Structure and architectureThe structure which spans the central
channels consists of three sets of balanced
cantilevers - a configuration which was
adopted because it could be built without the
need for temporary supporting structures. The
arrangement was essential due to the near
impossibility of providing supporting struc-
tures in the deep channels and due to the
need to maintain the shipping lanes free of
obstruction during the construction process.^6
The method of construction which was
adopted was to build three towers first and
then extend pairs of cantilevers simultaneously
on each side of these (Fig. 1.21). The structure
was therefore self-supporting during the entire
process of construction.
The basic form of the structure is shown
diagrammatically in Fig. 1.22 together with the
bending moment diagram^7 which results from
the peak load condition (a distributed load
Fig. 1.22 Forth Railway Bridge, Scotland. 1882-90,
Henry Fowler and Benjamin Baker, engineers.
(a) A diagrammatic representation (not to scale) of the
main elements of the bridge. The two river channels are
crossed by three sets of balanced cantilevers. Viaducts
consisting of short-span girders supported by closely
spaced piers provide the necessary link with the high
ground on the approaches to the bridge.
(b) The bending moment diagram which results from the
action of a uniformly distributed load across the entire
structure gives an indication of the variation which occurs
in the magnitudes of the internal forces across the span.
(c) The insertion of two extra hinges creates a cantilever-
and-suspended-span arrangement which alters the
bending moment diagram (d) and reduces the magnitude
of the maximum bending moment.
(e) The external profile which was finally adopted is closely
related to the bending moment diagram. The triangulation
of the internal geometry was necessary to achieve high
structural efficiency.6 The device of constructing the main elements of the
bridge on the shore and floating these into the final
position, which had been used earlier in the
nineteenth century at Saltash (I. K. Brunei) and the
Menai Straights (Robert Stephenson and William
Fairbairn) was impractical due to the very long spans
involved.
7 The bending moment is the internal force produced by
the load on the structure in the type of arrangement
shown in Fig. 1.22. The bending moment diagram is a
graph which shows how the intensity of this internal
force varies across the span. For an explanation of
bending moment see Macdonald, Structure and
Architecture, Chapter 2. 19(b)(c)(d)(e)(a)