STRUCTURAL DESIGN FOR ARCHITECTURE

Timber structures

known as the 'appearance' or 'defect' system of
grading in which limits are set for the sizes and
frequency of occurrence of various types of
defect such as knots, grain (its width, straight-
ness, angle of inclination to the principal
direction of the member, etc.) and the defects
which arise from conversion^6 and seasoning,
such as wane, splits, twist, cupping and
bowing (Fig. 6.17). Each piece of timber is
simply inspected and placed in a grade accord-
ing to the sizes and frequency of defects which
it contains. For each grade the limits on the
sizes of defects are related to the dimensions
of the piece of timber being considered. Thus,
large defects in a large plank can result in it
being placed in the same grade as a smaller
plank with smaller defects. If a large plank is
sawn up into two or more smaller planks the
timber must be regraded. Most countries have
adopted either four, five or six grades. Table
6.3 shows the relationship between the grades
which are used by a number of major sources
of structural timber.

The appearance or defects method of grading

is not entirely satisfactory as a means of classi-

fying structural timber because the appearance

of the surface of a specimen is not a reliable

enough guide to its strength properties. For

this reason a system of stress-grading has been

adopted in the UK, and all timber which is used

in structures which have been designed to

comply with BS 5268 must be graded by this

method, even though it may already have been

graded by the appearance method. The British

Standard in which the stress grading proced-

ures are specified is BS 4978.

The stress-grading system does in fact make

use of visual criteria but these are based on

the effects which knots and other defects have

on the cross-section of the member rather than

on the appearance of its surface and therefore

give a more realistic assessment of the effect

of defects on strength. Two visual stress

grades, general structural (GS) and special

structural (SS) are specified for each species of

sawn timber and three visual stress grades are

specified for laminated timber (LA, LB and LC).

The visual stress-grading system is now being

used in conjunction with 'machine-grading' in

which the timber is passed through a device

which imposes a bending load on it and simul-

taneously measures the resulting deflection. 197

6 Conversion is the name given to the process by which
felled timber in the form of tree trunks is sawn up into
planks.

Table 6.3 Relationship of 'appearance' grades for softwood timber

Norway, Sweden, I, II, III, IV V VI

Finland, Poland unsorted

and Eastern Canada

USSR I, II, III IV V

unsorted

Brazil No. 1 and

No. 2

British Columbia No. 1 Select No. 2 No. 3

and Pacific clear merchantable merchantable common

Coast of No. 2 No. I

North America clear merchantable

(R list) No. 3

clear

UK BS 3819 I Clear I,II III IV