Cable-stayed bridges - Computational Analysis and Design of Bridge Structures - page 349

covered in this chapter. However, a simple formula to estimate the critical
wind speed will be introduced in the dynamic analysis section.
As the span increases, the compression in the girder increases; addition-
ally, the stability of the girder becomes critical to the design and building
of a long-span cable-stayed bridge. Stability analysis in both lateral and
vertical directions is required, especially before the closure.
For the initial stresses, the axial forces in the girder under dead loads can
be used. The critical load can then be obtained from solving the eigenvalue
problem. Although the Class I stability result gives only the upper limit of
the critical loads due to the fact that a perfect stability problem rarely hap-
pens in actual engineering situations, it can serve as an initial guidance for
the stability analysis.
The process of static stability analysis with consideration of large dis-
placements can be the same as a regular static nonlinear analysis, except
that the loads should be selected to reflect the nature of the structure. Also,
the FEA system should allow for the increase of some of the loads step by
step in search of the ultimate loads. For instance, if the issue of temporary
construction loads is a concern for lateral stability before closure, minor
lateral wind loads, structural loads, and cable prestressing loads should be
applied as constant loads. The construction loads, as the main loads, should
be increased step by step. The level of the major loads at which the structure
fails is the critical load of the stability analysis.
A long-span cable-stayed bridge rarely fails in static geometric non-
linear analyses. Even for lateral stability, it is easy to understand that
the transverse components of the high-stressed cable tensile in a changed
geometry configuration will help to prevent large lateral displacements.
In terms of static stability, a full analysis by counting both geometric
and material nonlinearities is inevitable. More details will be discussed
in Chapter 14.
11.2.10 dynamic behavior
Compared with other girder-type bridges, cable-stayed bridges are rela-
tively slender and more flexible. In seismic design, a cable-stayed bridge
is preferable because of its low natural frequency. On the contrary, when
aerodynamic stability is of concern, a stiffer bridge is preferred. Certain
special measurements will have to be taken into account for a long-span
cable-stayed bridge for both seismic and aerodynamic requirements, for
example, installing damping devices in girders so as to improve responses
to dynamic loads from vehicles, adopting a wind-resisting girder
cross-sectional shape so as to improve aerodynamic response, and cross-
tying long cables to reduce wind and rain oscillations of cables. In both
aspects, the natural modes of a cable-stayed bridge should be investigated
carefully.