Cable-stayed bridges - Computational Analysis and Design of Bridge Structures - page 343

Cables to anchor pylons in
multiple spans

Counterweight

Counterweight

Secondary pier Secondary pier

Figure 11.18 Anchor of pylons.

configurations in improving the live load stiffness. In long-span cable-
stayed bridges, it is common that the anchor cables contain more strands
than other nonanchor cables or they are simply composed of two or three
cable stays. Increasing the working area of a cable will increase its axial
stiffness so that the horizontal stiffness in the upper part of the pylon will
be improved.
In cases that the secondary piers in side spans are used to improve the
main span vertical stiffness, sand boxes may be used as counterweight mea-
sures on the top of secondary pier areas. As the main span length increases,
the extreme live load reactions of secondary piers may exceed their reac-
tions due to structural weight and superimposed dead loads. Uplift may
happen without counterweight.

11.2.5 Backward and forward analyses
The ideal state is defined in the complete stage when a bridge is ready for
traffic. Although rejacking some particular cables after closure is possible,
retuning all cable stresses so as to reach a desired state is impractical. To
simplify the construction procedure and reduce each erection cycle, it is
best to jack a cable to the correct level at that stage when it is erected,
which guarantees its final stress level in the ideal state after the deck is
superimposed. How much is the correct jacking stress of each pair of cables
to reach the expected final ideal state? The answer to this question leads to
a unique analysis method or technique in cable-stayed bridge analyses—
backward and forward analyses.
Backward analysis simulates the reverse process of erection, and
forward analysis simulates the normal construction process of erec-
tion. Given the state after being superimposed, backward analysis will
show the state before each girder segment and cables are erected. Given
the erection parameters such as girder segment properties, structural
and other construction weighs, and jacking stresses, forward analysis
will show the state after the erection cables are jacked. Theoretically,
a full forward analysis using jacking stresses obtained from backward
analysis should meet the ideal state predefined at the stage after being
superimposed.