History Of Timber In Construction

Published Date: 02 Nov 2017

2.2.1 History of timber in construction

Right from the dawn of man, wood was used as a construction material to build structures for shelter. Man had to seek protection from predators and harsh weather conditions using protective cover that usually comprised of dugouts, caves, reeds, twigs, wood, mud, stone and snow. It is proposed that the first primitive structure was invented when early man pulled down a tree branch with full foliage (Guilhemjouan, 2013).

The use of timber in construction dates back to 500 to 100 B.C. Timber was spatially used in roof constructions by the ancient Roman and Egyptian civilizations that majorly used stone in buildings. Most noteworthy during the period is the development of the ten and mortise joints in timber framing. Over the subsequent thousand years in Europe, the use of timber frames heightened in areas with vast timber resources. Primitive construction techniques were employed and as a means of foundation, timber was either driven or laid onto the ground. Timbers were tied together using primitive rope of animal hides. Advanced joinery techniques were developed to build more permanent and decent houses using timber frames. Stone foundations provided superior support for the houses, and prevented rapid deterioration of the structural posts. Timber frames were permanently fastened using joinery techniques. In Europe, modern timber framing was developed in the 9th and 10th centuries and is characterized by exceptional building skills (BRTW, 2011). Timber framing techniques would later evolve across Asia, Africa and the undiscovered Americas.

Wood is usually cut longitudinally in either two planes: radial and tangential. Radial sections are formed along rays or the log’s radius and at right angles to annual rings. The logs are cut in quarters to form planes of quarter sawed lumber. The rings are parallel bands that are closely spaced and the rays have the appearance of scattered blotches. Tangential sections are formed tangential to annual rings and the log’s face and perpendicular to rays. Annual rings appear as wavy and irregular patterns (Armstrong, n.d.).

Figure1.Wood planes, A-Transverse, B-Tangential, C-Radial

Timber is categorized into two

Hardwood

Softwood

Hardwood

Hardwood comprises of tree and shrub species with heavy and dense wood. The trees are evergreen in subtropics and tropics, but deciduous and broad leafed in temperate. The hardwoods can also be subdivided further into very heavy, heavy, medium heavy and ironwoods (woods that sink in water). Hardwoods are mainly angiosperms (flowering shrubs and trees) and their wood has water conducting cells that are referred to as vessel elements and tracheids. Moreover, the fiber cells are thick walled and tightly packed unlike in conifers. Conifers such as firs, pines, redwood and spruce are softwoods because they bear tracheids and lack fiber cells. Wood hardness is dependent on cell wall lignin, cell density and the proportion of pores in the cell wall.

Popular hardwood species include cherry species, walnut, hickory, maple and oak. In the United Kingdom, hardwood trees are characterized by wood durability and hardness, and they comprise of Robinia, walnut, beech, oak, elm, ash and sweet chestnut. They are widely used for millwork, moldings, furniture, cabinets etc. Hardwoods species are greater in number than softwood species. In addition they are more expensive than softwoods. They are denser and have greater volume and calories than softwoods. They are highly preferred for projects where beautiful graining and strength is a prerequisite. Quercus pedunculata is a common oak in Britain and the Scotland lowlands whereas Quercus sessiliflora is less common but frequent in Northern England and Wales. Oak wood is the most durable and strongest amongst the timber trees in the United Kingdom. However, it’s not favored for planting due to slow growth. Oak trees are largely grown in the national forests by the governments of England and France. Oak is widely used in machines, furniture, ships and houses. Thus oak is the most preferred hardwood tree for planting.

Softwood

Timber is wood derived from gymnosperm tree species. Unlike hardwood, softwood is not porous and is generally less dense. Softwood trees are evergreen trees and mainly comprise of conifers such as cider, pine, Douglas fir etc. The wood is easily cut and has a wide range of uses e.g. furniture, building frames, chipboard, windows, staircases, doors, and paper. Coniferous forests also referred to as boreal or taiga forests are in the Southern hemisphere and are also common in North America, Asia and Europe. 80% of the timber used in the world is soft wood. According to the British Forestry Commission, there is sufficient supply for softwood for both current and future use. This largely attributed to extensive planting of conifers from 1960 to 1990. Thus the supply of timber is at its peak with the trees attaining maturity. This is well reported in Great Britain’s National Forest Inventory reports i.e. ‘25-Year Forecast of Softwood Availability’ and ‘25-Year Forecast of Standing Coniferous Volume and Increment’. The two reports reveal that the forest conifers range from 21 to 60 years old. Most important they will be viable for commercial harvesting in the subsequent 25 years (Rae, 2009). Thereafter, they supply of softwood may not satisfy the demand as relatively fewer confers have been planted in Great Britain from 1990 to 2010.

2.2.2 Benefits of timber in construction

Timber is ranked as the world’s most eco-friendly building solution. It is not toxic and chemical vapor is not leaked in the building. It ages naturally and is not degraded into toxic products that is environmentally damaging. In addition, timber is renewable as its continually grown in plantations and forests.

Relatively little energy is utilized in the conversion of wood to building timber. Energy in timber is the lowest in all building materials. Timber also serves as a good reservoir of atmospheric carbon that contributes to global warming. It is also a good insulator and this reduces the use of energy in heating buildings. Energy costs are reduced in winter and the buildings are cooler in summer. Energy needs are generally reduced when timber is used in the construction of floors, doors and windows. Timber is also readily available and has made a big impact on local economies. A wooden structure takes a short period to construct and yields a fast financial return. Moreover, the construction of a timber building is not subject to weather conditions. There is high thermal value/wall width as compared to other construction forms. The construction is lighter and this reduces the cost of the foundation.

2.2.3 Properties of Timber

The properties of timber vary depending on direction. The strength is high when it is parallel to the grain, but it is low when it is perpendicular to the grain (Punmia, Jain and Jain, 2005). Tensional strength is 40 times higher when the wood is parallel to the grain as compared to when is perpendicular to the grain. Thus it is easier to split wood along its grains/fibres than perpendicular to the grain. Timber is hygroscopic and its moisture content varies depending on climate. If the moisture content is lower than 30%, the shrinkage of timber is perpendicular to the grain, but the shrinkage experienced along the grain is negligible. The cross-section planes can experience 7% shrinkage. The moisture content of timber should be maintained at equilibrium similar to that of the product. In service, shrinkage deformations lead to tension on planes perpendicular to the grain and this presents a major failure. Owing to varied shrinkage in tangential and radial directions, there are splits when large planes of timber are dried rapidly. Kiln drying minimizes the incidence of splits.

2.2.4 Timber decay

Causes of wood decay

Moisture and sap in wood.

Moisture and heat.

Alternate wet and dry conditions.

No ventilation

Rot is the most common wood decay and is largely caused by microbes or chemical processes which are responsible for putrefaction and decomposition which is accompanied by the evolution of gases such as carbon dioxide and hydrogen sulphide. They are 2 types of rots: wet rot and dry rot.

Wet Rot

This is chemical decay that leads to decomposition of timber tissues, and is caused by alternate wet and dry conditions. The timber that is used for exterior works (e.g. windows, doors etc.) is highly susceptible to wood rot. The affected timber is degraded to grayish brown powder. This is prevented by using seasoned timber that is covered with paint or tar for both ground work and exterior work (Punmia, Jain and Jain, 2005).

Dry Rot

This mainly attributed to fungi and the most common fungal species is Merulium lechrymans. The wood is also degraded to powder (Punmia, Jain and Jain, 2005). Dry rot initially sets in the sap wood. The fungi breakdown the wood, it becomes brittle and the fibers have reduced cohesion before the ultimate degradation into powder. Fungal grows and proliferates when there is no ventilation. Poorly seasoned sap wood is highly susceptible to dry rot stored in warm damp conditions lacking ventilation is susceptible to dry rot. Favorable conditions for dry rot include: warmth, presence of sap, dampness, absence of sunlight and stagnate air. Dry rot is prevented by using seasoned timber that is devoid of sap. In addition, timber should be kept dry in places with adequate ventilation (Punmia, Jain and Jain, 2005).

Wood is also attacked by insects mainly white ants, marine borers and beetles.

2,2.5 Testing Timber

The commonly tested properties of timber include: tensile strength, moisture content, specific gravity, compressive strength and shrinkage & strength (Punmia, Jain and Jain, 2005).

Moisture Content

The test specimen should have a size of 50 mm × 50 mm × 25 mm of the entire specimen

Take mass M1 of the test specimen.

The specimen should be oven dried until it has a constant weight at a temperature 103 ± 2 °C and take M2 of the test specimen.

Moisture content mo = M1 - M2 × 100

M2

Shrinkage Test

The test specimen should have a size of 50 mm × 50 mm × 150 mm

Using the immersion method, V2 of the specimen should be taken

The end of the specimen should be dipped in hot paraffin and left to air dry until the specimen has a moisture content ranging from 12% to 15%.

V2 of the specimen should be taken again via the immersion method.

Volumetric shrinkage can be computed using the following formula

Volumetric shrinkage = V1 – V2

V1

Compressive strength test

The specimen should be prism shaped with a height of 30 mm and base of 20 mm.

The specimen is gradually loaded into the compression testing machine.

The failure load P is recorded.

Compressive strength = P N/ mm2

A

A is the test specimen’s cross-section area

Compressive strength perpendicular to the grains is lesser than parallel to the grain.

Tensile strength test

A test specimen of 50 mm × 50 mm × 200 mm should be prepared.

A tensile load is applied either perpendicular or parallel to the grain.

The maximum load P is noted at failure.

Tensile strength = P N/ mm2

A

A is the test specimen’s cross-section area

Tensile strength parallel to the grains is greater than perpendicular to the grain.

2.2.6 Timber Fire Proofing

Timber can be treated in order to render it fire resistant to a considerable extent. This is through covering it with a material or compound (Punmia, Jain and Jain, 2005). This is done using superficial layers and coatings of the preferred protective material on the timber surface. The coatings reduce the usual temperature increase during fire incidences thereby decreasing the rate at which the flame spreads. The flame penetration rate is also lessened as well as the timber surface in contact with fire. These coatings are only viable for interior purposes as they wear out upon exposure to the weather. The fire retardant water soluble chemicals are mainly a formulation of borax, sodium silicate or ammonium sulphate mixed with other materials with qualities that promote color, appearance, brushability and timber adherence. Chlorinated rubber is a protective layer that can be used as well as other fire retardant chemicals such as chlorinated paraffin or zinc borate.

Secondly, timber can be impregnated. Complete impregnation is done with chemicals that render the wood incapable of combustion. Partial impregnation may be sufficient but inappropriate if the wood is supposed to undergo milling. The chemicals comprise of dibasic ammonium phosphate, monobasic ammonium phosphate, sodium tetraborate (borax), zinc chloride and boric acid. Ammonium phosphates inhibit glowing and flaming. Borax inhibits flaming but does not retard glowing. Boric acid inhibits glowing but does not retard flaming effectively. In Burnett’s fire proofing process, timber is soaked in a mixture of water and zinc chloride. In Abel’s fire proofing process, a dilute sodium silicate solution is painted on timber followed by a cream of slaked lime, and finally a concentrated solution of sodium silicate (Punmia, Jain and Jain, 2005).