Figure : Classification of Seismic Protection Systems.
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METALLIC YIELD DAMPER
One of the effective mechanisms available for the dissipation of energy
input to a structure from an earthquake is through inelastic deformation
of metals. Many of these devices use mild steel plates with triangular
or X shapes so that yielding is spread almost uniformly
throughout the material. A typical X-shaped plate damper or ADAS
(added damping and stiffness) device is shown in Fig. 3. Other
configurations of steel yielding devices, used mostly in Japan, include
bending type of honeycomb and slit dampers and shear panel type. Other
materials, such as lead and shape-memory alloys, have also been
evaluated [16]. Some particularly desirable features of these
devices are their stable hysteretic behavior, low-cycle fatigue
property, long term reliability, and relative insensitivity to
environmental temperature. Hence, numerous analytical and experimental
investigations have been conducted to determine these characteristics of
individual devices.
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FRICTION DAMPER
Friction dampers utilize the mechanism of solid friction that develops
between two solid bodies sliding relative to one another to provide the
desired energy dissipation. Several types of friction dampers have been
developed for the purpose of improving seismic response of structures.
An example of such a device is depicted in Fig. 6. During cyclic
loading, the mechanism enforces slippage in both tensile and compressive
directions. Generally, friction devices generate rectangular hysteretic
loops similar to the characteristics of Coulomb friction. After a
hysteretic restoring force model has been validated for a particular
device, it can be readily incorporated into an overall structural
analysis [17].
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VISCO-ELASTIC DAMPER
Viscoelastic (VE) materials used in structural applications are usually
copolymers or glassy substances that dissipate energy through shear
deformation. A typical VE damper, which consists of VE layers bonded
with steel plates, is shown in Fig. 7. When mounted in a structure,
shear deformation and hence energy dissipation takes place when
structural vibration induces relative motion between the outer steel
flanges and the center plates. Significant advances in research and
development of VE dampers, particularly for seismic applications, have
been made in recent years through analyses and experimental tests [18,
19].
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FLUID VISCOUS DAMPER
Fluids can also be used to dissipate energy and numerous device
configurations and materials have been proposed. One class involves the
use of a cylindrical piston immersed in a viscoelastic fluid. Such a
system has been studied both experimentally and analytically by Makris
et al. (1993). Another proposed device involves the concept of a viscous
damping wall, again using a viscoelastic fluid (Arima et al. 1988;
Miyazaki and Mitsuaka 1992). Viscous fluid dampers, widely used in
aerospace and military applications, have recently been adapted for
structural applications (Constantinou et al. 1993). Characteristics of
these devices which are ofprimary interest in structural applications,
are the linear viscous response achieved over a broad frequency range,
insensitivity to temperature, and compactness in comparison to stroke
and output force. The viscous nature of the device is obtained through
the use of specially configured orifices, and is responsible for
generating damper forces that are out of phase with displacement.
A viscous fluid damper generally consists of a piston in the damper
housing filled with a compound of silicone or oil (Makris and
Constantinou 1990; Constantinou and Symans 1992). A typical damper of
this type is schematically shown in Fig. 7. It dissipates energy through
movement of the piston in the highly viscous fluid. If the fluid is
purely viscous (for instance, Newtonian), then the output force of the
damper is directly proportional to the velocity of the piston. Over a
large frequency range, the damper exhibits viscoelastic fluid behavior.
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TUNED MASS DAMPER
Early applications of tuned mass dampers (TMDs) have been directed
toward mitigation of wind induced excitations. Recently, numerical and
experimental studies have been carried out to examine the effectiveness
of TMDs in reducing seismic response of structures. It is noted that a
passive TMD can only be tuned to a single structural frequency. While
the first-mode response of a MDOF structure with TMD can be
substantially reduced, the higher mode response may in fact increase as
the number of stories increases. For earthquake-type excitations, the
response reduction is large for resonant ground motions and diminishes
as the dominant frequency of the ground motion gets further away from
the structure’s natural frequency to which the TMD is tuned. There has
been numerous application of TMDs over the world throughout the years
[20].