Suspension bridges - Computational Analysis and Design of Bridge Structures - page 378

starting from center of the main span is better as these changes are smaller.
Especially when the last segments close to the pylons are installed, most
deformation of the cable is completed if the erection is from center of the
main span to the pylons. However, to erect girders starting from the pylons
is easier.
No matter what sequence is adopted, one common goal should be
achieved during the erection of the stiffened girder, which is to minimize
the bending moment in the girder at the hanger locations. The ideal situ-
ation is that all the girder weight is evenly distributed onto the cables. To
reach this ideal situation, girder segments are usually connected only at the
top part of the girder and the bottom part is left unconnected during erec-
tion. These joint connections will be changed to rigid connections after all
girder segments are installed and the suspenders are adjusted. The change
from joints to rigid connections is done before the deck is superimposed.
As more and more segments of the stiffened girders are erected, bend-
ing moments at the bottom of pylons and deflections on the top of pylons
will be accumulated. Such a distribution of girder weights to pylons that
are designed to be flexible should be released by adjusting the horizon-
tal position of saddles on the top of pylons. Ideally, the pylons should be
under pure compression after all girder segments are erected. Figure 12.9
shows the adjustment of the saddle position so as to release the deflections
of pylons by jacking the saddles. This adjustment may be required several
times during the erection of girders.

Initial offset saddle

Displaced and final position

Jacking block

Δ Δ

Central of pylon after displaced
and before adjustment

Central of pylon before displaced
and after adjustment

Figure 12.9 Offset of saddle and release of the deflection of pylon by the jacking of
saddle.