Seismic performance of a friction pendulum-gap damper coupled system

Sliding devices with curved surface, also known as Friction Pendulum (FP) bearings, are widely used seismic isolation devices that exploit the self-centering mechanism related to the pendulum operating principle and provide energy dissipation in relationship to the frictional characteristics of the sliding interface. Displacement demand, energy dissipation and re-centering capability of FP bearings are affected by the frictional properties of the sliding pad. Indeed, higher friction coefficients increase the energy dissipation capability, thus reducing the displacement demand, but worsen the re-centering behavior. Potential residual displacements at the end of the earthquake influence the serviceability of the structure and may also cause accumulation of displacements in aftershocks and future events. On the other hand, FP bearings with lower friction coefficients exhibit a better re-centering behavior, but the corresponding energy dissipation is reduced in comparison with higher-friction FP bearings, which in turn implies higher displacement demand. In an attempt to efficiently combine the good re-centering behavior of low-friction FP bearings with a satisfactory energy dissipation typical of high-friction FP bearings, this contribution presents a friction pendulum-gap damper coupled system. Unlike a conventional damper, the gap damper introduces additional energy dissipation not throughout the range of displacements, but only when a threshold displacement or initial gap is exceeded, while not being engaged otherwise. The gap damper mechanical properties are chosen according to a performance-oriented design procedure, assuming a target displacement demand of the combined isolation system. The design procedure is numerically validated through a parametric study including a series of nonlinear response history analyses, different FP bearing properties, and two intensities of the earthquake excitation (extreme and serviceability) associated with two distinct performance requirements. The results of the parametric study demonstrate that the gap damper is effective in reducing the displacement demand during extreme earthquakes, while not impairing the re-centering capability of the FP bearing, even during minor serviceability earthquakes. Consequently, the proposed system outperforms both the high-friction FP bearings, which suffer from a low re-centering capability, and the low-friction FP bearings, which suffer from large displacement demand.