NATURAL CHARACTERISTICS OF TIMBER

Wood as a natural material is highly varied in its structure and has many natural characteristics or defects which are introduced during the growing period and during the conversion and seasoning process. Often such characteristics or defects can cause trouble in timber in use either by reducing its strength or impairing its appearance.

Knots

These are common features of the structure of wood. A knot is a portion of a branch enclosed by the natural growth of the tree, normally originating at the centre of the trunk or a branch. The influence of knots depends on their size, shape, frequency and location in the structural member. The presence of knots has adverse effects on most mechanical properties of timber as they distort the fibres around them, causing fibre discontinuity and stress concentrations or non-uniform stress distributions. Their effects are further magnified in members subjected to tensile stress either due to direct or bending stresses. For example, the presence of a knot on the lower side of a flexural member, being subjected to tensile stresses due to bending, has a greater effect on the load capacity of the member than a similar knot on the upper side being subjected to compressive stresses. The presence of knots in round timber has much less effect on its strength properties than those in a sawn timber. When a log is sawn, the knots and fibres surrounding them will no longer be continuous – thus, adversely affecting the strength properties; whereas in the round timber there are no discontinuities in the wood fibres and often the angle of grain to the longitudinal axis is smaller than that in the sawn timber.

Table 1.1 Effect of grain deviation on strength properties of timber

In general, the size, shape, frequency and location of knots influence the quality and hence the grade of softwood timbers for structural use, with better grades having fewer and smaller knots.

 Slope of grain

Wood grain refers to the general direction of the arrangement of fibres in wood and is expressed with respect to the longitudinal axis of the sawn timber or the round timber (log or pole). In general, the direction of the fibres does not lie truly parallel to the longitudinal axis of the sawn or round timbers. In softwoods, the deviation with respect to the log (longitudinal) axis is often constant, resulting in the production of spiral grain. Interlocked grains are often produced in tropical hardwoods where the grain direction changes routinely from one direction to another.

A cross grain occurs when the grain direction is at an angle to the longitudinal axis of the sawn section. A cross grain occurs during conversion (sawing process) as a result of conversion of a bent or heavily tapered log or a log with spiral or interlocked grain. Grain deviation can severely impair the strength properties of timber. Visual grading rules limit the grain deviation; in general, a grain deviation of 1 in 10 is accepted for high-grade timber whereas 1 in 5 often relates to a low-grade one. The effect of grain deviation on some properties of timber is shown in Table 1.1.

Reaction wood

Reaction wood is referred to abnormal wood tissues produced in tree trunks subjected to strong wind pressures. Horizontal branches and leaning branches are believed to form reaction wood in an attempt to prevent them from excessive bending and cracking under their own weight. There are two types of reaction wood: in softwoods it is referred to as compression wood and in hardwoods as tension wood. Compression wood, Figure 1.2, forms on the underside of branches of leaning softwoods and contains more lignin than normal wood. Tension wood forms on the upper sides of leaning hardwoods and contains more cellulose than normal wood. Reaction wood is much denser than normal wood with the specific gravity of around 35% greater in compression wood and 7% greater in tension wood. Longitudinal shrink-age is also greater, 10 times more than normal for compression wood and 5 times for tension wood. Timber containing compression wood is liable to excessive distortion during drying and tends to fail in a brittle manner. It is harder to drive a nail in com-pression wood, there is a greater chance of it splitting, and compression wood may take a stain differently than normal wood. Most visual strength grading rules limit the amount of compression wood in high quality grades.

Fig. 1.2. Compression wood (dark patch).

Juvenile wood

This is a wood that is produced early in the first 5–20 rings of any trunk cross-section (Figure 1.1) and, in general, exhibits lower strength and stiffness than the outer parts of the trunk and much greater longitudinal shrinkage than mature, normal wood. Juvenile wood is mainly contained within the heartwood. In this regard, in young, fast grown trees with a high proportion of juvenile wood, heartwood may be inferior to sapwood, but is not normally considered a problem.

Density and annual ring widths

Density is an important physical characteristic of timber affecting its strength proper-ties. Annual ring width is also critical in respect of strength in that excessive width of such rings can reduce the density of the timber. Density can be a good indicator of the mechanical properties provided that the timber section is straight grained, free from knots and defects. The value of density as an indicator of mechanical properties can also be reduced by the presence of gums, resins and extractives, which may adversely affect the mechanical properties. In this regard, the prediction of strength on the basis of density alone is not always satisfactory. Research studies show a coefficient of determination, R2, ranging between 0.16 and 0.4 for density and 0.2 and 0.44 for the annual ring width.

Specific gravity or relative density is a measure of timber’s solid substance. It is generally expressed as the ratio of the oven-dry weight of the timber to the weight of an equal volume of water. Because water volume varies with the moisture content of the timber, the specific gravity of timber is normally expressed at a certain moisture content. Basic oven-dry specific gravity of commercial timber ranges from 0.29 to 0.81, most falling between 0.35 and 0.60.

Conversion of timber

Once the tree is felled in the forest, the crown is removed and often it is also debarked in the forest. Logs are then classed and stockpiled under water sprays to prevent them from drying out. Some of the better-quality ones are sent to peeling plants for the manufacture of veneers but the majority (depending on the quality) are sent to sawmillers to convert round logs to sawn timber. There are many cutting patterns used to produce timber, but the first step in most sawmill operations will start by scanning the log for the best alignment and cutting pattern for optimum return; then removing one or two wings (slabs) from the logs to give some flat surfaces to work from. The log, referred to as a cant, is turned on a flat face and sawn through and through to give boards (sections) of the required thickness.

Each sawmill establishes its own cutting patterns for different sized logs; maximizing the number of pieces cut in the most popular sizes. Through conversion produces mostly tangentially sawn timber and some quarter sawn sections. Tangential timber is economical to produce because of the relatively less repetitive production methods.

Boxing the heart (Figure 1.3) eliminates the heartwood from the boards that would otherwise produce shakes, juvenile wood or may even be rotten. The quarter sawn techniques are more expensive processes, with more wastage, because of the need to double (or more) handle the log. They are, however, more decorative and less prone to cupping or distortion. There are several alternative variations of tangential and radial cuts to obtain the best or most economical boards for the end use. Examples of methods of log breakdown and different cutting patterns are shown in Figure 1.3. In growing trees, all cell walls including their voids, in both heartwood and sapwood, are saturated with water (moisture content in excess of 100%). When a tree is cut and its moisture content falls to around 27%, the only moisture left is the bound water, which is the moisture that is part of the cell wall. This state is referred to as fibre saturation point. Wood, in general, is dimensionally stable when its moisture content is greater than the fibre saturation point. The process of drying (seasoning) timber should ideally remove over a third of the moisture from the cell walls. Timber at this stage is referred to as seasoned with a moisture content of between 12 and 25% (depending on the method and duration of drying, i.e. air, kiln, solar, microwave, etc.). Wood changes dimension-ally with change in moisture below its fibre saturation point: it shrinks when it loses moisture and swells as it gains moisture. These dimensional changes are mostly in the direction of the annual growth rings (tangentially), with about half as much across the rings (radially) and as such mainly affect cross-sectional dimensions (perpendicular to the grain) and can result in warping, checking or splitting of wood. Longitudinal shrink-age of wood (shrinkage parallel to the grain) for most species is generally very small. The combined effects of radial and tangential shrinkage (differential shrinkage) can distort the sawn timber. The major types of distortion as a result of these effects after drying for various cross-sections cut from different locations in a log are shown in Figure 1.4.

 Fig. 1.3. Examples of log breakdown and cutting pattern.

Fig. 1.4. Distortion of various cross-sections.

The change in moisture content of timber also affects its strength, stiffness and resistance to decay. Most timber in the United Kingdom is air-dried to a moisture content of between 17 and 23% (which is generally below the fibre saturation point) at which the cell walls are still saturated but moisture is removed from the cell cavities. Figure 1.5 highlights a general relationship between strength and/or stiffness characteristics of timber and its moisture content. The figure shows that there is an almost linear loss in strength and stiffness as moisture content increases to about 27%, corresponding to the fibre saturation point. Further increase in moisture content has no influence on either strength or stiffness. It should be noted that although for most mechanical properties the pattern of change in strength and stiffness characteristics with respect to change in moisture content is similar, the magnitude of change is different from one property to another. It is also to be noted that as the moisture content decreases shrinkage increases.

Fig. 1.5. General relationship between strength and/or stiffness and moisture content.

Fig. 1.6. Defects in timber.

Timber is described as being hygroscopic, which means that it attempts to attain an equilibrium moisture content with its surrounding environment, resulting in a variable moisture content. This should always be considered when using timber, particularly softwoods, which are more susceptible to shrinkage than hardwoods. As logs vary in cross-section along their length, usually tapering to one end, a board that is rectangular at one end of its length might not be so at the other end. The rectangular cross-section may intersect with the outside of the log, the wane of the log, and consequently have a rounded edge. The effect of a wane is a reduction in the cross-sectional area resulting in reduced strength properties. A wane is an example of a conversion defect and this, as well as other examples of conversion or natural defects, is shown in Figure 1.6a.

Seasoning

Seasoning is the controlled process of reducing the moisture content of the timber so that it is suitable for the environment and intended use. There are two main methods of seasoning timber in the United Kingdom, air-drying and kiln-drying; other less common methods include solar and microwave techniques. All methods require the timber to be stacked uniformly, separated by spacers of around 25 mm to allow the full circulation of air etc. around the stack. Often, ends of boards are sealed by a suitable sealer or cover to prevent rapid drying out through the end grains. However, with air-drying it is not possible to obtain less than 16–17% moisture content in the United Kingdom. Further seasoning would require to be carried out inside a heated and ventilated building. The kiln-drying method relies on a controlled environment that uses forced air circulation through large fans or blowers, heating of some form provided by piped steam together with a humidity control system to dry the timber. The amount and duration of air, heat and humidity depend on species, size, quantity, etc.

 Seasoning defects

Seasoning defects are directly related to the movements which occur in timber due to changes in moisture content. Excessive or uneven drying, as well as the presence of com-pression wood, juvenile wood or even knots, exposure to wind and rain, and poor stacking and spacing during seasoning can all produce defects or distortions in timber. Examples of seasoning defects such as cupping (in tangential cuts), end splitting, springing, bowing, twisting, etc. are illustrated in Figure 1.6. All such defects have an effect on structural strength as well as on fixing, stability, durability and finished appearance.

 Cracks and fissures

They are caused by separation of the fibres along the grain forming fissures and cracks that appear on one face or at the end grain but do not necessarily continue through to the other side. Their presence may indicate decay or the beginnings of decay.

Fungal decay

This may occur in growing mature timber or even in recently converted timber, and in general it is good practice to reject such timber.