Suspension bridges - Computational Analysis and Design of Bridge Structures - page 388

displacements much. In most analyses of the suspension bridge, this can be
ignored. But in the backward analysis to obtain the initial state, the accu-
rate tangent point of the cable and the saddle influences the accuracy of the
zero-stress state, and the total cable length under no stress is derived from
the zero-stress state.
12.3.6 offset of saddles and release
of the deflection of pylons
When unbalanced horizontal forces in the main cables on the top of a pylon
exist during the erection of the stiffened girders in the main span and side
spans, the saddle would not be able to move freely due to frictions between
the saddle bottom and the top of the pylon, and such a move is forbidden
during the erection of girders. Thus the pylon will deflect toward the main
span as the dash lines shown in Figure 12.9.
For pylons that are designed as flexible, the deflection due to the erection
of stiffened girders and superimposed dead loads has to be released. The
release is accomplished by adjusting the horizontal position of the saddle.
Depending on the design of the pylons, usually the deflection has to be
released several times, during the erection and after the deck loads are
superimposed.
Because the final position of the saddle has to be centered, when a saddle
is installed, its initial position has to be offset from the center of the pylon
toward the side span. The offset value is the total horizontal displacements
of the saddle starting from the erection of cables to finishing of superim-
posed dead loads. When adjusting the position of the saddle, jacking force
is applied between the saddle and the jacking block on the side span side of
the pylon, as shown in Figure 12.9. Due to cables, the horizontal stiffness
of the saddle is much higher than the horizontal stiffness of the pylon at
the top. When jacking, the saddle will remain still and the pylon will move
toward the side span so the deflection is released.
12.3.7 Low initial stress stiffness
of the main cable close to pylon
As illustrated in Section 12.3.2, the initial stress of the main cable due to
dead loads plays an important role in the enhancement of live load stiffness.
The initial stress stiffness is perpendicular to the cable, and therefore the
vertical stiffness enhancement to the stiffened girder reaches maximum in
the middle of the main span. As the angle of the cable to the stiffened girder
increases in the area close to the pylon, the initial stress stiffness in vertical
projection decreases. One phenomenon relating to this behavior is that the
vertical displacements on the stiffened girder under live loads are larger in
the area close to the pylon than others.