Influence of the target earthquake design level on the dynamic response of damped structures

This work investigates how the energy dissipation properties of a structure equipped with nonlinear manufactured viscous dampers sized for a particular design point are influenced by the change of the earthquake intensity level, thus moving far away from the design point. In this preliminary investigation, Single-Degree-Of-Freedom (SDOF) systems are considered. First, the dynamic response of a non-linear damped SDOF system subjected to harmonic excitation is investigated to derive the analytical relationship between the non-linear damping coefficient, the equivalent damping ratio and the magnitude of the response. Then, the analysis is extended to SDOF systems subjected to earthquake ground motions. Numerical simulations are carried out through non-linear time-history dynamic analyses varying the intensity of the base earthquake input. The results indicate that, for SDOF systems under harmonic input, when Output Scaling ratios are smaller than 1, equivalent damping increases sharply as input intensity decreases. For Output Scaling ratios larger than 1, damping decreases with higher input. Forcing frequencies lower than the system’s one increase damping; higher ones reduce it. Under seismic input, nonlinear dampers are more effective for Input Scaling factors smaller than 1, and less effective for Input Scaling factors larger than 1. The results could be useful for the preliminary quantification of the dampers effectiveness under multiple seismic intensity levels, as typically requested in modern Seismic Performance Based Design approaches.