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In the last decades researchers in the field of structural engineering have challenged the idea of facing natural hazard mitigation problems by adding to structures particular systems which are designed to protect buildings, bridges and other facilities from the damaging effects of destructive environmental actions. Among most protective systems and devices, active structural control, although having already reached the stage of full-implemented systems, still need theoretical investigation to achieve a complete exploitation of its capacity in reducing structural vibrations. In most of the operating systems (e.g. Soong and Reinhorn, 1993), linear control laws based on some quadratic performance function criteria are used since the design process for these linear strategies are fully developed and investigated. Moreover, the performances of structural systems controlled by linear techniques bring about some question concerning the complete and wise utilization of the capacity of control devices. Indeed, some of these inefficiencies are evident such as the inability to produce a significant peak response reduction in the first cycles of recorded or simulated time histories. (e.g. Reinhorn et al., 1993). Realizing that the expected maximum value for the required control force is a fundamental parameter in all processes to design the complete control system, in this paper it is shown that appropriate nonlinear control laws can significantly enhance the reduction of the system response under the same constraints imposed on the control force. Energy evaluation on the performance of different kinds of nonlinearities are reported such that a common base is built to perform comparative studies. These techniques have been successfully experimented on a structural model with ground excitations supplied by shaking table (e.g. Gattulli et al., 1994).
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