timber .

1.  Timber is one of the oldest building materials known.                                                    
2.  It has been used in buildings in most parts of the world for thousands of years.                
3.  In low energy ages, timber was the natural material of choice.                                      
4.  Timber is a light construction material with a high strength to weight ratio.                    
5.  Timber is a natural product, drawn from the wood in the trunks of  trees.                      
6.  Its character is consistent with the species of tree and the form and growth over time of the wood in it.

     Classification of trees

Endogenous: (Grows inwards) Grow in longitudinal fibrous mass like plam, cane, bamboo, banana, etc. 

Exogenous: (Grow outwards) 

Conifers: Evergreen trees yielding soft wood. Deodar, chir, fir, kail etc. Distinct annual rings, straight fibers, light color, resinous and light weight.

Deciduous: Flat broad leaf trees yielding hard wood. Oak, teak, shishum, poplar and maple. Indistinct annual rings, non-resinous, dark in color and heavy weigh.

Tree Structure

       

Pith or Medula:  Innermost portion or core of tree.

Heart wood:  Inner annual rings surrounding pith.

Sap wood:  Outer annual rings that transmit sap from root to branches.

Cambium: Thin layer below bark, not converted to sap wood not as yet.

Bark (outer & inner):  Outermost protective layer of cells and woody fibers that splits and peels off. It is the skin of the stem.

Annual rings: These consist of cellular tissues and woody fiber arranged in distinct circles or rings around the pith generally one in every year.

Medullary rays: Thin horizontal veins from bark to pith to carry sap inwards. Only few are visible to naked eye and they hold the annual rings together.

                         MOISTURE CONTENT (MC)

1. Seasoning :

It is extraction of moist sap under controlled conditions (as nearly as possible at a uniform rate) from all parts of timber, and to leave inextricable moisture uniformly distributed throughout.

High moisture content of timber promote shrinkage, distortion and rot – factors which can result in serious and costly remedial work.Drying brings about shrinkage and also increasing strength and stiffness.

The mc of timber comes to equilibrium with the surrounding air, at first on the surface and then more slowly in the interior.  It is difficult to absolutely dry or saturate wood especially in constructional sizes.

Timber can be dried under cover to a mc of about 17 to 23% depending on temperature and relative humidity.  Lower mcs entail kiln drying or the use of dehumidifiers.

 2. Moisture content determination :

In most cases, mc can be determined by an electrical moisture meter.

Short pin electrodes give the mc of the surface wood but this is misleading if the surface has recently been wetted by rain or dried in the sun.  Deep probe electrodes are preferable. Where an accurate determination of the mc of timber is required, e.g. in cases of dispute, there is no alternative to destructive sampling and determination of moisture content by oven drying.

3 . Moisture content of timber in service :

Wood is always subject to moisture movement due to changes in temperature and relative humidity.In normal construction, some shrinkage of carcassing and internal joinery must be expected.

The mc of wood in heated buildings is the range 8 to 20% with normal heating and ventilation. In the case of special flooring and decorative paneling, there can be instances where wood is specified to be supplied at some particular and low mc. Such material should be delivered direct from the kiln to its prepared final location already maintained as a controlled environment. It is pointless to kiln dry timber if it is to be left outside in suppliers’ yards or on building sites.

Characteristics of good timber

• Obtained from heartwood area of tree and free from sap.
• Should have straight and close fibers.
• Dark uniform color with uniform texture.
• When struck, sonorous (loud & deep) ringing sound is produced.
• Narrow annular rings, closer the rings greater is the strength.
• Compact medullary rays.
• Sweet smell and a shining fresh cut surface.
• No clogging of saw teeth during sawing.
• Free from the defects like dead knots, shakes etc.
• Heavy weight.
• Bright and smooth surface when planed.