STRUCTURAL DESIGN FOR ARCHITECTURE

Table 6.4 Basic sawn sizes of softwood (from BS 4471 Part 1: 1978)

Thickness (mm) Width (mm)

75 100 125 150 175 200 225 250 300

16

19

22

25

32

36

38

44

47

50

63

75

100

150

200

250

300 X

X X X X X X X X X X

X X X X X X X X X X X X X

X X X X X X X X X X X X

X X X X X X X X X X X X X X

X X X X X X X X

X X X X X X X X X X X

X X X X X X X X

X X X X X X X X

X X X X X X X X

Structural Design for Architecture

This system is based on the fact that the

bending deformation which occurs to a speci-

men of timber is a reliable guide to its

strength.

The stress values which are quoted here in

Table 6.1 are the grade stresses which are

currently used in the UK. They are reproduced

from BS 5268. The prefix M indicates that the

material must be machine graded.

Table 6.5 Basic lengths of sawn softwood (m)

(From BS 4471 Part 1: 1978)

1.80 2.10 3.00 4.20 5.10 6.00 7.20

2.40 3.30 4.50 5.40 6.30

2.70 3.60 4.80 5.70 6.60

3.90 6.90

6.6 Timber components

6.6.1 Solid timber

6.6.1.1 Sawn timber and its derivatives

The simplest timber components are solid

elements of rectangular cross-section on which

no finishing was carried out after conversion.

These are referred to as 'sawn-timber'

elements. The range of cross-sectional sizes

(basic sawn sizes) and lengths in which

softwood timber is available in the UK are

given in Tables 6.4 and 6.5 and the dimen-

sional properties of their cross-sections are

given in Table 6.6. Note that the terminology

198 which is used to describe the cross-section is

to call the smaller of the two dimensions the

thickness and the larger the width.

Basic sawn-timber elements tend to have a

slightly irregular geometry and in situations

where this is undesirable, such as in trusses in

which the elements are joined by gusset plates

and must therefore be of equal thickness, the

timber is 'regularised'. This is a machine

process by which the thickness and/or width of

an element is made uniform throughout its

length. It is normally carried out on the critical

dimension only: that is on the thickness, for

elements which are to be used in trusses, and

on the width, for elements, such as floor joists

or wall studs, where this is the critical dimen-

sion. Regularising involves a reduction of

around 3 mm in the dimension which is

affected.