Cable-stayed bridges - Computational Analysis and Design of Bridge Structures - page 336

Figure 11.13 Mechanical behavior of a cable-stayed bridge.
11.2.1 Weakness of cable supports
However, the distribution of vertical loads from the girder to the cables
is far less than a continuous girder with intermediate rigid supports; in
another words, the spring supports' stiffness of cables are far less than
that of real supports. To illustrate the weakness of cable supports, a
five-span continuous girder and a girder stayed by four cables, as shown
in Figure 11.14, are taken as an example. Comparison between figures
demonstrates that a girder stayed by long vertical cables has less sup-
port stiffness than that supported by bearings. This is due to the fact
that cables are more flexible than regular bearings, and further, a girder
stayed by slanted cables has less support stiffness than that stayed by ver-
tical cables. In a fan cable system, the smaller the cable angle to the girder,
the less vertical support stiffness can cable provide. The stiffness that
anchor cables in side spans and end cables in the middle span can provide
is much less than that cables close to pylons can provide, which is one
factor that limits the main span capacity when the height of the pylons is
limited. Compared with the continuous girder, distributions of moment
and axial forces, as well as bridge displacements (under a uniform load
on the girder such as the girder's structural weight), reflect the weakness
of the cable's support capacity. Figure 11.15 shows this phenomenon. This
holds true in response to the live loads as well.
From these distributions shown in Figure 11.15, it is clear that the axial
forces in cables are low, bending moments along the girder are high, and the
vertical displacements of the girder are large. If the structural weight of the
girder and superimposed deck loads are not redistributed, as behaved in an
RC bridge that is built by casting concrete directly in its setting location all at
once, the span capacity of a cable-stayed bridge would be similar to a continu-
ous girder bridge. By using high-strength steel wires or strands as cables and
prestressing them at a much higher level than what it would be distributed due
to structural deformation under dead loads, as shown in Figure 11.15, the dead
loads will be transferred to the pylon so bending moments on the girder will be
reduced. Therefore, the weakness effect under dead loads is improved and the
span capacity is increased. The structural advantages of a cable-stayed bridge,