Geology Reference
In-Depth Information
Figure 2. Example of hybrid control system
Semi-Active Systems
Their operation is similar to that of active systems
but the actuators are only able to restrain the
structure instead of having also the capacity of
pushing it (in other words, they can take energy
from the system but cannot exert energy on it).
Consequently, these devices are much smaller (for
example hydraulic cylinders with an on/off valve)
than those required for active systems, and only
a minor amount of energy is required to operate
them (typically about 25 W, so some batteries
can supply it). Obviously, these systems are more
feasible and reliable than the active ones since they
do not depend on an external source of energy.
It can be said that the performance is better than
in the passive case and only slightly worse than
in the active one. The same system described in
Figure 1 can also represent a semi-active system.
energy. Though they are not as incredibly efficient
as the other systems, if properly designed, they
can provide excellent results in a wide range of
situations. Passive control is clearly more spread
than the other systems
Figure 3 shows a classification of the most
common systems to control the vibration of
structures. The list is not exhaustive because this
field is still under research and new mechanisms
and devices are being introduced.
Passive control systems, as previously stated,
could be defined as «inert» mechanisms added to
the structure to improve its behaviour in response
to the dynamic forces associated with wind and
especially earthquakes. The behaviour of these
systems is based on deforming inelastically in
response to the excitation.
It is possible to classify the mechanisms of
passive control of the seismic response in four
classes:
Hybrid Systems
These systems consist of a combination (series
of parallel) of active (or semi-active) and passive
systems. The efficiency of such co-operation lies
in the fact that the passive system can provide
the gross reduction of response (by absorbing or
deflecting energy) while the active one is used
for further lowering (for protection of sensitive
equipment, for example) of displacements or ac-
celerations. Figure 2 illustrates an example where
an actuator complements passive isolation to re-
duce high-frequency low-displacement vibrations.
As no big control forces are required, hybrid
systems are more feasible and reliable than active
ones.
It is remarkable that none of these devices
(active, semi-active or passive) is part of the main
structure, so they can be temporarily removed for
inspection, replacement or repair. Only the case
of base isolation is slightly different.
A comparison among the four categories previ-
ously described shows that the passive control sys-
tems are more feasible and reliable since devices
are simpler and do not depend on any source of
1.
Mechanisms for the Seismic Isolation
(Base Isolation) of Vibrations:
Generally,
they are put on the base of the structure, with
the aim of reducing the seismic forces enter-
ing the structure. The structure is partially
uncoupled from the foundation by using
flexible bearings instead of traditional (rigid)
connections. Vibration to be controlled can
be transmitted from the ground to the struc-
ture or, conversely, from the structure to the
ground (vibrating elements isolation).
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