STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

resistant to a variety of environmental condi-

tions is available. Both timber-to-timber

connections and joints between timber and

other materials can be made with glue. Glued

joints are designed so that the glue is loaded in

shear only and tests have indicated that when

this type of joint is loaded to breaking point,

the failure occurs in the timber adjacent to the

glued surface and not in the glue itself. The

allowable stresses on which the design of glued

joints are based are therefore the shear

stresses, parallel to the grain of the timber

concerned; they are subject to the normal

modification factors to allow for the effects of

duration of load and the direction of load in

relation to the grain. Although the rigorous

design of a glued joint is a complicated process,

the basic requirement is to provide an area of

contact between the components which is large

enough to maintain the shear stress at an

acceptable level. The feasibility of a proposed

joint, and therefore of the structural element of

which it forms part, is frequently determined by

the required size of the contact area. This must

obviously not be excessive in relation to the

sizes of the sub-elements being joined.

An additional factor which must be consid-

ered in relation to the feasibility of specifying a

glued joint is the condition of the environment

in which the joint will be made. This should

ideally be that of the workshop or factory. Most

glues have to be cured in conditions of close

environmental control and surfaces which are

to be glued must be carefully prepared to

achieve a glue line which is as thin as possible.

Also, the surface to which the glue is applied

should be freshly machined (not more than 48

hours old); this is the case because the open

ends of cut cells, whose presence is essential

to achieve effective bond with glue, tend to

close with time. The standards of both environ-

mental control and the type of supervision

which are necessary for the manufacture of

reliable glued joints are frequently difficult to

achieve on site, and it is normal practice to

specify mechanical fasteners rather than glue

for site-made joints.

Fig. 6.39 Typical timber floor deck

with closely spaced joists supporting

a floor consisting of boarding.

214

Herring-bone strutting

Joist ends on steel

bearing plate

Block partition

Chipboard or

plywood flooring

Brick outer leaf

to cavity wall

Wall-plate