Tuned mass dampers (TMD), active mass dampers (AMD) and hybrid mass dampers (HMD) have been widely applied for vibration control of tall buildings and bridges in the past decade. Recently, the first author and his coworkers have developed semiactive or smart tuned mass dampers (STMD) using semiactive variable stiffness systems. STMD's are superior then TMD's in reducing the response of the primary structure. In case the fundamental frequency of the primary structure changes due to damage or deterioration, then the TMD will be off-tune; hence, it will lose its effectiveness significantly, whereas the STMD is robust against such changes as it is always tuned. The author and his coworkers have shown that STMD can provide performance similar to AMD/HMD, but with an order of magnitude less power consumption. This paper presents the development of a new STMD to reduce the vibrations of structures. The new STMD is an adaptive length pendulum (ALP) damper. It is essentially an Adaptive Stiffness Device (ASD); wherein, the length of the pendulum is varied in real time to achieve the change in frequency or stiffness (with mass remaining constant) of the pendulum. The Two different control approaches to adjust the length of pendulum of the ALP-STMD are developed (1) a shape memory alloy (SMA) actuator, and (2) servomotor. In both the mechanisms the length of the pendulum is adjusted semi-actively based on the feedback signal displacement of the top floor of the structure. Experimental studies are carried on a two-storey scaled model building with ALP-STMD. Effectiveness of the proposed ALP-STMD in controlling the fundamental mode of the structure is validated experimentally using both the SMA and servomotor to change its length. Off-tuned ALP-STMD results are presented to demonstrate the importance of tuning the length of pendulum.
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