STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

The allowable stress which is used in the

design of a timber element is the grade stress

multiplied by one or more stress modification

factors, which allow for the effects noted

above.

Because timber is much stronger parallel to

the grain than in other directions most timber

components are designed so that the stress

which results from the principal load occurs

parallel to the grain.

6.4.4.3 Fire resistance

Although timber is combustible its perform-

ance in fire is good, compared to that of other

materials such as steel, aluminium or plastic.

There are two reasons for this. Firstly, the

structural performance of timber is more-or-

less independent of temperature (within the

range of temperatures likely to be experienced

in a building fire) so that a timber structure

will continue to support load during a fire until

the cross-sections of the elements are reduced

by combustion to the point at which the

stresses in them become excessive. It does not

collapse as a result of the effects of high

temperature alone. Secondly, the rate at which

timber is consumed in a fire is low and

relatively constant. It is therefore possible to

design a timber structure to withstand the

effects of fire for a predetermined period, by

oversizing the elements to allow for the mater-

ial which will be consumed in that period. This

method of design for fire resistance is known

Table 6.2 Charring rates of timber

Loss of timber from one

face of element (mm)

Minutes

15 30 45 60 90

Softwood including

laminated timber but

excluding western red cedar 10 20 30 40 60

Western red cedar 12.5 25 37.5 50 62.5

Hardwoods 7.5 15 22.5 30 45

as the 'sacrificial timber' or 'residual section'

design method. Charring rates for timber are

given in Table 6.2.

6.4.4.4 Durability

Timber structures are likely to suffer from two

types of decay: insect attack and fungal attack.

Fungal attack is the more destructive of these,

but this will only occur if the moisture content

of the timber is greater than 20% and can be

prevented by suitable design. Timber structures

are therefore detailed so that direct contact with

the ground is avoided and in such a way as to

minimise the risk of condensation forming on

timber elements. Features which will allow

water to collect and lie on timber are also

avoided. The likelihood of fungal and insect

attack can also be reduced by impregnating the

timber with various preservative materials.

6.5 Grading of timber

The properties of timber can exhibit consider-

able variability, even within samples from a

single species, and where the material is used

for structural purposes it is necessary that this

variability should be quantified. The system

which is adopted for dealing with variability is

one of grading. Processed timber is placed into

grades, which are simply categories, and

individual pieces of timber must comply with

certain performance requirements to qualify for

inclusion in a particular grade. All timber is

therefore inspected or tested at some stage

during its processing and allocated a grade

depending on its performance. The user of

timber specifies the grade which must be used

for a particular component as well as its

species, and can then be confident that its

properties will be within defined limits.

Most timber is graded in its country of origin

and, while the basic principles which are

adopted for grading are similar for all major

producing countries, there are differences

between countries in the details of the

methods. For softwoods, which account for

most structural timber in contemporary build-

196 ing practice, most countries operate what is