viaduct (similar to the approach viaducts) size of each cross-section, thickness of metal,
located within the primary structure and etc.). At this stage structural calculations were
supported by it at regular intervals (Fig. 1.23). employed to determine the magnitudes of the
The primary structure therefore provided internal forces which each element would carry
regularly spaced supports for this internal and to check that sufficient thicknesses of
viaduct rather in the manner of the equally metal were specified to maintain the stresses
spaced piers which carry the approach within acceptable limits. The detailed
viaducts. determination of the configuration of the joints
Figure 1.22e shows diagrammatically the between the elements, which could only be
final form of the bridge. It represents the done once the sizes of the elements were
culmination of the first stage of the design known, was then carried out.
process in which the form and general arrange- The description given above illustrates, in
ment of the entire structure were determined. much abbreviated form, the typical sequence
The sequence of decisions which led to this of decisions which is involved in the design of
proposal illustrates, in a much abbreviated any major civil engineering structure and
form,^8 the key stages of the preliminary design allows the two key stages in this - the initial
process and allows the nature of the process determination of form and the final realisa-
which concerns us here to be appreciated. In tion of the form - to be appreciated. A
particular, it shows that the basic form of this sequence of decision-making which is broadly
purely engineering structure was determined similar to this is involved in the design of all
by the designers from a consideration of the structures whether civil engineering or archi-
function of the bridge, from the constraints of tectural.
the site, from a knowledge of structural behav- In the case of an architectural structure, the
iour and from an awareness of the vocabulary form and general arrangement must also be
of structural possibilities which was available entirely compatible with that of the building
to them. The design was an imaginative which it will support. The preliminary stage in
response to these various influences. the design of an architectural structure is
The second stage in the design process, the therefore inseparable from the design process
realisation of the structure, led to the detailed of the building as a whole. Once the overall
specification of the various structural elements form of a building has been determined, the
from which the bridge was constructed. This choice of structural arrangement will normally
involved decisions on the precise shape of the be fairly narrow. The act of architectural design
individual elements (in longitudinal profile and is therefore also an act of structural design in
cross-section) and on the precise amount of which the most fundamental decisions relating
material which would be specified (the overall to the structure are taken.
Structure and architecture218 Several alternative arrangements were in fact consid-
ered by the designers before the chosen form was
adopted.