Figure : Working Process of Semi-Active Protection System[12].
  1. 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).
  1. 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].
  1. 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].
  1. 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].