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In agricultural buildings, wood is often used in the form in which it has grown, i.e., round poles. In some areas where enough trees are grown on the farm or in local forests, wooden poles can be obtained at very low cost. These poles have many uses in small building construction such as columns for the load bearing structure, rafters, trusses and purling. Smaller dimension sticks are often used as wall material or as framework in mud walls.
Where straight poles are selected for construction, it will be as easy to work with round timber as with sawn timber. However, somewhat crooked poles can also be used if they are turned and twisted and put into positions in which the effects of the bends are unimportant.
Round timber can generally be considered stronger than sawn timber of the same section area, since the fibres in round timber are intact. The pole is normally tapered and therefore the smallest section area, the top end, must be used in calculation of compressive and tensile strength.
A great number of species can be considered when selecting poles for building construction, but only a limited number are available on the commercial market. Some species are more suitable for silviculture (growing on farms) and silvipasture (growing on pastures) than others, but must always be selected to suit local climatic and soil conditions. Generally there are several species suitable for each location that are fast and straight growing and produce strong and durable timber. Some species will, in addition to building poles or timber, produce fodder for the animals, fruits, fuelwood, etc.
Many species of eucalyptus, from which gum poles are obtained, are very fast and straight-growing hard woods. However, they warp and split easily. Dimensions suitable for building construction are obtained by harvesting the still immature trees. Gum poles provide a strong and durable material if chemically treated.
In high altitude areas several species of acacia produce good building poles. Acacia melanoxylon (Australian Blackwood) is very resistant to attack by termites, but grows a bit slower than eucalyptus. In low to medium altitude areas with sandy soils and low rainfall, Casuarina Species produces straight and durable poles.
Cedar posts for fencing are obtained by splitting large logs. The posts are durable, resistant to rot and attack by termites. They are also suitable for wall posts in building construction.
In coastal areas, mangrove poles are widely used for posts in walls and trusses in roofs.
Unprocessed round wood material can be joined by being nailed or tied with string or wire. A special connector has been developed to join round wood in trusses where several members may have to be connected at each point.
The rate at which a tree grows varies with the season. The resulting growth rings of alternate high and low density form the grain in the sawed timber (lumber). The method of sawing has considerable effect on the appearance, resistance to warping, shrinking, paint holding ability and wear resistance of the final piece.
There are several methods of sawing a log into boards and planks giving different ways for the growth rings to relate to the surface, i.e., more or less parallel to the surface in plain sawn and at right angles in radial sawn.
Radially sawn boards shrink less and are less liable to cup and twist and are easier to season. Unfortunately, methods of cutting which produce a high proportion of quarter sawn timber are wasteful and therefore only used to produce material for high-class joinery work. See Figures 3.2 and 3.3.
Figure 3.2 Methods of sawing timber.
Because the tree is tapered and cylindrical and boards and planks are rectangular, the outer pieces will come off with tapered edges and less than full dimensions throughout the length. Such pieces, called offcuts, can be sometimes obtained at low cost and used for rough building.
Seasoning of Timber
The strength, stiffness and dimensional stability of wood is related to its moisture content. Hence, if wood is dried (seasoned) before use, not only can higher strength values be used in design, but a more durable structure will result. In developing countries, most timber is not seasoned and it is sold in what is called its green state.
Timber must be stacked, supported and sometimes retrained so as to minimize distortion during seasoning. If drying is too rapid, the outer parts, in particular the unprotected ends, shrink before the interior, surface checking and splitting result, and ring and heart shakes may extend. Some timber species are more difficult to season satisfactorily than others.
Figure 3.3 Effects on Cupping and Shrinkage of different methods of sowing.
Timber should be protected from rain and from the ground and stacked so that air can circulate freely around all surfaces. Thus the risks of twisting and cupping and attack by fungi and insects are minimized. In favourable conditions, thin softwoods can be air seasoned in weeks but in unfavourable conditions some hardwoods require a year or more.
Artificial seasoning can be either moderate or rapid depending on the temperature of the air injected into the chamber where the timber is piled, and the rate at which the air is circulated and extracted from the chamber. This method is expensive and can only be applied on small quantities of timber. Timber can be artificially seasoned from the green condition, but often hot air seasoning is used only at a later stage after most of the moisture has been removed with air seasoning.
Smoke seasoning is a moderate process and involves placing the timber over a bonfire. It can take a month or two depending on the size and type of wood being seasoned. This method is considered to be both a seasoning and a treating method for timber. Presumably it protects the timber against pest attacks and increases durability. However, it is not very reliable and can lead to splitting of the timber because of lack of control of the heat from the bonfire.
Care of Seasoned Timber
Timber should be protected from moisture on the building site. Close piling and covering with tarpaulins delays the absorption of atmospheric moisture, particularly in the interior of the pile.
Timber Grade Standards and Sizes
Grades are established by various government agencies. Even within one country more than one grading system may be in use. For small construction jobs, the grade may not be important, but in large projects where materials are bought by specification, it is important to indicate the grade standard being required.
Grades that provide specific information in structural design are most useful. For instance, the grade standard established by the Kenya Bureau of Standards illustrates this point in Table 3.1.
Table 3.1 Timber Grades and Applications
|F||Furniture, high-class joinery|
|S 75||Structural grade, having a value of 75% of basic stress|
|S 50||Structural grade, having a value of 50% of basic stress|
|C||A general construction grade for non-stressed construction|
|L||A low grade for low-quality work|
It is the S 75 and S 50 grades that are significant in building construction as will be seen in later sections.
Figure 3.4 Air drying of timber.
Timber in East and Southeast Africa is available in a number of S.l. metric sizes, but not all are available in all localities. The dimension indicates actual size as sawn. Smoothing will reduce the timber to less than dimension size.
Timber Measurement for Trade
Timber is normally sold in metre (or foot) running length; however, the price may be calculated per cubic metre when sold in large quantities. Basic lengths are from 1.8 to 6.3m although pieces more than about 5.1m are scarce and costly. Timber normally comes in running lengths, that is, not sorted for length.
Strength of Wood
Building materials of any type which are loaded are said to be subjected to a fibre stress. The safe fibre stress for a material is the load which the material will safely resist. Wood, like other materials has safe fibre stress values given in N/mm which have been determined by destructive testing to get first an ultimate stress, and then, by the use of various correction and safety factors, safe fibre stresses to use in designing.
Table 3.2 lists basic working stress values for several types of loading in 5 strength groups. Table 3.3 divides some representative species into the strength groups to be used in Table 3.2.
There are dozens of additional species of trees found in East Africa many of which are used only in very local areas. In order to obtain approximate working stress data for these indigenous species, their densities may be used to place them in the proper group for Table 3.2. If the density is not known, a good approximation can be found quite easily. A bucket, a graduated cylinder (millilitres) and an accurate scale for weighing a sample of the wood will be needed. The procedure is:
Table 3.2 lists basic working stress values. For design purposes these should be adjusted for a number of different variables including: grade, moisture content, duration of load, exposure, and use of the structure.
According to Kenya Forest Department the grades should be used as follows
|Grade 1||75% of basic working stress value|
|Grade 2||50% of basic working stress value|
|Grade 3||35% of basic working stress value|
|Grade 4||15% of basic working stress value|
Table values need to be reduced when timber is installed green and will remain wet and uncured continuously. Use Figure 3.5 to find a suitable stress value for green wood corresponding to the dry value in Table 3.2.
Table 3.2 Guide to Basic Working Stresses Values and Module of Elasticity for Timber
|Strength group||Strength rating||Density green||Density 12% M.C.||Max. bending strength and tension to grain||Modulus of elasticity||Maximum ompression strength||Max. shearing strength|
|II to grain||I to grain||Beams||Joints|
|1||Weak||< 520||< 400||10||4.0||2.5||U.6||1.0||0.4|
|2||Fairly strong||521 650||401-500||15||6.0||10.0||1.2||1.3||1.6|
|5||Exceptionally strong||> 1041||> 801||50||10.5||29.0||5.0||3.2||4.1|
Figure 3.5 Basic working stresses for timber.
Table 3.3 Some Representative Timbers Grouped According to Strength and Density
|Group||Latin Name||Common Name|
|1||Pinus radiate 1 2yr||Young pine|
|Pinus patula ( 17yr)*||Pine|
|Pinus radiate ( 17yr)||Pine|
|Cupressus lusitanica**||East African Cypress|
|Juniperus procera||African Pencil Cedar/Mutarakwa|
|Octea usambarensis||East African Camphorwood, Muzaiti|
|Acacia melanoxylon||Australian Blackvvood|
|Grevillia robusta||GrevilliA/Silky Oak|
|Vitex keniensis*||Vitex/Muhuru/Meru Oak|
|Khay anthot heca||African Mahogany|
|Eucalyptus regnans||Australian Mountain Ash|
|Eucalyptus saligna||Saligna gum/Sydney blue gum|
|Afzelia quanensis Afzelia|
|5||Olea hochstetteri||East African Olive/Musharagi|
* one group lower in compression perpendicular to grain
** one group lower in joint shear
Timbers exposed to severe weather and decay hazards should be designed using a 25% stress value decrease, particularly for columns and for bearing points.
The main structural softwood timbers of East and Southeast Africa are not naturally durable. If used in conditions subject to fungal, insect or termite attack, they will fail after some time. To avoid this, the timber used in permanent structures should be treated with a preservative.
Effective preservation depends on the preservative and how it is applied. An effective preservative should be poisonous to fungi and insects, permanent, able to penetrate sufficiently, cheap and readily available. It should not corrode metal fastenings, nor should the timber be rendered more flammable by its use. It is sometimes desirable to have a preservative-treated surface which can be painted.
If a structure is correctly designed and built, and the moisture content of its timber does not exceed 20%, then a preservative treatment is generally unnecessary as protection against fungal attack. Where the above conditions are not present, however, there will be a risk of fungal decay, and proper preservation is recommended.
Creosote is an effective general purpose preservative, cheap and widely used for exterior work and to a lesser degree inside. It is a black to brownish oil produced by the distillation of coal-tar, and has many of the properties required of a preservative, but it increases flammability, is subject to evaporation, and creosoted wood cannot be painted. It should not be used on interiors if the characteristic smell would be objectionable. Unfortunately creosote has been found to be a carcinogen and must be used with caution.
Coal-tar as a preservative is not as effective as the creosote produced from it. Tar is less poisonous, it does not penetrate the timber because of its viscosity, it is blacker than creosote and it is unsuitable for interior wood work.
Unleachable metallic salts are mostly based on copper salts. A combination of copper/chrome/ arsenate is used. The copper and arsenical salt are the toxic preservatives which are rendered nonleaching (cannot be washed out) by the chrome salt acting as a fixing agent. The timber is impregnated by a "vacuum-pressure" process. Preservation by metallic salt is being increasingly used since the treated surfaces are odourless and can be painted or glued.
Water-soluable preservarives are not satisfactory for exterior use, as they are liable to be removed from the timber by rain. They are, however, very suitable for interior work, as they are comparatively odourless and colourless, and the timber can be painted.
Used engine oil can often, at least in small quantities, be obtained free of charge. The oil contains many residual products from combustion and some of them will act as preservatives, but it is not nearly as effective as commercial preservatives. It can be thinned with diesel fuel for better penetration. The combination of 401 of used engine oil and 11 of Dieldrin is a viable alternative in rural construction.
Methods of Wood Preservation
To be effective preservation three main methods of preservation:
1 Pressure impregnation of timber placed in a horizontal steel cylinder is one of the best ways to apply preservatives into the wood. Creosote is the main preservative used, but unreachable metallic salts are also commonly applied by this method. Water-borne preservatives must be applied with the pressure treatment if the timber will be exposed to rain or ground moisture. Surface-applied water-borne preservatives quickly leach away leaving the timber unprotected.
2 Open tank treatment, known as steeping or soaking is used for relatively small quantities of timber. a Hot and cold steeping. The tank with the preservative and timber is heated to nearly boiling, held for one to two hours and then allowed to cool. During the heating period the cells and the air in the cells expand and some of the air is expelled. As the timber and preservative cool the timber contracts and the partial vacuum created causes the liquid to be gradually absorbed into the timber. b The timber can be steeped in either hot or cold preservative, but it is not as effective as hot and cold steeping. Creosote or metallic salts can be applied by these methods.
3 Superficial preservation includes dipping, spraying and brush application. None of these surface treatments are as effective as the pressure and open-tank systems, as the preservative only penetrates the timber slightly. The wood must be seasoned and the surface should be dry and clean before application. Greater penetration generally results if the preservative is applied hot, especially if creosote is used. The timber should have two coats at least; the first coating allowed to dry before the next is applied. Creosote is the most common preservative used for this method. Superficial treatment with clear liquids is not recommended since the proper application is difficult to control.
There are a number of building boards made from wood veneers or the waste products of the timber industry that are convenient and economical materials to use in building construction. In general, they offer excellent bracing for the building frame and a saving in labor because they are available in large sizes requiring a minimum of fitting.
Some manufactured boards are designed with rather specific characteristics such as fire resistance, ease of cleaning, high insulating value or resistance to weathering.
Plywood is produced by gluing together three to seven veneers that have been peeled from logs. The grain of each succeeding veneer is turned 90 from the previous one, resulting in a board that has considerable strength and rigidity in all directions. Waterproof glue is most commonly used giving a product that is highly resistant to moisture. Waterproof glue panels should always be chosen for farm buildings. As the wood itself is not waterproof, the panels are still subject to swelling and shrinking from moisture changes.
Grades of Plywood
Plywood is generally given 4 to 5 grades based on the appearance of the surface veneers. Each panel has a double letter grade, i.e., as to indicate the grade of the face of the panel and the back of the panel. The top-grade surface is generally free enough from defects to be finished naturally, while the second-best grade is good for painting. Lower grades are used for structural applications where appearance is of little significance. Theoretically from 10 to 15 different grade combinations are possible. In actual practice only part of them will be available from the timber merchants.
Sizes of Plywood Panels
The Kenya Bureau of standards lists twelve panel sizes and 9 different thicknesses. Combining grades, panel sizes and thicknesses, there are numerous possible combinations, however, only a few will be manufactured. The most common panel size is 2400 by 1200mm in thicknesses of 9, 12, 15 and 19mm.
Plywood as Structural Members
Plywood panels are made from many different species of wood and have a wide range in strength and stiffness. Either the manufacturer or a trade association which publishes grade standards to which manufacturers adhere, can provide specific strength characteristics for plywood. In general, plywood panels should equal or exceed the strengths shown in Table 3.4.
Table 3.4 Safe Spans for Plywood Panels Paralled to Grain of Plys
|Thickness||(170 kg/m)||(490 kg / m)|
Figure 3.6 Plywood.
Other Manufactured Boards
Blockboards and laminboards are made of strips of wood from 8 to 25mm wide, glued together and covered with one or more veneers on each side. At least one pair of corresponding veneers will have the grain at right angles to the grain of the core. Thus, if the finish grain is to run parallel with the core, there must be at least two veneers per side.
The same 12 panel sizes listed for plywood are also listed for the blockboard. However, the thicknesses are greater, ranging from 15 to 50mm in 5mm increments. The same appearance, grades and types of glue listed for plywood also apply to blockboards. Blockboard panels are often used for doors.
Figure 3.7b Particle board.
Particleboards are formed by pressing chips or flakes of wood between pairs of heated platens so that the particles lie in random fashion with their longer dimensions parallel to the surface of the board. The chips are bonded with thermosetting synthetic resins. Depending on the size of the particles, these boards are variously known as particleboard, chipboard or waferboard. Strength and rigidity generally increase with density, but that alone is not a measure of quality, as moisture resistance varies considerably and most particleboards should not be used in moist locations.
Softboards are made from uncompressed woodchips or sugarcane fibres mixed with water and glue or resins, giving a density below 350 kg/m. They are inexpensive and can be used for wall or ceiling surfaces that are not subject to high moisture conditions. Softboards have little resistance to rupture and must be supported frequently (300 to 400mm) when installed. The 2400 by 1200mm size is most common in thicknesses of 6.4 to 25mm.
Mediumboards, having a density ranging from 350 to 800 kg/m, are used for paneling, in particular those having a density at the higher end of the range. The 2400 by 1200 size is most common and thicknesses range from 6.4 to 19.0mm.
Hardboards are made of wood fibres compressed to over 800 kg/m. They are usually smooth on one surface and textured on the other. The 2400 by 1200mm size is most common in thicknesses of 3 to 12.7mm. An oil treated grade labeled "tempered" has good resistance to moisture.
Woodwool slabs consist of long wood shavings mixed with cement and formed into slabs 25 to 100mm thick and with a high proportion of thermal insulating voids. Although combustible, they are not easily ignited and provide good sound absorption.
Shingles are cut from clear rot-free timber logs. They are made about 2mm thick at the top end and 10mm thick at the bottom and usually about 400mm long. Some woods need treatment with preservatives before being used as roofing shingles, whereas others will last 10 to 15 years without treatment.
Sawdust is a by-product from sawmills. It is a good natural insulating material and also a good bedding material for use in animal housing.
Wicker made from shrubs, bushes and trees is used either directly for fencing or wall-cladding or can be sealed by smearing on mud, plaster, etc.
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