| dc.description.abstract | As Civil Engineering structures, e.g., tall buildings, long span bridges, deep water offshore platforms, nuclear power plants, etc., have become more costly, complex and serve more critical functions, the consequences of their failure are catastrophic. Therefore, the protection of these structures against damage induced by large environmental loads, e.g., earthquakes, strong wind gusts and waves, etc., is without doubt, a worldwide priority. However, structures cannot be designed to withstand all possible external loads and some extraordinary loading episodes do occur, leading to damage or even failure of the structure.
Protection of a structure against hazards can be achieved by various means such as modifying structural rigidities, increasing structural damping, and by attaching external devices, known as control devices. Control devices can be deployed either to isolate the structure from external excitation or to absorb input seismic energy to the structure (absorber) so as to mitigate vibration in the primary structure. Seismic base isolation is one such mechanism which isolates a structure from harmful ground excitations.
Seismic base isolation is a widely accepted and implemented structural control mechanism due to its robustness and ease in deployment. Following the Northridge earthquake (1994), and Kobe earthquake (1995), the interest of structural engineers in understanding near-source ground motions has enhanced. Documents published after these earthquakes emphasized the issue of large base displacements because of the use of none or little isolation damping (of viscous type only) prior to these events. More recent studies have investigated analytically and experimentally, the efficiency of various dissipative mechanisms to protect seismic isolated structures from recorded near-source long period, pulse-type, high velocity ground motions. Consequently, hybrid isolation systems, seismic base isolation supplemented with damping mechanisms, have become the focus of current research trend in structural vibration control.
Hybrid base isolation system incorporating passive supplemental damping devices like, viscous fluid dampers, etc., performs satisfactorily in minimizing isolator displacement but at the same time increases superstructure acceleration response. Furthermore, the passive system can be tuned to a particular frequency range and its performance decreases for frequencies of excitation outside the tunning bandwidth. In such a scenario, active control devices in addition to base isolation mechanism provide better performance in reducing base displacement and superstructure acceleration for a broad range of excitation frequencies. Tremendous power requirement and the possibility of power failure during seismic hazards restrict the usage of active systems as a supplemental device.
Semi-active devices provide the robustness of passive devices and adaptive nature of active devices. These characteristics make them better suited for structural control applications. The recent focus is on the development of magnetorheological (MR) dampers as semi-active device for structural vibration control applications. MR dampers provide hysteretic damping and can operate with battery power. The thrust of this thesis is on developing a hybrid base isolation mechanism using MR dampers as a supplemental damping device.
The use of MR damper as a semi-active device involves two steps; development of a model to describe the MR damper hysteretic behaviour; development of a proper nonlinear control algorithm to monitor MR damper current / voltage supply.
Existing parametric models of MR damper hysteretic behaviour, e.g., Bouc-Wen model, fail to consider the effect of amplitude and frequency of excitation on the device. Recently reported literature has demonstrated the necessity of incorporating amplitude and frequency dependence of MR damper models.
The current/voltage supply as the input variable to the MR damper restricts the direct use of any control algorithms developed for active control of structures. The force predicted by the available control algorithms should be mapped to equivalent current/voltage and then to be fed into the damper. Available semi-active algorithms in the literature used ‘on-off’ or ‘bang-bang’ strategy for MR applications due to nonlinear current/voltage-force relation of MR damper. The ‘on-off’ nature of these algorithms neither provides smooth change in MR damper current/voltage input nor considers all possible current/ voltage values within its minimum to maximum range. Secondly, these algorithms fail to consider the effect of the MR damper applied and commanded current/voltage dynamics.
The thrust of this dissertation is to develop semi-active control algorithms to monitor MR damper supply current/voltage. The study develops a Bouc-Wen based model to characterize the MR damper hysteretic phenomenon. Experimental results and modeling details have been documented. A fuzzy based intelligent control and two model-based nonlinear control algorithms based on optimal dynamic inversion and integral backstepping have been developed. Performance of the fuzzy logic based intelligent control has been explored using experimental investigation on a three storey base isolated building. Further the application of the proposed controllers on a benchmark building; a benchmark highway bridge and a stay cable vibration reduction have been discussed.
Experimental study has revealed that the performance of optimal FLC is better than manually designed FLC in terms of reducing base displacement and storey accelerations. The performance of both the FLCs (simple FLC and genetic algorithm based optimal FLC) is better than ‘passive-off’ (zero ampere current supply) and ‘passive-on’ (one ampere current supply) condition of MR damper applications. The ‘passive-off’ results have shown higher base displacements with lower storey accelerations, whereas, the ‘passive-on’ results have reduced base displacement to the least but at the same time increased the storey acceleration too much. The FLC monitored MR damper show a compromise between the two passive conditions. Analytical results confirm these observations. Numerical simulations of the base isolated building with the two model based MR damper control algorithms developed have shown a better performance over FLC and widely used clipped optimal algorithms.
The applications of the proposed semi-active control algorithms (FLC, dynamic inversion and integral backstepping) have shown better performance in comparison to that of control algorithms provided with the benchmark studies. | en_US |
