Let's look once more at bridge types.
Beams, flat trusses and
box girders impose only vertical forces on the ground. The total
force on the ground is the total weight of the bridge. With any
span, every metre of extra span is a metre of extra weight and extra
bending moment. The penalty
is that these designs are limited to spans far shorter than the maximum
possible bridge span.
Why is this? The reason is that
the bending moment produces strong forces in opposite directions, which
require material to withstand them.
Cantilevers, deep trusses and cable-stayed bridges can produce
large spans, such as those of the Forth bridge and the Quebec
bridge. This is partly achieved by using a suspended span between
two cantilevers. The bending moment referred to above is countered
by increasing the depth of the structure, reducing the weight of
By allowing the ground, which costs
nothing, to take horizontal forces, an arch can be made
quite light and slender, while reaching spans that are comparable with
big cantilevers. An arch need be no thicker than is needed to
avoid buckling, because it need not handle the bending moment that the
other bridges experience, apart from that needed for live loads.
How can the cable-stayed design do better than the arch and the
cantilever? Because it is supported at frequent intervals by
cables, against the pull of the earth, the cable-stayed span can be a
truss that has only to resist buckling against the dead weight and local
live loads. An arch and a cantilever have to provide inherent
stiffness without any cables.
But the longest spans of all are those of the largest suspension
bridges, again allowing the ground to absorb large horizontal forces.
Why does the suspension bridge do even better than the arch?
The reason is that the main stress bearing member is in tension and not in
compression. A tie is stable against perturbation,
whereas a strut is unstable. In the suspension bridge the ground
is used as the compression member. It is large, and it costs nothing,
though there is a necessity for ensuring that the interface between
bridge and ground spreads the load enough for the ground to cope.