Geology Reference
In-Depth Information
INTRODUCTION
or wind turbines, but they suffer from difficult
tuning, variable damping and a comparatively low
active mass. Tuned liquid column damper (TLCD)
overcome these drawbacks by a controlled and
guided liquid motion in a rigid piping system.
Originally developed to reduce the rolling mo-
tion of ships, they were first proposed for civil
engineering structures about 20 years ago. The
working principle is to transfer structural vibra-
tion energy into a liquid movement and dissipate
it by viscous and turbulent damping. Since the
restoring forces are due to gravity, the extremely
low natural frequencies in real size applications
are in the range of about 0.1 - 0.5Hz. Although
this excellent low frequency characteristic might
be feasible for very large structures, the invention
of the modified tuned liquid column gas damper
(TLCGD) expands the possible field of applica-
tions to structures with critical natural frequencies,
say up to 5Hz, see Hochrainer (2001). The classic
TLCD is closely related to the TMD of pendulum
type: both are only applicable for structures with
extremely low natural frequencies (high raised
buildings) and the restoring forces are due to grav-
ity only. The main advantages of TLCGD include
comparably low installation costs, easy application
to new buildings or in retrofitting existing struc-
tures, a simple tuning mechanism which allows
for adaptation to modified (degraded) building
dynamics, no moving mechanical parts, virtually
no maintenance requirements and little additional
weight in those cases where a water reservoir is
required, e.g. for the sake of firefighting.
TLCD have shown to be effective in reduc-
ing structural vibrations in recent years and the
research work of the last decade has culminated
in simple guidelines for optimal placement and
tuning of the TLCD. So far, all research results
indicate that the TLCD is competitive when
compared to TMD (spring-mass-dashpot type)
and it could replace the TMD in many structural
application. Due to their salient features, TLCD
have caused an increased research interest, result-
ing in both, analytical and experimental analyses.
The basic idea of most vibration reducing devices
is the absorption of vibrational energy, thereby
reducing the ductility demand of the main struc-
ture and thus preventing it from serious structural
damage. A well accepted damping principle is the
transfer of energy from critical building vibra-
tion modes to dynamic damping devices which
are designed to absorb and dissipate energy to
protect a structure from excessive dynamic loads.
This method of energy dissipation incorporates
dynamic absorber like tuned mass damper (TMD),
tuned liquid damper (TLD also called sloshing
motion damper) or tuned liquid column damper
(TLCD). A different concept is to prevent the
accumulation of seismic energy by uncoupling
the structure-base and the surrounding soil by
base isolation elements. This type of earthquake
protection is very effective because it reduces the
energy dissipation demand of the higher structural
vibration modes. If base isolation is combined
with other earthquake defending measures a high
level of protection can be achieved.
Besides base isolation, probably the most
commonly used passive device is the tuned mass
damper, which consists of a mass attached to the
vibrating structure through a spring and a dashpot.
TMD have been applied successfully in symmet-
ric high raised buildings where the motion of a
single mass can be used to absorb two bending
and a torsional motion. However, it is difficult to
guarantee a smooth, frictionless motion for huge
masses, and in order to avoid the application of
hydrodynamic bearings or friction compensating
actuators the mass is often suspended vertically
on cables, thereby forming a pendulum type mass
damper which is used in high raise buildings, e.g.
the Taipei 101 tower. Pendulum type absorbers
represent the ideal alternative to TMD of spring-
mass-dashpot type when considering symmetric
high raised buildings.
For other applications TLD are optimal, e.g. to
suppress wind induced vibrations of smokestacks
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