## 1 Introduction  
Base isolation is one of the most used techniques for the seismic protection of buildings. Experience from past earthquakes, as well as numerical studies, highlights the seismic vulnerability of a large portion of the existing Reinforced Concrete buildings. Replacement of existing damaged buildings by reconstruction is generally avoided due to the high economic costs [1]. It has long been known that seismic isolation technology is one of the most effective solutions to protect new and existing buildings at both serviceability and ultimate limit states [2, 3]. Correctly designed base-isolated buildings for design intensities earthquakes are not subjected to functionality loss and are immediately accessible in quakes aftermaths [4]. The isolated structures behaviour and the isolation system effectiveness have been widely investigated by several authors [5, 6, 7, 8, 9, 10]. Recent research works have highlighted the main dependency of the frictional properties on some important response parameters, such as the sliding velocity, the recentering capacity, the contact pressure, the temperature rise during the sliding motion and the consequences of mounting laying defects for curved surface slider devices [11, 12, 13, 14, 15]. Precisely, it has been observed that the friction coefficient decreases as the contact pressure rises, due to fluctuations induced by either the vertical component of the seismic event or overturning effects, while the recentering capability depends only on the characteristics of the isolation system. In addition, the coefficient of friction should be also influenced by the temperature on the sliding interfaces. Moreover, differences in the effects of radial and bidirectional motions should be noticed. Differently, studies and experimental investigations about isolated structures behaviour under collapse conditions and for high seismic intensities are limited [16; 17; 18; 19]. In the recent past, some unexpected results have been returned by seismic risk assessment procedures applied to base-isolated structures. More specifically, previous studies [20] have shown a lower seismic risk level can be achieved for the \"protected\" structural system, equipped with an isolation layer, in comparison to a conventionally designed fixed-base configuration. Such results may be addressed to the initial definition of collapse for isolation devices, which generally are assumed to reach failure as soon as the maximum displacement from numerical simulations achieves the displacement capacity [18]. This assumption somehow considers the design displacement as a conservative quantity, which leads to lower uncertainties (apparently) in the response, and no extra capacity for the isolation system is taken into account [17]. On the other hand, structures with fixed-base configurations are generally designed considering safety factors, which increase external loads and decrease material strengths, so that a certain extra capacity can be achieved if the collapse limit state is analysed [21; 22]. Experimental investigations on seismic isolation devices are conducted through testing protocols ruled by given standard codes, which aim at assessing the response of the isolators when subjected to the design displacement, and no further investigations are suggested for higher displacement demands. This aspect represents a very important issue to be investigated, especially when DCCCSS devices are studied. Sliding-based isolators can be designed according to different technologies, depending on the common practice of the country. In particular, in Europe, most of the cases present the stainless-steel sliding surface either welded to the backing plate or installed within a recess with a consequent small counterbore gap. Some experimental tests have shown that stable force responses can be obtained, even if the geometrical displacement threshold is overcome [23; 24; 25; 26].