STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

Fig. 6.37 Toothed-plate and shear-plate connectors are

available in single- and double-sided formats and can be

used either for timber-to-timber or timber-to-steel

junctions.

achieved by using a large number of small

fastening elements (Fig. 6.36a) (although there

are limits to the number of fasteners which can

be placed in a single line due to the uneven-

ness in load distribution which tends to

develop between them) or by using individual

components which are specifically designed to

reduce concentrations of stress, such as timber

connectors (Fig. 6.31). The required overall

width of a timber element is frequently deter-

mined by the need to provide sufficient space

to accommodate the fastening components

which are required at the joint rather than by

the need to provide a cross-section which is

large enough to resist the internal force which

the element carries.

6.6.6.2 loints with mechanical fasteners

The principal types of mechanical fastener are

nails, screws, bolts, timber connectors and

punched metal plates. Nails and screws, which

have similar dimensions, are capable of carry-

ing similar amounts of load. Nails are normally

preferred due to their lower primary cost and

to the lower labour cost involved in driving

them into the timber. Screws are used only

where high resistance to withdrawal is

required. Individual bolts carry higher loads

than nails but the safe working load for a bolt

is usually considerably lower than the strength

of the bolt itself, due to the inability of the

timber to accommodate the high concentration

of stress which occurs at a bolted connection.

Thus, although fewer bolts than nails may be

required for a particular connection the total

The selection of the joint type is usually

based on a number of considerations, such as

economy (both in the cost of the fastening

components and in the manufacture of the

joint), the type of workshop facilities which will

be available, the appearance of the joint, and

any special requirements which are imposed

by the way in which the structure will be

assembled. A fundamental decision is whether

to use mechanical fasteners or adhesive. Glued

joints produce more satisfactory structural

connections but are difficult to make on site.

Components which are fastened with glued

joints must normally therefore be produced

under factory conditions and considerations of

transport dictate the maximum size of these.

Very large elements must be assembled on site

and this will normally mean that mechanical

fasteners will have to be used.

A critical factor in the design of timber

joints, especially mechanical joints, is the

weakness of the material in crushing and

shear; the basic strategy which is adopted to

overcome this is to reduce the concentration of

stress which occurs by bringing the largest

possible area of timber into play. In the case of

a glued joint this is achieved by providing a

large contact area on which the glue can act.

212 Where mechanical fasteners are used it can be