Structural Design for Architecture
concrete structure therefore tends to be both
more time consuming and more costly than
that of an equivalent steel structure and these
factors can mitigate against its use in a par-
ticular situation. Another disadvantage is the
requirement for sufficient space for storage of
formwork and for the assembly of reinforce-
ment cages. This can be problematic if the
building site is very tight and congested.Strength
Although, as stated above, reinforced concrete
is one of the strongest of the four primary
structural materials it is nevertheless weaker
than steel, with the result that elements in
reinforced concrete structures tend to be
bulkier than steel equivalents. The relative
weakness also places restrictions on the spans
for which reinforced concrete is suitable. A
practical maximum span for floor structures is
around 20 m and spans greater than 15 m are
in fact rare.4.2.3 The selection of reinforced concrete
A structural material is selected on the basis of
the aesthetic opportunities which it allows and
the technical performance which it provides.
Many of the former are, of course, conse-
quences of the latter.
So far as the overall form of a building is
concerned, most of the possibilities which
reinforced concrete allows result from both its
relatively high strength and the high level of
structural continuity which it makes possible.
The high strength makes it suitable for skele-
ton-frame-type structures in which the internal
forces are relatively high. It is most suitable,
however, for structures in which fairly large
imposed loads are carried over relatively short
spans, that is spans in the range 6 m to 15 m,
and is therefore employed mainly for multi-
storey buildings where floor loads have to be
carried.
The high level of structural continuity which
the use of reinforced concrete allows gives the
designer considerably more freedom to ma-
nipulate the overall form of a framework than
is possible with steel (the principal alterna-
tive). The two-way-spanning capability of thefloor structure is especially significant and
allows both the adoption of an irregular
pattern of vertical support and the omission of
sections of floor to create volumes which run
through more than one storey. It also makes
possible the cantilevering of floors beyond
perimeter columns and the simple creation of
ramps or stepped changes in the levels of
floors. The structural continuity of reinforced
concrete also facilitates the creation of curvi-
linear plan-forms, either in the complete build-
ing or in the form of internal breaks in the floor
structure, and the adoption of complex, non-
rectilinear column grids. Reinforced concrete is
also ideally suited to the creation of form-
active or semi-form-active types of structure
such as shelis, vaults, domes and arches.
Reinforced concrete therefore offers the archi-
tect very great freedom in the matter of form.
The durability and fire resistance of concrete
are two very significant properties if the
expression of structure is an aspect of the
architectural programme. The range of surface
textures with which concrete can be produced
is an added benefit where this is done. Even
where the structure is not exposed, these
qualities will normally simplify both the initial
construction of a building and its subsequent
maintenance by removing the necessity for
fireproofing and corrosion protection
schemes.4.3 A brief introduction to concrete
technology
4.3.1 Introduction
Reinforced concrete is a composite material
whose constituents are concrete, which forms
the main bulk of the material, and steel, in the
form of reinforcing bars. Concrete itself is also
a composite material being composed of
cement and aggregate (fragments of stone),
The properties of concrete depend on those of
its constituents and on the proportions in
which these are mixed. Concrete can actually
be manufactured on the building site although
in modern practice it is normal for even in situ
118 concrete to be mixed in a separate factory and