What Are The Types Of Suspension
Bridges?
Several arrangements of suspension
bridges are illustrated in Fig. 1. The main cable is continuous,
over saddles at the pylons, or towers, from anchorage to anchorage.
FIGURE 15.9 Suspension-bridge
arrangements. (a) One suspended span, with pin-ended stiffening
truss. (b) Three suspended spans, with pin-ended stiffening trusses.
(c) Three suspended spans, with continuous stiffening truss. (d )
Multispan bridge, with pin-ended stiffening trusses. (e)
Self-anchored suspension bridge.
When the main cable in the side spans
does not support the bridge deck (side spans independently supported
by piers), that portion of the cable from the saddle to the anchorage
is virtually straight and is referred to as a straight backstay.
This is also true in the case shown in
Fig. 1a where there are no side spans. Figure 1d represents a
multispan bridge. This type is not considered efficient, because its
flexibility distributes an undesirable portion of the load onto the
stiffening trusses and may make horizontal ties necessary at the tops
of the pylons.
Ties were used on several French
multispan suspension bridges of the nineteenth century. However, it
is doubtful whether tied towers would be esthetically acceptable to
the general public. Another approach to multispan suspension bridges
is that used for the San Francisco–Oakland Bay Bridge (Fig. 2).
It is essentially composed of two three-span suspension bridges
placed end to end.
This system has the disadvantage of
requiring three piers in the central portion of the structure where
water depths are likely to be a maximum. Suspension bridges may also
be classified by type of cable anchorage, external or internal. Most
suspension bridges are externally anchored (earth-anchored) to a
massive external anchorage (Fig. 1a to d).
In some bridges, however, the ends of
the main cables of a suspension bridge are attached to the stiffening
trusses, as a result of which the structure becomes self-anchored
(Fig. 1e). It does not require external anchorages.
The stiffening trusses of a
self-anchored bridge must be designed to take the compression induced
by the cables. The cables are attached to the stiffening trusses over
a support that resists the vertical component of cable tension. The
vertical upward component may relieve or even exceed the dead-load
reaction at the end support. If a net uplift occurs, a pendulum link
tie-down should be provided at the end support.
Self-anchored suspension bridges are
suitable for short to moderate spans (400 to 1,000 ft) where
foundation conditions do not permit external anchorages. Such
conditions include poor foundation bearing strata and loss of weight
due to anchorage submergence. Typical examples of self-anchored
suspension bridges are the Paseo Bridge at Kansas City, with a main
span of 616 ft, and the former Cologne-Mu¨lheim Bridge (1929) with a
1,033-ft span.
Another type of suspension bridge is
referred to as a bridle-chord bridge. Called by Germans
Zu¨gelgurtbru¨cke, these structures are typified by the bridge over
the Rhine River at Ruhrort-Homberg (Fig. 15.11), erected in 1953, and
the one at Krefeld-Urdingen, erected in 1950.
It is a special class of bridge,
intermediate between the suspension and cable-stayed types and having
some of the characteristics of both. The main cables are curved but
not continuous between towers. Each cable extends from the tower to a
span, as in a cable stayed bridge. The span, however, also is
suspended from the cables at relatively short intervals over the
length of the cables, as in suspension bridges.
A distinction to be made between some
early suspension bridges and modern suspension bridges involves the
position of the main cables in profile at midspan with respect to the
stiffening trusses. In early suspension bridges, the bottom of the
main cables at maximum sag penetrated the top chord of the stiffening
trusses and continued down to the bottom chord.
Because of the design theory available
at the time, the depth of the stiffening trusses was relatively
large, as much as 1⁄40 of the span. Inasmuch as the height of the
pylons is determined by the sag of the cables and clearance required
under the stiffening trusses, moving the midspan location of the
cables from the bottom chord to the top chord increases the pylon
height by the depth of the stiffening trusses.
In modern suspension bridges,
stiffening trusses are much shallower than those used in earlier
bridges and the increase in pylon height due to midspan location of
the cables is not substantial (as compared with the effect in the
Williamsburg Bridge in New York City where the depth of the
stiffening trusses is 25% of the main-cable sag).
Although most suspension bridges employ
vertical suspender cables to support the stiffening trusses or the
deck structural framing directly, a few suspension bridges, for
example, the Severn Bridge in England and the Bosporus Bridge in
Turkey, have inclined or diagonal suspenders.
In the vertical-suspender system, the
main cables are incapable of resisting shears resulting from external
loading. Instead, the shears are resisted by the stiffening girders
or by displacement of the main cables. In bridges with inclined
suspenders, however, a truss action is developed, enabling the
suspenders to resist shear.
(Since the cables can support loads
only in tension, design of such bridges should ensure that there
always is a residual tension in the suspenders; that is, the
magnitude of the compression generated by live-load shears should be
less than the dead-load tension.) A further advantage of the inclined
suspenders is the damping properties of the system with respect to
aerodynamic oscillations.
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