must begin the process of selection and
adaptation. An insight into the working of this
process is given in Section 2.5.
An important factor in the selection of a
structure type for a building is the nature of
the relationship which has been adopted
between structure and architecture. If this is in
the 'structure ignored' category, then the
overall form of the building, and therefore of
the structure, will have been determined
without consideration of structural require-
ments and the structure will most probably
have to be of the semi-form-active type. This is
because, for a given load condition, form-
active and non-form-active structures have
unique geometries. A structure whose geom-
etry has been determined without regard to
structural considerations is therefore likely to
have a semi-form-active shape. In this situ-
ation the choices available to the structural
designer may be very limited.
The structural material will almost certainly
have to be either reinforced concrete or steel
because a material with the ability to resist
significant bending will be required. The choice
between these two materials will be influenced
by the nature of the building. Concrete results
in a more massive structure but allows the easy
creation of curvilinear forms. Steel is stronger
and is therefore capable of producing a lighter
structure (both literally and in appearance) but
must normally be used in the form of a skele-
ton framework. The distinction between their
respective suitabilities can be seen by compar-
ing the Vitra Design Museum building by Frank
Gehry (Fig. 4.20) with the rooftop office in
Vienna by Coop Himmelblau (Fig. 2.1).
In the Vitra Design Museum building the
presence of strongly curvilinear forms, of
cantilevered volumes of complex shape and of
the limited number of openings in the build-
ing's envelope favoured the use of reinforced
concrete. The mouldability of the material and
the level of structural continuity which it
allows are particularly useful properties. In the
Rooftop Office in Vienna the extensive areas of
faceted glazing favoured the adoption of a
skeletal structure of slender steel elements.
The minimal amount of curvature in the roof
structure was accommodated by steel and the
general arrangement of the structure, despite
its apparent novelty, conforms to the fairly
standard primary/secondary beam format.
If the relationship between structure and
architecture is other than 'structure ignored' it
will be necessary to select a structure type
which is compatible with the aesthetic and
programmatic aspects of the building and
which is also sensible from a structural point
of view. The choice of structure type should,
however, be larger because there should be a
readiness to adjust the aesthetic programme
to accommodate structural requirements and
thus produce a building with a structure which
is satisfactory technically. The factors on which
this depends are now reviewed.2.3.2 The effect of scale
The span of an architectural structure, which is
determined by the required sizes of the spaces
which are enclosed by it, has a very significant
effect on both the generic type of structure
which should be used and on the selection of
the types of structural element of which it
should be composed. The underlying principle
which governs the relationship between span
and structure type is that the ratio of self-
weight to load carried should be satisfactory.
For a given type of structure the strength-to-
weight ratio tends to become less favourable
as the span increases.^15 More efficient types of
structure must therefore be specified as spans
are increased to maintain the ratio at an
acceptable level.
Table 2.1 shows the ranges of span for which
basic types of structure are most suitable and
therefore normally specified. The figures must
be regarded as approximate but serve to give
an indication of the applications for which
each is most appropriate. Separate figures are
provided for floor and roof structures to allow
for the variations which are caused by the
significantly different levels of gravitational
load to which they are subjected.15 See Macdonald, Structure and Architecture, Chapter 6.^35Structural design for achitecture