Suspension bridges with cables made of
high-strength, zinc-coated, steel wires are suitable for the longest
of spans. Such bridges usually become economical for spans in excess
of 1000 ft, depending on specific site constraints.
Nevertheless, many suspension bridges
with spans as short as 300 or 400 ft have been built, to take
advantage of their esthetic features. The basic economic
characteristic of suspension bridges, resulting from use of high
strength materials in tension, is lightness, due to relatively low
dead load.
But this, in turn, carries with it the
structural penalty of flexibility, which can lead to large
deflections under live load and susceptibility to vibrations. As a
result, suspension bridges are more suitable for highway bridges than
for the more heavily loaded railroad bridges.
Nevertheless, for either highway or
railroad bridges, care must be taken in design to provide resistance
to wind- or seismic-induced oscillations, such as those that caused
collapse of the first Tacoma Narrows Bridge in 1940.
A pure suspension bridge is one without
supplementary stay cables and in which the main cables are anchored
externally to anchorages on the ground. The main components of a
suspension bridge are illustrated in Fig. 15.8.
Most suspension bridges are stiffened;
that is, as shown in Fig. 15.8, they utilize horizontal stiffening
trusses or girders. Their function is to equalize deflections due to
concentrated live loads and distribute these loads to one or more
main cables.
The stiffer these trusses or girders
are, relative to the stiffness of the cables, the better this
function is achieved. (Cables derive stiffness not only from their
crosssectional dimensions but also from their shape between supports,
which depends on both cable tension and loading.)
For heavy, very long suspension spans,
live-load deflections may be small enough that stiffening trusses
would not be needed. When such members are omitted, the structure is
an unstiffened suspension bridge.
Thus, if the ratio of live load to dead
load were, say, 1:4, the\ midspan deflection would be of the order of
1⁄100 of the sag, or 1/1,000 of the span, and the use of stiffening
trusses would ordinarily be unnecessary. (For the George Washington
Bridge as initially constructed, the ratio of live load to dead load
was approximately 1:6. Therefore, it did not need a stiffening
truss.)
FIGURE 15.8 Main components of a
suspension bridge.
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