STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

Fig. 6.54 Beam grid for single-storey timber skeleton

frame. Closely spaced joists (built-up-beam sections) are

carried on primary beams of relatively short span.

Fig. 6.55 Beam grid for single-storey timber skeleton

frame. Strong primary structural elements (one-off trusses/

portal frameworks) carry a secondary structure of purlins.

and columns. In floor structures the floor joists

should span parallel to the long side of the

rectangle and the main beams parallel to the

short sides.

The most successful timber skeleton frames

are single-storey structures carrying low levels

of imposed load (Fig. 6.53). Fairly long spans

are possible with this type of arrangement,

especially if built-up-beams or trusses are used

for the principal elements, but the sizes of the

elements are likely to be large compared to

equivalents in steel.

Two strategies may be adopted for the

planning of single-storey frames (Figs 6.54 and

6.55). In Fig. 6.54 the main structural elements

span in the long direction in a rectangular

column grid and are spaced close together to

minimise the load carried by individual

elements, and to allow them to carry the

cladding directly without the use of a

secondary structure. The geometry of this type

of arrangement is similar to that of a steel

frame with lightweight joists (Figs 3.25 and

3.27). Columns are positioned at every third or

fourth beam and intermediate members are

carried on 'primary' beams. The latter are more

heavily loaded than the main beams but can

usually be of the same depth due to their

smaller span. The exact configuration which is

adopted in a particular case depends on the

type of beam being used.

The second type of arrangement is one in

which the primary beams are placed at a wider

spacing and a secondary structure, on which

the cladding is mounted, is provided to span

between these (Fig. 6.55). The column grid

coincides with the primary beams, which must

be much stronger elements than those in the

system described previously. Proprietary

beams (at least in the simply supported form)

are rarely suitable for this type of arrangement

and large trusses are normally used. Built-up-

beam-type cross-sections can be used as

primary elements if a 'strong' structural config-

uration, such as a portal frame (Figs 6.3 and

6.53) is adopted. This type of structure usually

has a plan grid which is similar to those which

are used in single-storey steel frames (Fig.

3.30) but in the case of timber the primary

226 elements are usually placed closer together

10-20m

3-5 m

Timber built-up-beam

(e.g. Plyweb beam,

Trussed rafter,

Laminated beam)

Bolted truss or

laminated beam

15-30m

3-5 m

Purlins of sawn or

laminated timber

Primary member

(bolted truss,

built-up or laminated

portal frame)