STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

Table 6.12 Simple rules for preliminary sizing of

stressed-skin panels

Parameter

Minimum overall

depth

Minimum stringer

width

Maximum stringer

spacing

Tension skin

Compression skin

0.75 kN/m^2

span

40

50 mm

600 mm

8 mm

9 mm or 12

\mposed load

1.5kN/m^2

span

35

50 mm

400 mm

9 mm

mm 12 mm, 15 mm

or 18 mm

stressed-skin panels are normally regarded as

components which must be prefabricated

rather than made on site. They are used where

longer spans are required than are possible

with traditional forms of timber deck or where

a smaller constructional depth must be

provided. Data for stressed-skin panels are

given in Table 6.12.

The second development of the traditional

deck is the use of built-up-beams in place of

sawn-timber joists for the principal elements

(Fig. 6.41). Plyweb and laminated timber

beams, as well as various proprietary lattice-

beam types have been used in this role and

these allow much larger spans (up to 20 m) to

be achieved than are possible with sawn-

timber elements. Economy in the use of these

more sophisticated components requires that

they be spaced further apart than traditional

joists and beam spacings of around 1.5 m to

3.5 m are usually adopted. This requires that a

fairly thick form of boarding be used to form

the deck skin.

6.7.2 Pitched roof structures

In the traditional pitched roof structure, sawn-

timber rafters are positioned at close centres

(450 mm to 600 mm) supporting a roof skin of

sarking boards or tile battens (Fig. 6.42). The

vertical support structure is usually of the

loadbearing-wall type and, because the rafters

produce an outwards thrust at the wallhead,

the system is suitable for small spans only (up

to 3.5 m). The span range can be extended by

use of tie elements, and in traditional

construction various arrangements of semi-

trussing were devised to allow longer spans to

be achieved (Fig. 6.43). Traditional roof forms

of this type require fairly large sizes of timber,

however, because the principal elements are

stressed in bending as well as axially, and so

must have large cross-sections to resist this.

Also, many of their geometries are fundamen-

tally unstable and depend on the continuity of

elements through joints for stability. Elements

Fig. 6.41 Plyweb built-up-beams

at close spacing are used here to

form the principal structural

elements of a roof deck. The

arrangement is similar to that of a

traditional joisted deck with sawn-

timber elements. The use of built-

up-beams allows longer spans to

be achieved.

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