STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

Fig. 4.40 Coffered slab.

The coffered slab is a two-

way-spanning flat-slab

structure (i.e. a slab which

is supported directly on

columns). The coffers

improve the efficiency by

removing concrete from the

tensile areas where it

contributes little to the

structural performance

[Photo: Pat Hunt].

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used but the maximum economic spans are

higher; solid slabs are used for spans of up to

8 m and this can be extended to 16 m if the

weight is reduced by removing some of the

concrete on the tension side of the cross-

section, and to 20 m if pre-stressing is used. A

system of intersecting ribs, which form a

square grid, is then created on the soffit of the

slab; the resulting form is said to be a coffered

or 'waffle' slab (Fig. 4.40).

Stairs

Stairs are designed as reinforced concrete

slabs whose depths are equal to the waist

thickness of the stair, and which are normally

an integral part of the structure (Fig. 4.41). The

requirements in respect of the ratio of span to

depth which is specified are the same as for

one-way-spanning slabs.

Precast components

The precasting technique, in which concrete

components are manufactured in a factory and

transported subsequently for erection on site,

allows higher quality control to be achieved

than is possible with in situ concrete. Among

the advantages which this offers are higher

concrete strength for given mix proportions,

better quality of finish, greater dimensional

accuracy and the economies of scale which are

associated with the factory process. The factory

method of manufacture also allows more

complicated shapes of cross-section to be

achieved than are possible with in situ

concrete. This, in turn, makes possible higher

levels of structural efficiency and can also fa-

cilitate the use of structural elements, such as

beams and columns, as ducts for services.

Precast concrete has therefore been widely