STRUCTURAL DESIGN FOR ARCHITECTURE

Fig. 4.60 In this hybrid struc-

ture precast beam and slab units

act compositely with an in situ

concrete topping slab.

4.4.4 Curved forms and structures of
complex geometry
The mouldability and strength of concrete
allow it to be cast into a wide variety of shapes
and it has been extensively used to create
envelopes which are based on curved forms
(Fig. 4.61). The fact that these are more
efficient structural forms than the post-and-
beam structures which have already been
described, either because they are close
approximations to the form-active^14 shape for
the loads which are applied to them or
because they allow the proportion of under-
stressed material which is present to be
reduced in some other way, means that large
spans can be achieved with very small volumes
of structural material. Spans of up to 70 m are
possible with shell thicknesses in the range
40 mm to 250 mm (Tables 4.5 and 4.6) and this
represents a considerable saving compared
with the volume of material which would be
required to achieve the same span by using
post-and-beam types of structure. A high
degree of expertise is necessary, however, both
in the design and in the construction of struc-
tures which have a complicated geometry and
this tends to make them more expensive than
forms with simpler geometries, despite the

14 See Appendix I for an explanation of the form-active

concept.

large saving in material which is involved.

Considerable reductions in the cost of the

design and construction of shells is possible if

some degree of regularity is introduced. For

this reason most curvilinear or folded forms of

structure are given a very simple basic geom-

etry. The most favoured shell forms are those

Table 4.5 Typical thicknesses of hyperbolic

paraboloid shells in reinforced concrete

Span Shell thickness (mm)

(m) At crown At edges

10 40 50

20 40 75

30 40 100

40 75 130

Table 4.6 Typical thicknesses of elliptical

paraboloid shells In reinforced concrete

Span Shell thickness (mm)

(m) At crown At edges

10 40 50

20 50 80

30 60 120

40 70 170

50 80 200

60 100 220

70 130 250

Reinforced concrete structures

Pre-stressed concrete

edge beam

Precast trough units

Shear links

Pre-stressed soffit slab

to main beam

In-situ concrete structural topping

Bars in edge of

trough unit acting as

reinforcement in

secondary beams

145