STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

Fig. 4.58 Where rigid joints are required in precast

arrangements, so that the structure is self-bracing, this is

frequently accomplished by positioning the junctions

between the elements at different locations from the

beam-to-column connections.

Fig. 4.59 Hybrid in situ/precast structure. In this structure

precast concrete perimeter units are used in conjunction

with an in situ concrete framework. The superior standard

of finish which precasting allows is exploited to allow the

units to be exposed on the exterior of the building. [Photo:

British Cement Association]

ways. In the first of these the structure consists

of a mixture of element types each of which is

either precast or in situ (Fig. 4.59). The ratio of

precast to in situ components can vary widely.

At one extreme an in situ framework of beams

and columns, with rectangular shapes of cross-

section, might be combined with precast stair

and ribbed-slab elements whose more compli-

cated profiles are more easily achieved under

factory conditions. At the other extreme, the

use of in situ concrete might be confined to the

making of continuous joints in a structure in

which all of the principal elements were

precast.

A second type of hybrid structure is one in

which the individual elements are formed by a

combination of precast and in situ concrete

acting compositely. With this type of arrange-

ment the precast parts of the structure are

invariably used as permanent formwork on

which the in situ parts are cast (Fig. 4.60). Thus

a ribbed slab may be formed from a slender

precast soffit of complex shape on to which an

in situ top is cast. Beams of complex shape can

be formed in the same way. Composite hybrid

structures are particularly well suited to the

realisation of the advantages of both precast

and in situ concrete.

The general arrangements of hybrid struc-

tures are similar to those for precast forms.

They are either beam-column frameworks or

flat-slab arrangements with column grids

which are either square or rectangular depend-

ing, respectively, on whether the floor slabs are

144 two-way or one-way-spanning systems.