Effects of seismic sequences on structures with hysteretic or damped dissipative behaviour

Repeated seismic events strongly affect the building capacity in earthquake-prone regions, as its resilience, intended as the capacity of a system to quickly revert to a fully operational state after a damage due to a significant event, depends on the ability to withstand cumulated damage. This paper investigates the effects of repeated seismic sequences on structures characterized by different hysteretic behaviour. To this aim, non-linear single-degree-of-freedom (SDOF) systems were subjected to ten recorded seismic sequences taken from literature. The elasto-plastic and pivot hysteresis rules were analysed first, considering both hardening and softening behaviour. From each analysis, the inelastic spectrum of the seismic sequence was computed for different ductility levels, and the ductility demand was calculated and compared with the values for an only seismic event. It was shown that the effect of seismic sequences is quite significant, and a reduction of the behaviour factor from 15% for bilinear with hardening and pivot hysteretic rules to 35% for elasto-plastic systems with high ductility should be adopted in design to increase the seismic resilience. The use of linear and non-linear viscous dampers was also analysed in SDOF systems subjected to seismic sequences, demonstrating the effectiveness of this mitigation measure. Nonlinear viscous dampers with an initial friction force were found to dramatically reduce the acceleration and displacement demand, although it cannot avoid residual displacements at the end of the seismic events, and can be recommended for structures with short vibration periods. Nonlinear and linear dampers have the advantage of allowing full recentring of the structure at the end of the seismic events, thus significantly improving resilience.