Argenziano, Mario (2022) Mass damping-based solutions for reducing the seismic response of new and existing buildings: analytical methods and optimization criteria. [Tesi di dottorato]

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In the recent years, the need of improving safety standards for both existing and new building structures against earthquake and wind excitations has created a growing interest in conceiving new energy absorption or dissipation mechanisms, capable to control or redirect, in active or passive way, the dynamic response of structures. In this perspective, mass-damping systems are employed with the aim to control and reduce excessive vibrations in civil structures. Inspired by other engineering fields, the concept of the tuned mass damper (TMD) has been applied to tall building applications for the reduction of lateral deformation caused by wind loads since 1976, with the first example being the John Hancock Tower in Boston, USA. Classical TMDs are conceived as simple mass-spring-damper systems attached to the so-called primary structure to control the response. It should be pointed out that conventional TMDs which have an absorber mass that’s much smaller than the primary building mass (in most cases, less than 1%) are quite successful at reducing the dynamic response of buildings where wind excitation is prevalent. However, two main concerns arise for the case of seismic input: the TMD’s high sensitivity to the design parameters and the impact of the earthquake frequency content on the system’s efficiency. To solve these issues and improve the robustness of the overall structure, non-conventional TMD systems with large mass ratios should be employed. In this light, two engineering solutions based on the basic idea of utilizing a part of the building mass as a giant absorber have been proposed in literature: the intermediate isolation system (IIS) and the mega-subcontrol system (MSCS). The IIS is realized by placing the isolation system at an intermediate level, thus dividing the building into three portions: a lower structure, an isolation layer, an upper structure. In terms of dynamics, this system combines both the effects of isolation and mass damping. On the one hand the isolation interface acts as a filter for the inertial forces rising to the upper structure, on the other hand, the relative displacement between the structural portions leads to a reduction of the global seismic response. Moreover, in the MSCS, the building is divided in a main system (mega frame) and several secondary systems (sub-configurations), disconnected from the primary structure by isolation layers. The reduction of the global seismic response is achieved by the mass damping effect exerted on the mega frame. Within this framework, after introducing the mathematical fundamentals for approaching the differential problem governing the dynamic response of n-DOF structural systems, the present Thesis investigates how to properly identify geometrical and mechanical parameters in order to formally set the optimization of tuned mass dampers considering both the main visco-elastic configurations, i.e. the Kelvin-Voigt and the Standard Linear Solid models, and thus deriving general criteria for designing such systems. In order to assess the analytical and numerical optimization methods previously defined, ad hoc designed 3D-printed structures modelling a 2-DOF TMD system are also realized in scale and tested under several dynamic actions, highlighting some asymptotic behaviours and providing insights into cases of presence of nonlinear elastic elements. Finally, the analytical and numerical optimization criteria defined for simplified 2-DOF TMD models have been first adopted for the preliminary design phase of some civil engineering structures and then validated, through finite element approaches, to applications involving the design of new tall buildings, through MSCS and IIS configurations. Furthermore, by exploiting the mass-damping principles, a novel strategy for seismically retrofitting existing masonry buildings is also proposed and suggested first for a wide-scale application of the architectural heritage of a historical city and then applied to the design of the interventions on an exemplar case-study building.

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