STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

18

overall form of the structure must obviously be

compatible with, if not identical to, that of the

building which it supports. The preliminary

stage of the structural design is therefore virtu-

ally inseparable from that of the building, taken

as a whole. It is at this stage that architectural

and structural design are most closely related

and that the architect and engineer, be they

different persons or different facets of the same

individual, must work most closely together.

The second stage of the design of the structure,

which is principally concerned with the sizing of

the elements and the finalising of details, such

as the configuration of the joints, is principally

the concern of the structural engineer.

The different aspects of the structural design

activity are most easily seen in relation to

purely engineering types of structure, such as

bridges. It will be instructive here, before

looking at the process as it takes place in the

case of a building, to consider the design of a

prominent example of this type. The Forth

Railway Bridge in Scotland (Fig. 1.20), despite

being now over 100 years old, provides a good

illustration of the various stages in the evolu-

tion of a structural design. The issues involved

are broadly similar to those which occur in any

engineering design project, including those of

the present day. The same issues will be

considered again in Chapter 2, where they are

discussed in relation to architectural design.

This preliminary review, in the context of

engineering, serves to identify the essential

aspects of the structural design process.

As is normal in bridge design, the most

significant sets of factors which influenced the

design of the Forth Bridge were those

connected with its function and with its

location. The ground level at each side of the

estuary of the River Forth, where the bridge is

situated, slopes steeply up from the shore and

the railway therefore approaches from a level of

approximately 50 m above water level at each

end. This, together with the requirement that

the busy shipping channels which the bridge

crosses should not be blocked, dictated that

the railtrack level should also be relatively high

(50 m above sea level). At one shore a broad

strip of low lying ground occurs close to the

Fig. 1.21 Forth Railway Bridge, Scotland, 1882-90, Henry

Fowler and Benjamin Baker, engineers. The main part of

the structure consists of three pairs of balanced

cantilevers. In this shot the central tower of one pair of

cantilevers has been completed and the first elements of

the cantilevers themselves have been added. The arrange-

ment was adopted so that the uncompleted structure

could be self-supporting throughout the entire period of

construction. [Photo: E. Carey; copyright: British Rail

Board Record Office]

edge of the water. At the other, the ground

rises steeply from the water's edge but a broad

strip of shallow water occurs close to the shore.

Between these two flanking strips of relatively

level ground the estuary consists of two very

deep channels separated by a rocky island. The

bridge was therefore broken down into three

parts and made to consist of two long approach

viaducts, each with a sequence of girders

spanning relatively short distances between

regularly spaced piers, and a massive central

structure spanning the two deep channels.