A new type of tuned liquid damper and its effectiveness in enhancing seismic performance : numerical characterization, experimental validation, parametric analysis and life-cycle based design.
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2015
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Abstract
In the last decades the use of seismic protection devices in Chilean buildings has gained popularity for reducing earthquake losses. Mass dampers (also referenced as inertia dampers), with the most popular representative being the Tuned Mass Damper (TMD), are a potential device for facilitating these tasks; they consist of a secondary mass attached to the primary structure through an equivalent spring and dashpot. Through proper tuning of frequency/damping characteristics, the movement of this secondary mass counteracts the vibration of the primary mass (structure) providing the desired energy dissipation for this vibration. Among the general class of mass damper devices, Tuned Liquid Dampers (TLDs), which consist of a tank filled with some liquid (typically water) whose sloshing within the tank provides the mass damper effect, have some attractive characteristics such as low cost, easy installation and tuning, bidirectional control capabilities and alternative use of the secondary mass (liquid in this case). Their popularity, though, has been hindered by the facts that (i) their dynamic behavior is highly non-linear due to wave breaking and (ii) their inherent damping is usual lower than the optimal one, requiring the introduction of submerged elements (to increase this damping) that make the overall behavior even more complex. Other type of liquid dampers that share some of the TLD advantages, the liquid column dampers, offer a simpler modeling but their dynamic behavior is still non-linear since their damping ends up being amplitude dependent, whereas they are strictly restricted to one-directional applications. Additionally, the advantages of any such type of mass dampers particularly for seismic applications in the Chilean region have not been clearly demonstrated; this pertains to both their efficiency, acting as an inertia device, to allow significant energy dissipation for the ground motions common in the region but more importantly to an explicit discussion of the life-cycle cost improvement they can facilitate. The research presented here introduces a new type of liquid mass damper, called Tuned Liquid Damper with Floating Roof (TLD-FR) which combines the favorable characteristics of both TLDs and liquid column dampers, and further examines its efficiency for seismic applications for Chile. The TLD-FR consists of a traditional TLD (liquid tank filled with liquid) with the addition of a floating roof. The sloshing of the liquid within the tank is what still provides the inertia damper effect, but the roof prevents wave breaking phenomena and introduces a practically linear response and a dynamic behavior in a dominant only mode. This creates a vibratory behavior that resembles other types of a linear mass dampers and a framework is developed to characterize this behavior with a simple parametric description that can facilitate an easy comparison to such dampers. Within this framework, focus is given on a theoretical/computational characterization of the new device, coupled with an experimental validation of its capabilities and of the established numerical tools. To support these advances an efficient computational approach is formulated to describe the dynamic behavior of liquid tanks and is then extended to describe the behavior of the TLD-FR (address the inclusion of the roof). The aforementioned parametric formulation is then used to develop an approach that facilitates a direct design in the parametric space, as well as an efficient mapping back to the different tank geometries that correspond to each parametric configuration. During this process the efficiency of mass dampers for seismic applications in Chile is also examined by comparing the performance across different types of ground motions, representing different regions around the world. Finally, a versatile life-cycle assessment and design of the new device is established considering risk characterizations appropriate for the Chilean region, so that the cost-benefits from its adoption can be directly investigated. This involves the development of a multi-criteria design approach that considers the performance over the two desired goals: (i) reduction of the total life-cycle cost considering the upfront damper cost as well as seismic losses and (ii) reduction of the consequences, expressed through the repair cost, for low likelihood but high impact events. Through this approach the financial viability of the TLD-FR (competitiveness against TMDs) for enhancing seismic performance is demonstrated.
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Tesis (Doctor in Engineering Sciences)--Pontificia Universidad Católica de Chile, 2015