2.3.4.6 TUNED LIQUID DAMPER
The basic principles involved in applying a tuned liquid damper (TLD) to reduce the dynamic response of structures is quite similar to that discussed above for the TMD. In effect, a secondary mass in the form of a body of liquid is introduced into the structural system and tuned to act as a dynamic vibration absorber. However, in the case of TLDs, the damper response is highly nonlinear due either to liquid sloshing or the presence of orifices. TLDs have also been used for suppressing wind-induced vibrations of tall structures. In comparison with TMDs, the advantages associated with TLDs include low initial cost, virtually free of maintenance and ease of frequency tuning. The TLD applications have taken place primarily in Japan for controlling wind-induced vibration. Examples of TLD-controlled structures include airport towers and tall buildings [17].
  1. ACTIVE PROTECTION SYSTEM
The active protection system applies counteracting control forces externally or by creating reactive internal forces in the structure to reduce structural response[21]. The counteracting control forces are applied externally by actuators driven by an adequate power source. The regulation of the control force is done according to an appropriate algorithm. In comparison with passive control, active control of structural response is characterized essentially in terms of the following two features: (i) a certain amount of external power or energy is required; and (ii) a decision-making process based on real-time-measured data is involved. In this regard, active control includes a wide range of technologies.
An active control system is one in which an external source powers control actuator(s) that apply forces to the structure in a prescribed manner. These forces can be used to both add and dissipate energy in the structure. In an active feedback control system, the signals sent to the control actuators are a function of the response of the system measured with physical sensors (optical, mechanical, electrical, chemical, and so on)