Numerical steady-state and transient responses of a SDOF system constrained by two optimally designed bumpers

Seismically isolated structures can be subjected to large horizontal displacements relative to the ground, especially in Near-Fault earthquakes, which are characterized by one or more intense pulses of velocity and displacement of long period. One strategy to mitigate the problem of large displacements, which occurs in linearly isolated structures, is the use of deformable and dissipative devices (bumpers). The impact between the structure and the bumpers, if the bumpers are appropriately designed, can produce beneficial effects on the dynamic response of the system, both on displacements and accelerations. In this paper the response obtained from a numerical model of isolated single-degree-of-freedom (SDOF) systems constrained by two bumpers, arranged symmetrically on both sides of the mass of the system with an initial gap, subjected to base harmonic excitation, is studied. This model is called Vibro-Impact Isolation System (V-IIS). The objective of this work is to define a methodology for choosing the design parameter defining the V-IIS (mechanical characteristics of the bumpers, gap and isolation frequency of the system) by observing both steady-state and transient responses of both the system and the bumpers. The study of the transient response is compared with that obtained in the steady-state to assess how representative the latter is of the V-IIS transient response. From the definition of the methodology for choosing the parameters of the V-IIS, through optimal design, the only design parameters are the gap and the isolation frequency of the system. Therefore, an appropriate choice of the gap makes it possible to bring frequency-selective viscous damping in V-IISs, introducing two advantages over linear systems: the reduction of the peak intensity of the responses in the resonance range (both displacement and acceleration) and the reduction of the static displacement of the system, but keeping the dynamic response with which the system is designed unchanged.