STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

6.4.4 Mechanical properties

6.4.4.1 Introduction

Due to the fibrous nature of the material and

the phenomenon of grain, the strength of

timber and its behaviour in respect of deform-

ation are non-isotropic (not the same in all

directions).^5 Many of the features of the

mechanical properties of timber can in fact be

understood by imagining its internal structure

to be similar to that of a bundle of drinking

straws.

6.4.4.2 Strength

In the direction parallel to the grain the

strength of timber is slightly less in compres-

sion than it is in tension. This is due to the

tendency for the cell walls, which are aligned

parallel to the grain, to buckle under the action

of compressive loads. The strength at right

angles to the grain is considerably less than

that parallel to the grain. This is due to separ-

ation of the fibres when a tensile load is

applied and crushing of the cells when a

compressive load is applied. A number of

different values are therefore normally quoted

for the strength of a timber depending on the

direction in which a load is applied with

respect to the grain.

The grade stress values which are given in

the British Standard (BS 5268), are reproduced

here in Table 6.1. These are derived from the

results of tests carried out on structural-sized

elements rather than small specimens of the

species concerned, but they are subject to

modification to allow for differences between

the test conditions and the conditions which

are likely to prevail in a real structure in

service. Examples of the latter are the duration

of the load (the ability of timber to sustain a

load depends on the length of time for which it

is applied), the moisture content of the timber,

the particular dimensions of the cross-section

of the element and a number of other factors.

5 The strength and elasticity of a specimen of timber

depend on the orientation of the load with respect to

the grain. Strength and modulus of elasticity are

greater parallel to the grain than normal to the grain.

Fig. 6.18 Seasoning defects which occur due to the

difference between the rate of radial and tangential shrink-

age.

be entirely eliminated. The extent and

incidence of seasoning defects is reduced if

small cross-sections (i.e. cross-sections in

which one of the dimensions is 75 mm or less)

are used; these factors are also affected by the

original location and orientation of a plank

within the tree (Fig. 6.18)

The equilibrium moisture content of the

timber in a building depends on the internal

environment of the building. In the UK it is

usually around 15% and is sometimes less in

buildings in which high temperatures are

maintained. Air-seasoned timber is supplied

with a moisture content of around 22% and will

therefore suffer further drying and shrinkage

after installation in a building. Kiln-seasoned

timber can be supplied with a lower moisture

content and is less likely to suffer a large shift

in moisture content after installation. All

timber is likely to undergo small shifts in

moisture content, with corresponding dimen-

sional changes, during the life of a building,

because of changes in the environmental

194 conditions.

Cupping of plain sawn

boards caused by greater

shortening of longer

growth rings during drying

Radial sawn

Diamonding

Plain sawn