# Enhancing Friction Pendulum Isolation Systems Using Passive and Semi-Active Dampers
## 1 Introduction  
Base isolation systems are implemented to mitigate the damage and minimize the risk of collapse of structures due to earthquake vibrations. The base is designed to exhibit low shear stiffness and to be able to cope with large displacement in such a way that the input energy is concentrated and dissipated into it [1, 2]. Thus, the isolators are usually designed to move the first-mode period of the structure out of the maximum acceleration of the design response spectrum through a higher fundamental period of the isolator system. This concept has given satisfactory results, allowing the development of different types of isolators, with linear and/or nonlinear behavior such as lead-rubber bearings, high damping rubber bearings, and friction pendulum systems (FPSs), these being the most common ones [3]. However, due to the uncertain nature of the earthquakes, the behavior of these passiveisolation devices may not be as desirable as expected. That is, if the frequency content of the excitation is significantly greater than the fundamental frequency of the base-isolated structure, the ground movement will be effectively filtered out. On the contrary, if the excitation shows frequency content in the vicinity of the fundamental frequency, large undesirable oscillations may take place. The FPS allows us to unlink the foundation from the structure and to dissipate energy from the earthquake through its displacement. Greater dissipation on the device is achieved when large isolator displacements are allowed. The isolator displacements will initially depend on the dry friction, then, the displacement will be governed by the pendulum stiffness directly related to the concave plate radius of curvature and the dynamic friction. The later may affect the existence of residual displacements after the seismic event [4]. Three generations of FPSs have been studied until now. The first generation consists of two plates, where one of them has a concave surface, and the second plate (bearing plate) slides over the first one. The bearing displacements are limited by the concave plate-size. The second generation is characterized by having both concave surfaces, obtaining double friction capacity, double stiffness and more bearing displacement capacity [5]. The third generation is a compound of two isolators of the type of the second-generation, one inside the other. This generation develops more bearing displacement capacity, that is, allows the dissipating of more energy as compared to the second generation by having the double of concave surfaces [6]. Figure 1 illustrates these generations. Much research has been done on the analysis of the FPS performance and its design. Some studies are focused on investigating the different variables that affect the behavior of FPSs, such as the influence of the FPS temperature when it is working, or the performance of FPSs under bidirectional effects [7]. Others studies are mainly focused on simplified methodologies that allow designers to analyze isolated-structures with FPSs [8]. Most of the design methodologies focus on finding the optimal pendulum geometry by minimizing the peak acceleration of the structure [9; 10]. Generally, to achieve lower structural accelerations, higher displacement demands of the FPS will have to be accommodated. Thus, a larger size isolator will be required. To reduce the isolator displacement demand, and consequently the isolator size, devices with a higher friction coefficient may be used. Although the displacement demand is reduced, the re-centering capacity is drastically affected [4]. That is, under low-intensity earthquakes, residual displacements may take place, increasing thus the risk for future shocks [11]. Additionally, high friction FPSs may show problems related to the pre-sliding phase; that is, the FPS may not be engaged even for a moderate earthquake.