STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

4 Precasting of components leads to a much

simpler site operation than is possible with

in situ concrete. The building can therefore

be erected more quickly with fewer tem-

porary structures.

Disadvantages

1 One-off precast concrete systems tend to be

more expensive than in situ equivalents. This

favours their use for large-scale projects

where economy of scale reduces the differ-

ential. It is claimed by the precast concrete

industry, however, that the overall building

cost is reduced if a precast structure is used

instead of in situ concrete, due to the

simpler and shorter site operation.

2 Inflexibility. Precast concrete units must

normally be manufactured some time in

advance of their being transported to the

site and installed in a building to give time

for the concrete to gain adequate strength.

(Sometimes the highest load to which a

component will be subjected will occur

during transportation or installation.) Late

changes to the design of the building

cannot therefore be readily accommodated.

3 Standardisation. Another aspect of inflex-

ibility is that, because precast structures are

assembled on site from components which

are fairly large, it is convenient to maintain

the overall geometry of the structure in as

simple a form as possible.

There is considerable incentive to

standardise components so as to obtain the

maximum re-use of moulds and to facilitate

erection. Thus, while precasting offers the

possibility that individual elements can

have complex cross-sections and profiles,

the overall form of the building must

normally be relatively simple and repetitive.

This is a disadvantage which precast

concrete shares with steel.

4.4.3.2 Precast frame structures

Planning principles for precast concrete frames

are the same as for in situ concrete frames:

floors are normally of the one-way-spanning

type, in which case a rectangular beam-

column grid is used but two-way-spanning

floors, on a square column grid, are also possi-

ble. Normally the structure consists of individ-

ual beam, column and slab units which are

erected on site in a similar way to a steel frame

(Fig. 4.56); the joints between elements can be

of the hinge or rigid type, depending on the

detailing. If hinge-type joints are used (Fig.

4.57), additional bracing components are

required and these can take the form of infill

walls, either in situ or precast, or diagonal

bracing. Where the beam-column joints are

rigid the frames are self-bracing. Frequently in

precast frame construction the joints between

the individual units do not coincide with the

beam-column junctions (Fig. 4.58). The units

then have a fairly complex geometry which

makes transport and stacking on site more

difficult; the advantage is that it makes pos-

sible the achievement of a self-bracing frame

without the need for site-made joints which

are of the rigid type.

Fig. 4.56 Layout of a basic precast concrete framework.

(a) Typical plan arrangement in which a beam-column

frame supports a one-way-spanning floor slab.

(b) and (c) Slab units can be of rectangular or ribbed cross-

142 section depending on the span.

Slab span

Beam span (a)

(b) (c)