Civil Engineering Reference
In-Depth Information
9.9.4
Tuned liquid dampers
Tuned liquid dampers (TLD) are relatively new devices in building and structures
applications, although similar devices have been used in marine and aerospace
applications for many years. They are similar in principle to the tuned mass damper, in
that they provide a heavily damped auxiliary vibrating system attached to the main
system. However, the mass, stiffness and damping components of the auxiliary system
are all provided by moving liquid. The stiffness is in fact gravitational; the energy
absorption comes from mechanisms such as viscous boundary layers, turbulence or wave
breaking, depending on the type of system. Two categories of TLD will be discussed
briefly here:
tuned sloshing dampers
(TSD) and
tuned liquid column dampers
(TLCD).
The TSD type (Figure 9.17) relies on the motion of shallow liquid in a rigid container
for absorbing and dissipating vibrational energy (Fujino
et al.,
1988; Sun
et al.,
1989).
Devices of this type have already been installed in at least two structures in Japan (Fujii
et al.,
1990) and on a television broadcasting tower in Australia.
Although a very simple system in concept, the physical mechanisms behind this type
of damper are in fact quite complicated. Parametric studies of dampers with circular
containers were carried out by Fujino
et al.
(1988). Some of their conclusions can be
summarized as follows:
• Wave breaking is a dominant mechanism for energy dissipation but not the only one.
• The additional damping produced by the damper is highly dependent on the amplitude
of vibration.
• At small to moderate amplitudes, the damping achieved is sensitive to the frequency of
sloshing of liquid in the container. For dampers with circular containers, the
fundamental sloshing frequency is given by:
n
=(1/2π)
√
[(1.84
g/R
) tanh(1.84
h/R
)]
(9.29)
where
g
is the acceleration due to gravity,
h
the height of the liquid and
R
the
radius of the container, as shown in Figure 9.17.
This formula is derived from linear potential theory of shallow waves.
• High viscosity sloshing liquid is not necessarily desirable at high amplitudes of
vibration, as wave breaking is inhibited. However, at low amplitudes, at which energy
is dissipated in the boundary layers on the bottom and side walls of the container,
there is an optimum viscosity for maximum effectiveness (Sun
et al.,
1989).
• Roughening the container bottom does not improve the effectiveness because it has
little effect on wave breaking.
The above conclusions were based on a limited number of free vibration tests with only
two container diameters. Further investigations are required, including the optimal size of
TSD for a given mass of sloshing liquid. However, the simplicity and low cost of this
type of damper makes them very suitable for many types of structure.
Variations in the geometrical form are possible, for example Modi
et al.
(1990) have
examined TSDs with torus (doughnut)-shaped containers.
