Structural Design for Architecture
compared to structural timber in buildings and
of the aluminium alloys used in aircraft
construction. The high relative costs of the
motor car, the sailing yacht and the aeroplane
are not due to the basic costs of the materials
but to the costs of fashioning these materials
into complex shapes and configurations. By
comparison with a vehicle, of whatever type,
most buildings are very simple objects, being
composed of basic components assembled
into relatively simple geometries.
The virtually unlimited range of forms which
is available to the designer of the motor car or
the ship or the aeroplane is therefore at the
disposal of the architect. The factors which
have mitigated against the use of complex
forms have for the most part, as stated above,
not been technical and therefore not real, in
the sense of ultimate physical, constraints.
There is, however, one technical factor
which, in the context of buildings, does inhibit
the freedom to invent form. This is scale, or
more particularly the size of the span involved.
If this is small, then the architect can indeed
assume virtually unlimited freedom in the
matter of form. The larger the span the more
restrictive are the structural constraints on
form.
The scale of a building is therefore of critical
importance if its form is to be determined free
from any consideration of structural perform-
ance. This is because the building will have to
contain a structure with sufficient strength to
resist the loads applied to it. If the form has
been determined 'freely' in the visual sense
(i.e. without consideration of technical implica-
tions) it is likely that the structure will be
subjected to bending-type internal forces
which will result in an inefficient use of struc-
tural material.^5 It is probable also that the
magnitudes of these internal forces, for a given
application of load, will be high, which will
further compromise efficiency. If the internalforces are so high that, even with strong
materials such as steel or reinforced concrete,
the sizes of the cross-sections required to
contain stresses within acceptable limits are
excessive, the structure will not be feasible. As
is stated above the critical factor here is scale,
and more particularly span.
This situation may be appreciated by con-
sidering the effect of size on any physical
object. If the object is small - say less than one
metre across (a model of a building for
example) - it would be possible to make it from
a weak material such as cardboard or papier
mâché. If the span is 10 m (small building
scale) a stronger material (reinforced concrete,
steel, timber, plastic) would be required. The
limits of what was possible would depend on
the material and on the nature of the form.
With reinforced concrete, for example, the limit
could be reached at a span as low as 20 m but
might not be reached until the span was 200 m,
depending on the form. If steel were used the
limits would be higher.
Scale is a significant consideration with any
type of structure but it is particularly important
if the relationship between structure and archi-
tecture is that of 'structure ignored'. The larger
the span the greater is the possibility that the
form will not be feasible. With the materials
currently available, virtually any shape is possi-
ble up to a span of around 30 m. Most shapes
are possible up to a span of around 60 m. For
spans greater than 60 m there is an increasing
possibility that a shape which has been chosen
without regard to structural considerations will
not be feasible. Built forms which have been
determined without regard to structural
considerations are likely to be significantly
more costly to produce, however, than those
which have.
To summarise, the architect who wishes to
disregard structural considerations when deter-
mining the form of a building must be mindful
of the considerations of cost and scale. If any
significant departure is made from the basic
forms of structure outlined in Chapters 3 to 6
here, then a cost penalty is likely to be
involved. If the span is large there is also a
possibility that the form chosen will not be5 It will, in other words, most likely be either a semi-
form-active or a non-form-active structure, neither of
which is structurally efficient. See Appendix I for the
28 explanation of this.