Cable-stayed bridges - Computational Analysis and Design of Bridge Structures - page 350

The connections between girders and pylons influence the dynamic
behaviors. If girders and pylons are rigidly connected, as they do in single
cable-stayed plane bridges, the girder is stiffer and the first mode will
be the bending mode with higher frequency. If the girder is designed to
move free longitudinally and independently from the pylons, as com-
monly adopted in long-span cable-stayed bridges, the girder will behave
like a suspending pedal and the first mode will be the horizontal swing-
ing mode. Many cable-stayed bridges use special connection mechanisms
such as thin concrete blocks to limit the girder horizontal movements.
These are carefully designed so that when there are severe movements,
such as earthquakes, they will break and lose their function, and the
girder therefore behaves more effectively to absorb dynamic energy. For
long-span cable-stayed bridges, as the longitudinal displacements due to
temperature and wind loads are both significant, more sophisticated con-
nection systems are needed to resolve these contradictory requirements.
Figure 11.21 shows the horizontal damping systems used in the Sutong
Bridge, in which gradual displacements due to temperature are released,
dynamic displacements due to wind loads are reduced by the viscous
damper systems, and excessive displacements are blocked by their move-
ment-stopping mechanism.
In regard to the aerodynamic behavior of a slender structure, the shape of
the girder cross section plays an important role. Because the height is much
less than its other two dimensions, the girder can be treated as a flat slab
in most cases. The side of the cross section is usually cosmetically modified
sharply toward the outside, as shown in Figure 11.10, or wind fairings are
installed, which can be the first technique used to improve the aerodynamic
Figure 11.21 Horizontal damping systems between girder and pylon in Sutong Bridge.
(Data from You, Q. et al., “Sutong Bridge—A Cable-Stayed Bridge with
Main Span of 1088 Meters, ABSE Congress Report, 17th Congress of
IABSE, Chicago, 2008, pp. 142-149.)