STRUCTURAL DESIGN FOR ARCHITECTURE

Table 4.4 Span range and principal dimensions
of basic precast concrete frames

Slab span Beam span Slab depth Beam depth
(m) (m) (mm) (mm)

4 6.0 140 450

5 7.5 140 600

6 9.0 150 700

7 10.5 190 800

8 12.0 190 1000

9 13.5 190 1150

10 15.0 250 1300

11 16.5 250 1400

12 18.0 250 1500

It is normal for precast concrete frameworks to
have a regular, rectilinear form so as to
maximise the standardisation of components
but it is possible to adopt irregular grids. Typical
element sizes for the normal span ranges of
rectilinear frames are given in Table 4.4.
Columns are normally rectangular in cross-
section but other shapes can be provided if
this is necessary to accommodate irregular
beam layouts or for architectural effect. The
most favoured beam cross-section is the
inverted T, as this facilitates the carrying of
simple slab types. More complex shapes are
used to accommodate changes in floor level.
Precast floor slabs are normally of the one-
way-spanning type and are either solid,
hollow-core or with a T-profile (Fig. 4.56c). All
of these are ideally suited to a rectangular
beam layout. Rhomboid plan-forms are fairly
straightforward to produce but where the plan
shape is highly irregular the section of floor
involved is cast in situ.
Where vertical-plane bracing is provided by
structural walls these can be precast units and
can be used to provide support for floors as
well as to resist lateral load. They are normally
arranged as bracing cores around lift or stair
wells. A reasonable number of bracing walls
must be provided in two orthogonal directions
and these should be arranged as symmetrically
on plan as practicable.

4.4.3.3 Hybrid in situ and precast forms
Hybrid structures, in which both in situ and

Fig. 4.57 Joints in precast concrete frameworks. All of the

joints shown are capable of transmitting shear and axial

force only. They are therefore hinge-type joints.

precast concrete are used, allow the advan-

tages of both forms of construction to be

realised. The precast components will bring

the benefits of factory production (high

strength and efficiency, durability, good

appearance, complex element cross-sections,

dimensional accuracy, rapid erection) and the

in situ parts allow complex or irregular overall

forms and structural continuity between

elements to be easily achieved.

The in situ and precast components in hybrid

structures can be combined in two principal 143

Reinforced concrete structures