Timber a Building Material

TIMBER A BUILDING MATERIAL

Timber as a building material falls in two major classes such as natural and man- made. With the help of science and latest technology, wood in its natural form as timber, lumber, etc. is being rapidly replaced by man-made composite wood materials. Apart from formwork timber has many applications in buildings as doors, windows, frames, temporary partition walls, roof trusses and ceilings. Timber is a good shock absorber and hence it is suitable for constructions in hilly areas which are earthquake prone areas.

 

1. CLASSIFICATIONS OF TIMBER

Considering various important factors, the timbers are classified as follows:

 

2. CLASSIFICATION BASED ON ITS POSITION

Trees that produce good timber for construction are called Standing Timber. If the branches of a tree are cut and is approximately converted into pieces of required length, then it is known as rough timber. Through sawing, rough timber is converted into different marketable parts like planks, battens, posts, beams etc. then it is called as converted timber.

 

3. CLASSIFICATION BASED ON MODE OF GROWTH

Based on the mode of growth, timbers are classified as exogeneous and endogeneous.

❖Exogeneous: These are trees which grow outward by introducing successive rings annually. These rings are called as annual rings. By counting these rings, the age of the timber can also be identified. These types of trees are further classified into coniferous trees which have conical type leaves, fruits and deciduous trees which have broad leaves. Coniferous trees yield softwood whereas deciduous trees produce strong wood and are mostly used for carpentry and construction works. Some of the difference between soft wood and hard wood are as follows.

Coniferous Trees

❖ Annual rings are seen distinctly

❖Colour of soft wood is light and are less in weight

❖ The structure of soft wood is resinous.

❖ Soft woods have lesser strength in compression and shear.

❖ They have poor fire resistance.

Deciduous Trees

❖ Annual rings are seen indistinct

❖ Colour of hard wood is dark and are heavy

❖ Hard wood is generally close grained.

❖Hard woods have good compression and shear strength.

❖ They are more fire resistant.

❖ Endogeneous: These are trees which grow inwards, where fresh fibrous mass is in the inner most portion of the tree. Bamboo and sugarcane belongs to this type. These type of trees cannot be used for structural works.

 

4. CLASSIFICATION BASED ON GRADING

Based on the structure and utility, timbers are classified into structural grading and commercial grading.

❖ Structural grading: This type is also known as stress grading, which is based on the visible defects in the timber and the effects on the material strength properties. It follows the standard by which the material is graded by considering the maximum principle stresses to which it can be subjected. They are also further sub divided into grading based on known effects of defects and estimating accumulative value and machine grading.

❖ Commercial grading: It is also known as utility grading in which the material is graded by consideration of usage of the material and price. They are further divided in the following types:

Grade A: This type is based on the dimensions and general appearance. Lengths, widths and thicknesses of converted materials are measured and graded.

Grade B: This type is based on the decisive use of the material. Here, each grade is further divided into A, B and C classes to designate the defects. Only two lengths are recognized, long (L) which is 5m and above, and short (S) that is under 5m. Each timber is imprinted as BAL (Beam, A-class, long) and PBS (Plank, B-class, short).

Grade C: This type is based on qualitative evaluation of defects and rough estimate of cost of material usage.

Grade D: This type is based on assessment of units of defects and fixing the permissible number of standard volume of area or the material in each grade.

 

5. CLASSIFICATION BASED ON MODULUS OF ELASTICITY

Based on the bending test results, value of E is obtained and the timbers are classified as follows:

Group A: If E> 12.5 kN /mm²

Group B: If E = 9.8 kN/mm² to 12.5 kN/mm²

 Group C: If E = 5.6 kN/mm² to 9.8 kN/mm²

 

6. CLASSIFICATION BASED ON DURABILITY

Durability tests are conducted by half burying the test specimens and observing the conditions periodically for several years. Based on the durability test results, the timbers are classified as follows:

High durability: If average life > 10 years.

Moderate durability: If average life = 5 to 10 years.

Low durability: If Average life < 5 years.

 

7. CLASSIFICATION BASED ON AVAILABILITY

Based on the availability, timbers are classified as follows:

X - Most common, If availability is 1415 m³ or more per year

Y- Common, If availability is between 355 m₃ to 1415 m³ per year

Z-Less common, If availability is less than 355 m³ per year

 

8. STRUCTURE OF TIMBER

The structure of a timber can be seen from the cross section of a tree as shown in

❖ Pith: It is the inner most part of a tree having soft tissues near about the center of log of a timber. When the tree becomes old, the pith will be dark and fibrous.

❖ Heart Wood: Inner part of log of a tree surrounding the pith is called Heart. This is the dead part of wood, which is strong and dark in colour. This portion consists of several annular rings.

❖ Sap Wood: This layer is present next to heart wood, and is also known as alburnum. Outer part of log of a tree surrounding heard wood & upto the bark which contains living cells is called Sapwood. It denotes recent growth and contains sap. The annual rings of sap wood are light in colour and are less sharply divided.

❖ Cambium Layer: Layer of liquid material deposited below the bark & outside the sap wood in the log is called as Cambium Layer. If the bark is removed and cambium layer is exposed to atmosphere, activeness of cells comes to an end and tree dies.

❖Inner Bark or Bast: It is the inner skin surrounding the cambium layer. This skin feed & covers the Cambium Layer.

❖ Outer bark: It is the covering, outside the log of a tree which consists of wood fibres.

❖ Annual ring: The concentric innumerable rings in the log of a timber, indicating its growth are called annual rings.

❖ Medullary Rays: Horizontal thin fibrous tissues which extended radially from the cambium layer towards the core or from the pith towards the bark. They hold annular rings together.

 

9. PROPERTIES OF TIMBER

The following are the properties of a good timber:

❖ Colour: Colour should be uniform.

❖ Odour: When freshly cut, odour should be pleasant.

❖ Texture: Good timber should have fine and even texture. Grains: Grains are close in good timber.

❖ Density: If density is more, timber is said to be stronger.

❖ Warp: Even under changing environmental conditions, good timbers do not warp.

❖ Soundness: When struck together, clear ringing sound indicates the timber is in good quality.

❖ Hardness: If timber is hard, they are strong and durable.

❖ Toughness: Timber should be capable enough to resist shock loads.

❖ Abrasion: Good timber does not weaken due to wear.

❖ Strength: Timber should have good strength in compression, bending and shear.

❖ Fire resistance: A good timber should have greater resistance to fire.

❖ Permeability: Good timber should have low water permeability.

❖ Workability: Timber should be easily workable and should not congest

the saw.

❖ Durability: Good timber should have high resistance against fungal and insects attack.

❖ Defects: Good timber should be from defects like dead knots, shakes and cracks.

 

10. CHARACTERISTICS OF GOOD TIMBER

The principal characteristics of timber are strength, durability and finished appearance:

❖ Narrow annual rings, closer the rings greater is the strength.

❖ Heavy weight and compact medullary rays.

❖ Dark colour and uniform texture.

❖  Sweet smell and a shining fresh cut surface.

❖When struck sonorous sound is produced.

❖  Free from the defects in timber.

❖ No woolliness at fresh cut surface.

 

11. SEASONING OF TIMBER

The process of drying out the water from "wet" or "green" timber is termed as "seasoning" of timber. Drying the moisture out of wood enhances its properties to such an extent that the resulting timber is given the special name "seasoned" rather than "dried" although the terms are identical. The process of seasoning of timber may be of the following types:

 

12. NATURAL SEASONING

Seasoning of woods or timbers using natural elements is called natural seasoning. may be of two types namely water and air seasoning.

(i)Water seasoning: Removal of wood sap by immersing logs into water flow is called water seasoning. It is carried out on the river banks while thicker ends are kept towards upstream. After that, the logs are allowed to dry. This process is time consuming such as 2 to 4 weeks generally.

(ii) Air seasoning: This process involves exposing of woods to air for seasoning. At first, a platform is required that is built on the ground at 300mm height above the ground. Next process is the arrangement of woods in layers. Air circulation is maintained between logs because it helps to reduce the moisture which is important for seasoning. A clean, shady, dry, cool place is preferred for this process. Sometimes coated by the impermeable substance to reduce extreme moisture. To improve the quality oil coating, thick paint coating is maintained. To prevent fungal infection logs are treated with petrol or gasoline. Using this process good quality of seasoned wood can be obtained.

 

13. ARTIFICIAL SEASONING

In this method timber is seasoned artificially in a chamber with regulated heat, controlled humidity and proper air circulation. Seasoning can be completed in 4 to 5 days and it may be of the following types:

(i) Boiling:

Seasoning by boiling wood logs in hot water is called seasoning by boiling. Drying is done after proper boiling. For a large amount of wood, it is done in an enclosed place where hot steam is passed.

Advantages

❖ It takes a short amount of time. Generally, 3-4 hours is good enough.

 ❖ Develops the strength and elasticity.

Disadvantages

❖ It is serviceable basically for a small quantity of wood, not convenient for a large amount.

❖ The cost is high.

(ii) Kiln seasoning:

Seasoning of wood by using a large chamber or kiln where there is a good process for the circulation of hot air. Fully saturated air with a temperature 35°C to 38°C is forced in the kiln. Kiln seasoning can be done by 2 processes such as:-

Progressive kiln Seasoning: Wood log is entered through the kiln and the temperature and humidity differentials are maintained through the length of the kiln to maintain proper drying.

Compartmental Seasoning: It is maintained by enclosed container or buildings. It accelerates the process because external energy is used

Advantages

❖ Most effective and economic seasoning.

(iii) Chemical seasoning

Reduction of moisture using salt solution is called chemical seasoning. After the absorption of water by the chemical solution logs are let to dry.

Advantages

❖ It increases the strength of the timber.

❖ It is less time-consuming.

Disadvantages

❖ Chemical reagents can sometimes reduce strength.

❖ It can cause a problem in gluing or finishing or corrosion while using.

(iv) Electrical seasoning

Dry wood is non-conductor of electricity while green timber is a conductor, so, can pass alternating current. Thus in this method alternating current is used for drying the cells of wood by creating heat. As electricity is used, it's called electrical seasoning.

Advantages

❖ Using this method quick drying is obtained. a French electrical seasoning method is used to season overnight.

Disadvantages

❖ The equipment required is very costly.

❖ It is an uneconomic process as a high rate of electricity is consumed.

 ❖ During heating the cells of wood or timber they lose their strength and become weak.

 

14. PRECAUTIONS DURING SEASONING

Following precautions must be taken during seasoning of timber:

❖ During seasoning, the moisture should be removed under an environment maintaining conditions.

❖ Seasoned timber should be protected from exposure to the rain and excessively high humidity during air seasoning

❖ During seasoning, a proper gap between logs should be maintained for easy and uniform air or water or hot air passing.

❖ Moisture should be extracted almost at an equal rate from all logs because differentiated dryness causes irregular shape.

 

15. SAWING OF TIMBER

Sawing is the cutting of timber from logs into different shapes and sizes. Sawn timber is generally cut into varying rectangular widths and lengths, but may also be wedge-shaped. The following are the two types of sawing of timber:

Ordinary or Slab Sawing: This is the most economical method of sawing, as the wastage of timer & cost of sawing is minimum. Cuts are made into the log of wood according to the required thickness, not necessarily tangential to the annual rings.

Tangential Sawing: In this method the board & planks are sawn out of wood tangentially to the annual rings. This method is economical due to less wastage &involves less Labor. But this method do not suit for heavy works as for flooring.

Quarter Sawing: Here, logs are cut into four quarters. These quarters have their flat faces essentially radial. Annual rings are cut at angles not less than 45 degrees. This method requires more labour & also involves more wastage.

Radial or Rift Sawing: Logs are cut out of quarter logs, parallel to the medullary rays and perpendicular to the annual rings. It produces tiny end grains which are sometimes known as "Silver Grains", by sawing parallel to the medullary rays. Used for high class decoration and joinery work. This method is not economical.

 

16. DEFECTS IN TIMBER

Different defects which are liable to occur in timber can be classified as follows:

(i) Defects due to Natural Forces:

Defects due to natural forces in wood are as follows:

❖ Wind cracks are the shakes or split on the side of the bark of timber due to the shrinkage of the exterior surface. It happens due to the exposure to atmospheric agencies like sun. wind etc

❖ Knots are the roots or bases of small branches of the tree. These may stay alive or may become a dead part of the tree but they break the continuity of fibers. In the beginning, the base portion gets food from the stem, but finally it results in the formation of the dark and hard ring known as the knots. These knots are not very harmful when these are small in size, feel hard, look round, and these timbers are mostly used for members other than tie bars and beans. Knots are classified being based on diameters such as pin knot, small knot, medium knot, and large knot. Timber containing large dead knots or many smaller ones should be rejected as these weaker points lack in strength and impact on the appearance negatively.

❖ Shakes are the abnormal cracks which wholly or partially get separated by the fibers of the wood. Different types of shakes are seen, some of the common shakes are cup shakes, heart shakes, ring shakes, star shakes, ib boradial shakes etc.

(i) Cup Shakes in Timber: Cup shakes are caused by the rupture of tissue in a circular direction. It is a curved crack, and it separates partly one annual ring from the other developing one due to non-uniform growth or due to excessive bending of a growing tree during a cyclonic weather. It may prove to be harmful if it covers only a portion of the ring.

(ii) Ring Shake in Timber: When cup shake covers the entire ring of timber, they are known as ring shakes. Ring shake in timber is a tangential separation of wood fibers along with the parts of the annual rings. It often looks so fine that is not found visible in greenwood and can be detected in dry wood later.

(iii) Heart Shakes in Timber: Heart shakes occur in the center of a cross- section of timber due to the shrinkage of the interior part of a tree, which is approaching toward its maturity. It divides the tree in cross-sectional way bon into two to four pieces.

(iv) Star Shake in Timber: Star shake cracks start from the bark and extend. toward the sapwood. It is usually confined up to the plane of sapwood. These cracks are more extensive on the extreme ends and get narrower on the inside ends. Star shake cracks are typically formed during the growth of the tree.

(v) Radial Shake in Timber: Radial shakes are same as star shakes, but as compared to star shakes, radial shakes are quite irregular, fine and numerous in shape. Generally, these cracks occur when timber is exposed to the sun for seasoning after falling. Radial shakes cracks start from a short distance from the bark and gets extended toward the center, then follow the annual ring direction and then run toward the pith

❖ Burls are also known as the excrescences. These are mainly formed when a tree has received shock or injury in its young age. Due to such damage, the growth of a tree may get completely upset and irregular projections start appearing on the body of timber.

❖ Upsets are ruptures of a fiber of timber caused by some major impacts, injury, pressure, due to incompetent chopping and related potent effect.

❖ Twisted fibers are also known as the wandering hearts. The twisting of young trees causes these being impacted by fast blowing wind, and the fibers of timber gets twisted in one direction. The wood with twisted fiber is inapt for sawing.

❖ Rind Galls: The meaning of word rind is bark, and gall is a kind of abnormal growth. Therefore, an unusual cured swelling is found on the branch of a tree. This is known as the rind galls. Rind gall develops at the points from where branches are improperly cut off or removed. Rind galls are rarely found in a tree, and the timber in this part of tree becomes very weak and mostly fragile.

(ii) Due to defective seasoning

Due the defective process of seasoning, the following defects might occur in wood:

❖ Bowing: The bow is a timber defect indicated by the curvature formed in the direction of the length of timber.

❖ Cup: It is a defect of timber which is indicated by the curvature formed in the transverse direction of the wood.

❖ Check: It is a crack which forms due to the timber seasoning process and it tends to separate the fiber layer of the wood.

❖ Split: When check extends from one end to the other in a wood log, it is known as a split. Splits are lengthwise separation of the wood caused by either mishandling or seasoning.

❖ Twist: When a timber piece gets spirally distorted along its length, it is known as a twist.

❖ Collapse: Due to uneven shrinkage, wood sometimes get flattened during the drying process. This is known as a collapse.

❖ Timber Warping: When a piece of timber has twisted out of its shape, it is called the warp.

(ii) Defects due to Conversion

The following defect occurs in wood, while converting the timber to commercial form:

❖ Diagonal Grain: It is a timber defect that is formed because of improper timber sawing indicated by a diagonal mark on the straight-grained surface of the wood.

❖ Torn Grain: It is also known as a form grain, and it is usually caused by a small depression formed on the finished surface of the timber by accidental falling of tool.

❖ Chip Mark: It is indicated by the marks or signs placed by chips on the finished surface of the timber. The parts of a planning machine may also stand responsible for forming these marks.

❖ Wane Wood: It is a timber defect that is denoted by the presence of real rounded surface on the manufactured piece of timber.

(ii) Due to Fungi and Insects

Due the fungal attack, the following defects might occur in wood:

❖ Dry Rot: Certain types of fungi feed on wood and during feeding they attack timber and convert it into dry powder form, which is known as the dry rot. It occurs at the places where there is no free circulation of air is experienced. The unseasoned softwoods and sapwood easily get attacked by dry rot. The most favorable conditions for the rapid growth of fungus, which is responsible for dry rot, is the absence of sunlight, sogginess, presence of sap, stagnant air and warmth. If timber is not properly stacked after cutting and shaped, it may get affected by an attack of dry rot.

❖ Wet Wood Rot: Some varieties of fungi cause chemical decomposition of timber and convert wood into a grayish brown powder, which is known as wet rot. The main reason for the development of wet rot is alternate dry and wet conditions. If unseasoned timbers are exposed to rain and wind, they become easily liable for attacks of wet rot. The well-seasoned timber should be used for exterior work to prevent wet rot problem.

❖ Brown Rot: It is used to indicate decay and underlying disease of timber. Specific Types of fungi remove cellulose compounds from wood, and hence the wood takes the brown color, which is known as brown rot.

❖ White Rot: White rot defect is just the opposite of brown rot defect. In the case of white rot defect, certain types of fungi attack lignin of wood, and the wood assumes the appearance of a white mass consisting of cellulose compounds, hence the name white rot.

❖ Heart Rot: Heart rot defect is formed when branches have come out of a tree. Hence, the heartwood is exposed to the attack of atmospheric agents. Ultimately the tree becomes feeble, and it gives out hollow sound when the hammer hits the log.

❖ Timber Sap Stain: The particular types of fungi do not cause the complete decay of timber, but they feed on cell contents of sapwood, and the sapwood loses its color and texture gradually This is known as sap strain. It generally occurs when moisture content gets equal to higher than 25%.

The insects usually found responsible for the decay of timber are termites, beetles, and marine borers.

❖ Termites: Termites are also known as white ants, and they are found in tropical and sub-tropical countries. These insects live in a colony and are truly fast in eating away the wood from the core of the cross-sections. These insects make tunnels inside the timber in different directions and usually do not disturb the outer shell or cover. Very few woods such as teak, sal etc. can resist the attack of termites.

❖ Beetles: Beetles are small insects, and they cause rapid decay of timber. They form holes of size about 2 mm diameter in wood and attack the sapwood of all species of hardwoods. Tunnels are formed in all directions in sapwood by the larvae of beetles and wood gets converted into a fine flour-like powder. They usually do not disturb the outer layer of the timber. Hence the timber piece gets attacked by beetles may look sound externally till it completely gets withered inside.

❖ Marine Borers: Marine borers are generally found in salty water. Most of the varieties of marine borers do not feed on wood but they make holes or bore tunnels in wood for taking shelter. The approximate diameter and length of holes or bores are between 25 mm and 60 mm. The timber attacked by marine borers loses strength and colour. It is noted that no wood is completely protected by the attack of marine borers.

 

17. COMMON MARKETS FORMS & SIZES OF TIMBER

Some of the commonly available market forms of timber are explained below:

❖ Log: Stem or trunk of a tree which is felled is known as a log. It can be converted and used as firewood and lumber ,etc.

❖ Bolt: A Short log 1.25m or less in length.

❖ Cant: A thick piece of timber with or without squared edges.

❖ Baulk: Baulk is a piece of timber obtained by removing the bark and sapwood, which looks roughly square-shaped. The cross-sectional dimensions are usually more than 50 mm on one side and more than 200 mm on the other side.

❖ Deal: Deal is a type of softwood, usually sawn with parallel sides of thickness 50 to 100mm, and of width 200 to 250mm.

❖ Plank: It is a long timber piece with parallel sides of width more than 50 mm and thickness less than 50 mm.

❖ Pole: Pole is also known as Spar, which is a long log of diameter less than or about 200 mm.

❖ Post: Timber member used in upright position in Building fencing or structural work is called a post. It is a rectangular wooden piece used as a compression member in buildings, having a diameter less than 300 mm.

❖ Strip: Strips are wooden pieces of a thickness not more than 50 mm and width not exceeding 100 mm.

❖ Batten: It is a strip of timber, of thickness 50 to 100mm and breadth 150 to 200mm. It is used in roofing, flooring, ceilings, wall and other construction applications.

❖ Boards: It is a plank of timber with parallel sides, thickness not exceeding 50 mm and width exceeding 200 mm.

❖ Scantling: Scantlings are pieces of timber with different sizes, with thickness and breadth are more than 50 mm but limited to 200 mm. Depending on the type of work they are sawn out of a log and cut into required sizes.

❖ Quartering: These are square pieces of timber, of length varying from 50 mm to 150 mm.

❖ End: The small pieces of battens, deal, poles or scantlings are referred to as Ends.

❖ Sleeper: Transverse supports under rails in a Railway track of size 250mm x 125mm or 200mm x 115mm.

❖ Slat: Wood about 185mm x 65mm x 6mm in size.

 

18. USES OF TIMBER

Timber is used for the following purposes:

❖ It is ready to use material available naturally and easy to transport and handle.

❖ It has more thermal insulation, sound absorption and electrical resistance.

❖ It can be easily worked. Hence repairs and alterations to wood work can also be done easily.

❖ It possesses excellent strength and a good preference for making load bearings like columns, beams, trusses and piles.00

❖ Timber is widely used for doors, windows, flooring and roofing as it is easily convertible into any shape and size.

❖ It is used for temporary works in construction like scaffolding, centering, shoring and strutting, packing.

❖ It is also used for other permanent works like for railway sleepers, fencing poles, electric poles and gates.

❖ Timber is widely used in ornamental works like showcases, furniture, sports goods and musical instruments

❖ It is used in body works of buses, lorries, trains and boats and for industrial uses like paper pulps card boards and wall papers.