STRUCTURAL DESIGN FOR ARCHITECTURE

vertical-plane diaphragms. As with skeleton

frames in other materials, the braced panels

should be positioned as symmetrically as

possible on plan and the disposition of the

vertical-plane bracing is therefore a factor

which affects the internal planning of the

building.

6.7.5 Shells and other surface forms

Surface structures are those in which the

elements which form the skin of the structure

have an important, sometimes dominant,

structural role. Stressed-skin panels (Fig 6.40)

are in this category; folded forms and curved

shell structures are other examples and these

are described briefly here.

Folded forms are arrangements of flat struc-

tural panels; there are two basic types, the

'prismatic' type, which consists of flat-sided

corrugations whose depth is constant across

the structure (Fig. 6.58) and more complex

types in which the depth of the corrugations is

varied. The latter are normally based on a

system of triangular panels. In both cases the

panels which form the structures usually

consist of a double skin of plywood sheets

separated by solid timber stringers. Nailed and

glued joints are used to assemble the panels,

which are then bolted together to form the

complete structure. The structures are highly

efficient, in the sense that they achieve a high

strength and stiffness for the amount of mater-

ial which they contain, and they are therefore

suitable for long-span structures (up to 40 m)

or for situations where self-weight must be

minimised. They are sophisticated forms,

however, which are difficult to design and

construct and they are not in common use.

Timber shells, which are curved stressed-

skin structures of the form-active type, are also

highly efficient structures which achieve high

strength and stiffness for a given weight of

material. Like folded forms they are difficult to

construct and are not in common use. Unlike

folded forms, timber shells are usually solid

and constructed in layers. Strips of thin board-

ing are laid across a formwork or mould of the

required shape and several layers, which are

nailed and glued together, are used. These are

normally used and, to avoid eccentricity in the

transfer of load between elements, a 'spaced'

arrangement can be adopted for either the

beams or the columns (Fig. 6.57). In frames

with solid timber beams the strength of the

beams is usually a critical factor and so spaced

beams are employed, with single-element

columns. Where the horizontal elements are

trusses or built-up-beams a spaced column

can be used. Where tie-beams are required in

the direction at right angles to that of the

principal members, these will frequently have

to be placed at a different level from the princi-

pal elements, so as to allow sufficient space

for the two sets of lap joints. This increases the

total depth of the structural zone and can be a

critical factor in the design.

Beam-to-column joints in timber frames are

rarely rigid and vertical-plane bracing is there-

fore required in skeleton frames. This can be

provided either in the form of diagonal bracing

(usually of the tensioned-wire type) or by

228 suitably positioned wall panels which act as

Structural Design for Architecture

Fig. 6.57 Typical joint arrangements for timber skeleton

frames. One of the features of this type of construction is

the space required for joints.