THE STRUCTURE OF TIMBER

Structural timber is sawn (milled) from the trunk of the tree, which provides rigidity, mechanical strength and height to maintain the crown. Trunk resists loads due to gravity and wind acting on the tree and also provides for the transport of water and minerals from the tree roots to the crown. Roots, by spreading through the soil and acting as a foundation, absorb moisture-containing minerals from the soil and transfer them via the trunk to the crown. Crown, comprising branches and twigs to support leaves, provides a catchment area producing chemical reactions that form sugar and cellulose that cause the growth of the tree.

As engineers we are mainly concerned with the trunk of the tree. A typical cross section of a tree trunk, shown in Figure 1.1, illustrates its main features such as bark, the outer part of which is a rather dry and corky layer and the inner living part. The cambium, a very thin layer of cells underside the inner bark, is the growth centre of the tree. New wood cells are formed on the inside of the cambium (over the old wood) and new bark cells are formed on the outside and as such increasing the diameter of the trunk. Although tree trunks can grow to a large size, in excess of 2min diameter, commercially available timbers are more often around 0.5 m in diameter. Wood, in general, is composed of long thin tubular cells. The cell walls are made up of cellulose and the cells are bound together by a substance known as lignin. Most cells are oriented in the direction of the axis of the trunk except for cells known as rays, which run radially across the trunk. The rays connect various layers from the pith to the bark for storage and transfer of food. Rays are present in all trees but are more pronounced in some species such as oak. In countries with a temperate climate, a tree produces a new layer of wood just under the cambium in the early part of every growing season. This growth ceases at the end of the growing season or during winter months. This process results in clearly visible concentric rings known as annular rings, annual rings, or growth rings. In tropical countries, where trees grow throughout the year, a tree produces wood cells that are essentially uniform. The age of a tree may be determined by counting its growth rings.

The annular band of the cross-section nearest to the bark is called sapwood. The central core of the wood, which is inside the sapwood, is heartwood. The sapwood is lighter in color compared to heartwood and is 25–170 mm wide depending on the species. It contains both living and dead cells and acts as a medium for transportation of sap from the roots to the leaves, whereas the heartwood, which consists of inactive cells, functions mainly to give mechanical support or stiffness to the trunk. As sapwood changes to heartwood, the size, shape and the number of cells remain unchanged. In general, in hardwoods the difference in moisture content of sapwood and heartwood depends on the species but in softwoods the moisture content of sapwood is usually greater than that of heartwood. The strength and weights of the two are nearly equal.

Sapwood has a lower natural resistance to attacks by fungi and insects and accepts preservatives more easily than heartwood. In many trees and particularly in temperate climates, where a definite growing season exists, each annular ring is visibly subdivided into two layers: an inner layer made up of relatively large hollow cells called springwood or earlywood (due to the fast growth), and an outer layer of thick walls and small cavities called summerwood or latewood (due to a slower growth). Since summerwood is relatively heavy, the amount of summerwood in any section is a measure of the density of the wood; see Figure 1.1.

TYPES OF TIMBER

Trees and commercial timbers are divided into two types: softwoods and hardwoods. This terminology refers to the botanical origin of timber and has no direct bearing on the actual softness or hardness of the wood as it is possible to have some physically softer hardwoods like balsa from South America and wawa from Africa, and some physically hard softwoods like the pitch pines.

 Softwoods

Softwoods, characterized by having naked seeds or as cone-bearing trees, are generally evergreen with needle-like leaves (such as conifers) comprising single cells called tracheids, which are like straws in plan, and they fulfil the functions of conduction and support. Rays, present in softwoods, run in a radial direction perpendicular to the growth rings. Their function is to store food and allow the convection of liquids to where they are needed. Examples of the UK grown softwoods include spruce (whitewood), larch, Scots pine (redwood) and Douglas fir.

Softwood characteristics

·        Quick growth rate (trees can be felled after 30 years) resulting in low-density

·        timber with relatively low strength.

·        Generally poor durability qualities, unless treated with preservatives.

·        Due to the speed of felling they are readily available and comparatively cheaper.

Hardwoods

Hardwoods are generally broad-leaved (deciduous) trees, which often lose their leaves at the end of each growing season. The cell structure of hardwoods is more complex than that of softwoods with thick-walled cells, called fibres, providing the structural support and thin-walled cells, called vessels, providing the medium for food conduction. Due to the necessity to grow new leaves every year the demand for sap is high and, in some instances, larger vessels may be formed in the springwood, these are referred to as ‘ring-porous’ woods such as in oak and ash. When there is no definite growing period, the pores tend to be more evenly distributed, resulting in ‘diffuse-porous’ woods such as in poplar and beech. Examples of the UK grown hardwoods include oak, beech, ash, alder, birch, maple, poplar and willow.

 Hardwood characteristics

·        Hardwoods grow at a slower rate than softwoods, which generally results in a timber of high density and strength, which takes time to mature, over 100 years in some instances.

·        There is less dependence on preservatives for durability qualities.

·        Due to the time taken to mature and the transportation costs of hardwoods, as most are tropical, they tend to be expensive in comparison with softwoods.

British Standard BS 5359:1991 provides a list of some 500 timbers of economic interest in the United Kingdom and tabulates softwoods and hardwoods including their standard names, botanical names/species type and also, where relevant, their alternative commercial names with sources of supply and average densities