- wood which is suitable or fit for engineering construction or engineering purpose is called timber. Example : building houses and making furniture
- should have structural as well as aesthetic merits
what is the uses for the timber?
- building construction
- house post construction
- beams and rafters
- construction of bridges and boats
- furniture and instruments
- railway sleepers and packing cases
- toys and engraving work
- railway coach wagons
- formwork of cement concrete
what is the general properties of timber?
- Color – a darker color in wood indicates greater durability.
- Odor – it is present only on freshly cut trees.
- Hardness – is the ability of wood to withstand indentations caused by harder bodies.
- Density – densest woods are generally the strongest.
- Texture - In hardwoods, the texture depends upon the size and distribution vessels and rays. In softwoods, it is determined by the size and distribution of trachea
- Grain - Depending on the actual alignment, the grain may be straight, spiral, interlocked, wavy or irregular. a) Spiral Grain, b) Interlocked Grain, c) Wavy Grain
Identifications of Timber
- Luster- is the ability to reflect light beam pointedly.
- Workability- the relative case in which wood is shaped cut and fastened together than the others.
- Warping- is the general term used to describe any variation from a true surface.
- Moisture content- is a percentage of the mass of water over the mass of wood fibre in a piece of timber.
- Specific Gravity- is the ratio found by dividing the weight of a substance by the weight of an equal volume of pure water
what is seasoning of timber?
process by which moisture content in the timber is reduced to required level
why seasoning is important for timber?
By reducing moisture content, the strength, elasticity and durability properties are developed. A well-seasoned timber has 15% moisture content in it.
There are 2 types of seasoning of timber. What is natural seasoning and artificial seasoning?
Natural Seasoning of Timber
Natural seasoning are water seasoning or air seasoning. Which timber is seasoned by subjecting it to the natural elements such as air or water.
Water Seasoning ( 2 - 4 weeks )
timber is immersed in water flow which helps to remove the sap 清除汁液 present in the timber. It will take 2 to 4 weeks of time and after that the timber is allowed to dry. Well-seasoned timber is ready to use.
Air Seasoning ( months - year )
timber logs are arranged in layers in a shed 棚. The arrangement is done by maintaining some gap with the ground. The logs are arranged in such a way that air ( and wind ) circulated freely between logs. By the movement of air, the moisture content in timber slowly reduces and seasoning occurs. Even though it is a slow process few moths to years. It will produce well-seasoned timber.
Artificial Seasoning
Natural seasoning gives good results but takes more time. So, artificial seasoning of timber is developed nowadays. By artificial seasoning, timber is seasoned with in 4-5 days. Here also different methods of artificial seasoning are there and they are as follows.
Kiln seasoning ( 3 - 12 days )
-In this method timber is subjected to hot air in air tight chamber. The hot air circulates in between the timber logs and reduces the moisture content. The temperature inside the chamber is raised with the help of heating coils. When the required temperature is obtained moisture content and relative humidity gets reduced and timber gets seasoned. Even though it is costly process it will give good results strength wise.
Chemical seasoning ( 30 - 40 days )
-chemical seasoning, timber is stored in suitable salt solution for some time. The salt solution used has the tendency to absorb water from the timber. So, the moisture content is removed and then timber is allowed to drying
Electric seasoning ( 5 - 8 hours )
timber is subjected to high frequency alternating currents is passing through the timber. The resistance of timber against electricity is measured at every interval of time.
When the required resistance is reached seasoning, process is stopped because resistance of timber increases by reducing moisture content in it. It is also called as rapid seasoning and it is uneconomical.
What is the advantages of timber seasoning?
Reduced weight - saving in freight 節省運費
resistance to decay and rot 腐爛 - no water no fungus
Strong and durable - seasoning before treatment greatly increases the effectiveness
Takes high polish
Easier to work with
Lasts longer
Defects caused in timber may be basically classified into two categories:
Internal Defects – due to factors affecting the growing tissue or the tree.
External Defects – due to external agents or subsequent treatment of timber.
What is the purpose of preservation of timber?
To increase the life of timber structures.
To make the timber structures durable.
To protect the timber structures from the attack of destroying agencies such as fungi, insects, etc
How many type of timber preservation? Briefly name it
The main classes of timber preservatives are :
Ascu treatment
Chemical salts
Coal tar is highly effective against fungi
Cresote oil
Oily substances insoluble in water
Water soluble salts
Zinc chloride is the most extensively used of this type. It is clean and odourless.
Availability and uses of timber
Common Timber
Teak - The sapwood is pale yellowish or greyish-white in color and is distinguished from heartwood.
The heartwood is golden brown and turns a dull deeper brown on exposure.
Deodar - The sapwood is white to creamy white and is distinct from the heartwood which is light yellowish brown turning to pinkish brown on exposure.
Chir - The sapwood is white to creamy white and is distinct from the heartwood which is light yellowish brown turning to pinkish brown on exposure.
Kail - The yellowish in white to greyish sapwood is white to pale-yellowish white in color and is distinct from the heartwood which is light pinkish-red to light red.
Sisso - Sapwood of sisso is pale yellowish or greyish white and is distinct from the heartwood which is golden brown or dark brown in color.
Sal - The sapwood is pale-yellowish or brownish white and is distinct from the heartwood which is brown or reddish brown. Mango - This is brown in color.
Industrial timber
Veneers
Plywoods
Fibre boards
Impreg timbers
Compreg timbers
Block board and lamin board
Glulam
Flushdoor shutters
Particle board or clip board
Hardboard
what is the advantages of plywood?
- It has a good strength both across as well as along the grain.
- Plywood has better splitting resistance due to grains in adjacent layers running at the right angle and nailing and screwing even closed to the edges in safe.
- Can easily be bending to a curvature limited by the tensile and the comprehensive strength of the other plies.
- The face piles made at different heartwoods show great aesthetic variety in grain pattern and color
