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
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