Geology Reference
In-Depth Information
INTRODUCTION
mechanical actuators which supply control forces
to the structure. Control forces are developed
based on feedback from sensors that measure
the excitation and/or the response of the structure
(Symans & Constantinou 1999). Active tuned
mass damper (ATMD) or hybrid mass damper is a
kind of these control systems, which is considered
when the required response reduction exceeds the
capacity of the TMD. ATMD systems are more
costly, complex, needs careful maintenance, as
well needs huge source of energy difficult to
provide in severe earthquakes. Moreover, the
control forces which these systems apply to the
structure may cause unforeseen behavior of the
structure. These disadvantages made them to be
less reliable than TMDs and are being used only
for certain cases (Chey et al., 2009).
The limitation of passive and active control
systems result in developing semi active control
systems. Semi-active control systems maintain
the reliability of passive control systems while
taking advantage of the adjustable parameter char-
acteristics of an active control system (Symans &
Constantinou 1999). The semi-active tuned mass
damper (STMD) with variable damping is a kind
of these systems. Various studies confirm the
efficiency of STMDs and show that the applica-
tion of TMDs is much better when they behave
as STMDs, especially in wind and earthquake
excitations. In these systems, the stiffness or the
damping ratio of the control device changes pro-
portional to the relative displacement or relative
velocity, by receiving information from sensors
in every second (Mulligan, 2007). Therefore, they
do not require large power supply, and they do
not add additional energy to the main structure
and guarantee stability of the system. In order to
regulate the stiffness or the damping ratio of the
STMDs, fuzzy systems can be utilized.
Hybrid control systems are the combination of
some passive systems with active or semi active
systems, resulting to better performance of the
control device in reducing the structural responses.
One of the most popular systems of them is smart
One of the most important tasks in structural
engineering is to reasonably minimize the un-
desired vibrations of the structures due to the
environmental dynamic loads such as earthquake
excitations. Various strategies and theories have
been investigated and developed to approach this
goal over the years. Use of the control systems
is one of these methods to enhance the structural
performance against vibration excitations. The
main purpose of these methods is to reduce the
structural responses such as displacement, veloc-
ity and acceleration. Control systems are divided
to four groups of passive, semi active, active and
hybrid systems based on the performance and rate
of the energy consumption and the kind of their
installation to the main structure.
The passive systems dissipate the vibration
excitations without using any external power
source for operation and utilize the motion of the
structure to develop the control force. Since the
properties of these types of control systems can-
not be modified after installation, these systems
are regarded as passive. These systems add no
energy to the structure, and therefore are not able
to make the structure unstable. These systems are
undoubtedly simple, inexpensive, and reliable to
suppress the undesired vibrations of the structures.
Another advantage of these systems is low cost
of repairing and maintenance. Passive tuned mass
dampers (TMDs) and base isolations are two
kinds of these systems (Pinkaew &Fujino 2001;
Yang & Agrawal 2002; Cao et al. 1998; Soong
& Constantinou 1994; Soong & Dargush 1997).
Although TMD control systems may be considered
a hybrid of a tuned dynamic absorber, including a
mass block and spring, combined with a viscous
damper, however, in engineering is known as a
passive control system (Pinkaew & Fujino 2001;
Soong & Dargush 1997).
An active control system may be defined as
a system which typically requires a large power
source for operation of electrohydraulic or electro-
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