Timber Project Essay, Research Paper
1. Timber has been a building material since man first came out of his cave. The reasons for this are due to timber being a plentiful product and the properties it holds, being that timber is high in strength, light weight, and reasonably durable. Timber can be cut and shaped easily without the use of machines. Prehistoric man found that these properties were ideal for building houses, boats, and bridges. Timber is now has many different uses, including paper, furniture, doors, windows, decorative objects and structural members etc. The uses for timber is endless and as long as the replanting continues and our resources are retained generations to come will enjoy the benefits timber has given us.
There are two classes of timber:
Gymnosperms (Evergreen, softwood, conifer)
These are the most primitive type of tree, the leaves are in the form of needles they have one main stem and a conical shape to the crown e.g. Douglas fir quite quick to grow.
Angiosperms (deciduous hardwood)
These are flowering plants and are more advanced in evolution than conifers. They have broad leaves, multiple stems and a globular crown shape. E.g. Oak, Walnut. Slow to mature and expensive to purchase compared to softwood.
In a living tree the sapwood is just under the bark of the tree. The sapwood is the trees transportation system for nutrients, sugar and water. The heartwood in the centre of the trunk provides structural stability for the tree. The sapwood next to the heartwood is in the process of being converted to heartwood. The sapwood has a higher MC than the heartwood.
2. Properties of timber
Tensile strength
The tensile strength of wood is not great. The molecular structure of the cells of the wood tear apart easily but can be compressed with a high amount of force before the wood disfigures. Beams used in extreme tension are commonly made of steel or re-enforced concrete. Timber can have extreme variations in strength with different species of wood and different parts of the tree.
3. Compressive strength
The compressive strength of wood in an average piece of timber with a density of 513 kg/m| has the compressive strength of 30 – 40N/mX
Timber has a higher compressive strength than tension strength due to the wood being cellular. The cells can grip on to each other holding the wood together. Concrete has a greater compressive strength than timber but less in tension. Years ago the wood strength was not known, this lead to the over specification of many buildings, and wasting valuable resources. With today s technology materials can be used to their maximum potential minimising waste and over specifying.
4. Elasticity
Timber is an elastic material, which means when a mass is put on a piece of timber it deflects and when the mass is removed the timber returns back to its original position, up to a certain well defined limit of stress. The yield stress point can vary on the size, and type of timber. Timber can vary in strength. So often the stress grading of the timber is greatly under estimated.
5. Water Absorption
It is recommended that the moisture content of timber be kept below 20%, if any greater amount of moisture is present then rotting and insect infestation can occur; the normal average moisture content is 12%. The ventilation of the wood is a very important aspect of design, if timber is kept dry and ventilated it can last a considerable amount of time.
The weight of dry wood = Wd
The moisture content = MC
The moisture weight = Wm
MC = Wm Wd . 100%
a. Wd
Timber which has moisture content of 100% means that the weight of the water is equal to the weight of the wood. It is possible for timber to have several hundred percent MC.
The loss and gain in moisture within the wood causes shrinking and swelling in all directions this can cause many problems and have severe consequences
6. Water repellent
Naturally wood absorbs water and does not repel, the wood needs sealing with a water resistant material i.e. paint or varnish. The wood can be treated with preservative to attempt to combat both insect and fungi attack. Preservative treatment of timber involves introducing sable chemicals into the wood structure protecting it from fungi attack. Preservation of timber is insurance, not an excuse for cutting corners or turning a blind eye to faults in design or workmanship. No timber is completely immune to decay and in much the same way no preservative treatment provides absolute protection. (TRADA wood information) A large number of treatments available for wood are in the appendix.
7. Conduction of Electricity
Timber does conduct small amounts of electricity it is not the cells of the wood, which conducts, it is the moisture content in the timber, which conducts the electricity. Moisture meters use this principle for measuring the MC of wood.
8. Thermal insulation
Thermal conductivity (k) is a measure of the rate of heat transfer through a given thickness and area of the material from FACE to FACE. The units for this are W = watts, m = metre, mX = area, K = Kelvin, there for W/mK+. Timber is not often used in the construction industry for its thermal properties it is more commonly used for structural and decorative reasons. (See figure 1)
Figure 1
Bulk Densitykg/m3 Material Thermal conductivity(k) W/mK Thermal resistivities(lk) mK/W
513 Softwoods & plywood s 0.124 8.07
769 Hardwoods 0.16 6.25
961 Plaster board 0.16 6.25
2260 Concrete 1:2:4 1.44 0.69
1700 Brickwork 1.45-0.73 0.69-1.38
16; 24 Expanded polystyrene 0.035 28.6; 30.4
Timber is a natural insulator. Air pockets within its cellular structure make timber a natural barrier to heat and cold. Wood combined with fibreglass insulation provides an excellent insulator.
Since thermal conductivity increases with density, lightweight timber is a better insulator than dense timber. Thermal conductivity varies slightly with moisture content and natural characteristics such as checks, knots and grain. The thermal conductivity of steel is approximately 400 times that of timber; concrete 10 times, brick and glass 6 times. Mineral wool has 1/3 of the conductivity of timber.
References:
Timber Datafile P1 Timber Species and Properties of the NAFI Timber Manual Timber Species and Properties
9. Thermal Movement
Generally the thermal movement of timber is small about 13.3mm / metre which make this an ideal material for an area with a large temperature range i.e. a roof space with the temperature ranging from as low as 10|C up to 40|C timber is an ideal material for this purpose. Expansion joints are not normally required even in large structures. A rise in temperature of 1 .C can reduce the strength of timber by 0.3 %.
10. Acoustic properties of timber
Acoustics is the science of all audible sounds. Room acoustics is control of wanted sounds within a room. The control of unwanted sounds in a building is sound insulation. Timber can play a large part in both room acoustics and sound insulation.
An important acoustical property of timber is its ability to damp vibrations. The cellular network of interlocking pores converts sound energy into heat energy by friction, the vibration of the fibres causes heat.
Wood has more damping capacity than most structural materials. Damping reduces the tendency of structures to transmit vibrations long distances; it also reduces the magnitude of resonant vibrations, improving the performance of wood panelling as a reflective surface.
Reference: Timber Datafile P1 Timber Species and Properties, NAFI Timber Manual.
11. Cell structure
Structural load can be supported by using cylinders in the form of pipes, this is true for the structure of a tree within the monocular structure of the tree there are hundreds of tubular cells running longitudinal to the tree trunk, this type of cell structure gives the tree its natural strength. The timber fibres are composed of cellulose and hemicelluloses and these are bonded together essentially by lignin.
Softwoods generally contain more lignin than hardwoods
The average proportions in dry wood are:
45 to 60 % Cellulose
10 to 20 % Hemicelluloses
20 to 35 % Lignin
(Ref 7) (pg 31)
12. Durability
The durability of timber is only concerned with the heartwood and not the sapwood. The sapwood of most species is not very durable and should not be used in exposed conditions. In BS EN 350-1, is the guide to natural durability, which is split into 5 classes for the durability of the timber?
Class 1. V Durable Ebony
Class 2. Durable Mahogany American
Class 3. Moderately durable Mahogany African
Class 4. Slightly durable Maple
Class 5. Not durable European Ash
The most common type of degradation is fungi attack which can happen anywhere. When the moisture content of the wood reaches over 20% the common way to prevent the moisture reaching this high is to ventilate the timber, so any moisture can evaporate. Preservative treatment is discussed in the appendix.
13. Suitability
Joinery of timbers; both hardwoods and softwoods can be used for joinery works. Softwoods can be selected for its properties. If the finish is to be painted the type of wood is not too important due to fact that the grain will not be seen. Softwood timbers are usually used in this instance. Hardwoods are usually left their natural colour and treated with preservative or coated with a clear varnish to extend their life expectancy. The colour and type of grain are important when selecting a type of hardwood. When high-class joinery work is involved. Timber can be prepared; rough sawn or planed, each is suitable for a different type of works i.e.:
Rough sawn timber would be used for roof trusses and floor joists which are purely structural and not usually seen. Inexpensive softwoods are usually used for this type of work. This timber is treated due to it being part of the structure of the building.
Planed timber, which is more expensive to produce whether softwood or hardwood can be used in, furniture, windows, skirting boards, door linings, stair cases etc and many more different products.
Workings of a windmill in Poland the main beams are oak and the teeth are apple wood.
14. Economic considerations
Timber is a natural product, a renewable energy resource it can be regenerated over time, the softwoods like pine are relatively quick to grow but the hardwoods are slower to grow and therefore more expensive to purchase.
Transport and buildings are the major producers of carbon dioxide because they are the main energy users. In the UK, buildings account for around 50% of the country’s carbon dioxide emissions.
Timber can help to meet the threat of global warming in three ways:
Trees absorb carbon dioxide and release oxygen into the atmosphere through the natural process of photosynthesis (See fig 1, ref 7). A young tree, growing in a properly managed forest, is a more effective vehicle for carbon dioxide absorption than a mature forest.
Converting timber into a usable building material takes far less energy, it generates less carbon dioxide, than virtually any other alternative including aluminium, steel and concrete.
Once installed, wood is an extremely effective insulating material with a very high energy efficiency. Indeed, timber framed houses are widely recognised as being top performers. (Ref: 5)
(Ref 7)
Britain has an active, flourishing forest sector, but the fact remains that we currently import around 80% of all the timber we use in this.
+ 69%, derives from coniferous forests located mainly in Scandinavia, Canada and Russia.
+ 3% derives from deciduous forests in Europe and North America.
+ 8%, is imported from tropical regions.
+ There are an estimated 223 sawmills in the UK the forest area amounts to approximately 10% of land area
+ Forests Forever is a campaign by Britain’s timber industry to help safeguard the forests of the world, by supporting forest management and regeneration projects. Correctly harvested, the timber is not damaging the global environment. Wood is a renewable resource; it is organic, non-toxic, recyclable and biodegradable.