Figure : Working Process of Semi-Active Protection
System[12].
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STIFFNESS CONTROL DEVICE
These devices are utilized to modify the stiffness, hence, the natural
frequency of the system. This establishes a new resonant condition
during earthquakes. The devices used are stiffness bracings, which are
engaged or released so as to include or not to include the additional
stiffness in the system, and operate generally through fluid control
within tubes by valves (Yang et al., 1996; Nagarajaiah, 1997; He et al.,
2001).
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ELECTRO-RHEOLOGICAL DAMPER
Electrorheological (ER) dampers typically consist of a hydraulic
cylinder containing micron-sized dielectric particles suspended within a
fluid (usually oil). In the presence of a strong electric field, the
particles polarize and become aligned, thus offering an increased
resistance to flow (a change from viscous fluid to yielding solid within
milli-seconds). By varying the electric field, the dynamic behavior of
an ER damper can be modulated. Electrorheological dampers have been
investigated for seismic response control by Ehrgott and Masri [22],
Gavin et al. [23], Makris et al. [24].
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MAGNETO-RHEOLOGICAL DAMPER
Magnetorheological (MR) dampers are essentially magnetic analogs of ER
dampers. Qualitatively, the behavior of the two types of dampers is very
similar except that the control effect is governed by the application of
an electric field in one case and by a magnetic field in the other. MR
dampers typically consist of a hydraulic cylinder containing
micron-sized, magnetically polarizable particles suspended within a
fluid (usually oil). MR fluid behavior is controlled by subjecting the
fluid to a magnetic field. In the absence of a magnetic field, the MR
fluid flows freely while in the presence of a magnetic field, the fluid
behaves as a semisolid. The maximum yield stress that can be resisted by
MR fluids is on the order of 50–100 kPa [25]. MR dampers have been
investigated for seismic response control by Spencer et al. [25],
Dyke et al. [26], and Gordaninejad et al. [27].
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FRICTION CONTROL DEVICES
Semi-active friction control devices are utilized either as energy
dissipaters within the lateral bracing of a structure or as components
within sliding isolation systems. Such systems have been investigated
for seismic protection of structures by Akbay and Aktan [68–70],
Pandya et al. [71], Feng et al. [28], Fujii and Feng
[76,77], Dowdell and Cherry [78,79] and Fujita et al. [80].