Section Sizes

In general, it is possible to design timber structures using any size of timber. However, since the specific use is normally not known at the time of conversion, sawmills tend to produce a range of standard sizes known as 'customary' sizes. Specifying such customary sizes will often result in greater availability and savings in cost.

There are a number of alternative sizes and finishes of cross-sections. BS 4471:1978 [13] provides sizes and tolerances for three types of surface finish: sawn, planed and regularised. For example, the deviation in sawn sections at a moisture content of 20% are as follows: for thicknesses and widths up to 100 mm, -1 mm and +3 mm, and for over 100 mm sizes, -2 mm and +6 mm. Sawn sections should only be used in situations where dimensional tolerances are of no significance. Planing two parallel edges to a specified dimension is referred to as regularising and if all four edges are planed to specified sizes, the process is referred to as planed all round. The requirements of EC5 for timber target sizes (i.e. specified sizes) are those given in BS EN 336:2003 [14] and in its National Annex. This standard specifies two tolerance classes: tolerance class 1 (T1) is applicable to sawn surfaces, and tolerance class 2 (T2) applicable to planed timber. Regularised timber can be achieved by specifying T1 for the thickness and T2 for the width. For T1, dimensions of up to 100 mm are limited to —1/+3 mm and dimensions of over 100 mm to —2/+4 mm. For T2, dimensions of up to 100 mm are limited to —1/+1 mm and those over 100 mm to —1.5/+1.5 mm.

The commonly available lengths and cross-section sizes are also listed in the UK National Annex of BS EN 336, and are referred to as target sizes. The 'target size' is defined as the specified timber section size at a reference moisture content of 20%, and to which the deviations, which would ideally be zero, are to be related. The target sizes can be used, without further modification, in design calculations.

In general, the differences between BS 4471 and BS EN 336 are minor and should not present any problems to specifiers and suppliers in the United Kingdom [4]. The customary target sizes, whose sizes and tolerances comply with BS EN 336, for sawn softwood structural timber, for structural timber machined on the width and for structural timber machined on all four sides are given in Table 1.4. In Table 1.5 the range of lengths of sawn softwood structural timber are detailed.

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