Phd in STRUCTURAL AND GEOTECHNICAL ENGINEERING

Thesis title: Improvement of seismic isolation performances via negative stiffness and super-elastic hysteresis based damping

Among the passive vibration control strategies, base isolation is definitely the most effective. This strategy consists in decoupling the motion of the structure from that of the substructure/soil by introducing a layer of highly flexible elements between them. The high flexibility of the isolation layer interposed between the foundation and the structure causes an increase of the modal periods of the latter, which produces a dual effect: a strong reduction of the accelerations transmissibility and an increase of the displacement transmissibility. The last effect is usually contrasted by the introduction of auxiliary damping devices. A recently explored concept is the amplification of the damping of a structure by the parallel application of negative stiffness devices, i.e., devices that exert a force in the same direction of displacement. The main advantages that a negative stiffness mechanism can provide to a seismic isolation system are: - The possibility of reaching levels of flexibility of the isolation layer over the limit represented by the deformability of the material of the bearing devices, i.e. the possibility of obtaining acceleration transmissibility reductions otherwise not achievable with the existing devices; - The possibility to introduce high levels of hysteretic damping without performance losses caused by the initial stiffness increase, since this increase is cancelled by the negative stiffness properly tuned. In the light of the above, the intention is to exploit the characteristics of bistable mechanisms together with super-elastic hysteresis to obtain an ideal seismic isolation system, with high static stiffness, low dynamic stiffnesses and self-recentering capabilities. The research objectives are, therefore, the study of the effects and limits of applying mechanisms with negative stiffness and super-elastic hysteresis to seismic isolated systems and the design of a new multidirectional and compact damper with negative stiffness and super-elastic hysteresis. The performance of the proposed isolation system has been evaluated by studying the response of a one-degree-of-freedom oscillator having as restoring force the sum of the contributions given by the elastomeric isolators, the bistable mechanism and the super-elastic hysteresis, described by appropriate hysteretic models. The investigation first involved the static characterization of the response of the dimensionless system in terms of stability, stiffness and equivalent damping, highlighting the presence of different types of stability in the space of the design parameters and the possibility of obtaining almost zero stiffness together with amplifications of the damping up to overdamped responses. The second phase of the investigation involved the dynamic response under impulsive and harmonic excitation and the search in the space of the design parameters of the optimal configurations in terms of reduction of the forces transmissibility, revealing the possibility of obtaining strong improvements of the seismic isolation performances. These regions of optimal design parameters have been validated through the study of the dynamic response of a MDOF system, representative of a seismic isolated building, under seismic forcing. Finally, the performances obtained by NS-SMA damping are compared with the ones exhibited by the baseline isolation system equipped with classic auxiliary damping devices. The last phase of the study was dedicated to the development of a rheological device which would allow to achieve the dynamic response of the proposed isolation system . Starting from the main weaknesses of the existing negative stiffness dampers, i.e. the large size given by the prestressed element, the monodirectional response or the dependence of the response on the weight of the mass to be isolated, a new compact multidirectional damper with super-elastic hysteresis has been designed. Finally, the analytical equations of the force-displacement law of the device were derived and validated by comparison with the response provided by a three-dimensional numerical model developed on the program Abaqus.