Dottorato in INGEGNERIA STRUTTURALE E GEOTECNICA

Titolo della tesi: Numerical and experimental analysis of the Vibro-Impact Isolation Systems under dynamic excitations

Seismic isolation is a widely used strategy for controlling the dynamic response of bridges, artwork, equipment, and nonstructural elements as well as both new and existing buildings. The concept on which the underlying isolation system is based is to reduce the natural frequency of the structure to values far from the frequencies of seismic excitation that have strong energy contributions. However, reducing the natural frequency of the system, results in greater flexibility of the structure, which can experience greater displacements of the isolated floor. In Pulse-Like earthquakes, these displacements may be excessively large, exceeding the limit displacement or seismic design gap. This phenomenon can result in damage to isolation devices on the one hand, as they exceed the ultimate deformation limit, and on the other hand, pounding of the structure with surrounding structures if the seismic gap is insufficient. An unconventional methodology for mitigating the negative effects, which occur as a result of large displacements, is through the insertion of end-stop devices that limit the displacements of the isolated system. Relatively few works address the issue of large displacements in isolated structures by proposing an optimal design of these end-stop devices (bumpers) and evaluating their positive effects on the structures themselves. Moreover, particularly for multi-degree-of-freedom systems, experimental studies examining the effectiveness of bumpers in reducing the negative effects of large displacements under seismic excitation conditions are even rarer. The main focus of this Ph.D. thesis is oriented toward the control of Vibro-Impact Systems (V-IISs), in particular, evaluating their potential beneficial effects. Thus, V-IIS with a single-degree-of-freedom (SDOF) subjected to harmonic actions (as in the case of vibrating machinery) and seismic actions (considering that an isolated structure generally has a predominant response at the main frequency) are initially analyzed. These studies were later extended to multi-degree-of-freedom (MDOF) systems excited by seismic actions, with a systematic evaluation of the response as a function of parameters such as gap size, intensity and type of seismic input. A key contribution of this work is to develop a methodology for choosing the design parameters of the V-IIS (gap and isolation frequency) based on an optimal design of the bumper parameters (stiffness and damping). This was achieved through the analysis of both steady-state and transient responses of SDOF V-IIS subject to harmonic excitations. Another novelty concerns the study of gap uncertainties, which may arise from bumper placement errors or accidental displacements due to dynamic action, and the influence of these uncertainties on the dynamic response of V-IIS SDOFs with optimally designed bumpers. Another relevant aspect of this thesis is a systematic study of an experimental nature that allowed the analysis of the response of both V-IIS SDOF and MDOF subjected to seismic inputs, as a function of the gap and the characteristics and intensity of the seismic input itself. The results were enriched by comparison with experimentally validated numerical models, which allowed for in-depth investigations of phenomena that emerged in the experimental campaigns, rarely treated in the literature. The studies conducted, both experimental and numerical, highlighted the potential critical issues associated with large displacements in isolated structures, particularly the problems associated with rigid impacts. These observations made it possible to formulate interesting considerations on vibration control, showing how an accurate choice of bumper parameters (through optimal design) can direct the response of the system to achieve specific objectives, avoiding undesirable scenarios and favoring others, exploiting the impact for beneficial purposes.