Civil Engineering Reference
In-Depth Information
points to promote flow separation on the vibrating cable, as shown in Figure 12.9. In this
figure, the effective cross-wind shape is postulated to be elliptical. Other observations
have suggested that the circumferential motion was not two-dimensional, and that the
width and depth of the rivulet on the upper surface were less than that on the lower
surface.
Wind-tunnel tests in France for the Normandie Bridge (Flamand, 1994) showed that
carbon combustion products deposited on the surface of the casing were necessary for
aerodynamic instability to occur, indicating the role played by surface tension in allowing
the water rivulet to be maintained.
Fundamental wind-tunnel model studies of inclined cable aerodynamics, with and
without rain, have been made at various angles of pitch (inclination), yaw and rivulet
position. It was found that aerodynamic oscillations were of either the 'velocity-
restricted' type (i.e. occurring over a narrow range of wind speeds) and produced by
vortex shed-ding or of the 'divergent' or galloping type (Section 5.5.2)—i.e. vibration
triggered at a particular wind speed and rapidly increasing in amplitude. However,
instabilities usually commenced at reduced wind velocity (
U/n
c
b,
where
U
is the wind
velocity,
n
c
the cable frequency and
b
the diameter) of about 40. In the case of the vortex-
induced vibrations,
Figure 12.9
Flow separations produced by
rivulets of rain water.
these tended to occur in narrow bands of wind speed centred around 40 or multiples of
40, i.e. 80, 120, etc. (Matsumoto
et al.,
1993).
12.5.3
Solutions
The solutions that have been successful in eliminating or mitigating rain-wind induced
vibration of bridge cables can be divided into the following categories:
• aerodynamic treatments—i.e. geometrical modifications of the outer cable casing;
• auxiliary cable ties; and
• auxiliary dampers.
