## 1 Introduction  
The safety assessment of existing structures placed in areas with a relevant seismic hazard has been one of the challenges for engineers over the last few years [1, 2]. In particular, during the 1960s and 1970s, many buildings realized in reinforced concrete (RC) were designed with disregard for details in comparison to the current code specifications for seismic areas [3, 4]. In fact, a major part of such buildings is conceived for resistance mainly relating to gravity actions, assuming a minor influence of horizontal ones (e.g., seismic events). In this framework, the assessment of seismic actions is a crucial issue with the aim of identifying proper dispositions to retrofit existing buildings [5, 6] with particular care for under-designed structural systems [7]. In recent years, with the aim to enhance the seismic response of RC buildings, the system of seismic isolation adopting friction pendulum (FPS) devices proved to be among the most diffuse and effective solutions [8, 9, 10, 11]. Concerning the application to buildings constituted by RC structural frames, the major benefits relate to achieving a fundamental period not dependent on the mass of the structure placed up to the isolation level (i.e., superstructure). In particular, these devices allow significant energy dissipation capacity during the seismic motion by means of a friction mechanism [12; 13]. Moreover, with reference to post-earthquake resilience, the self-recentering capability of the devices can permit fast recovery of building functionality [14]. For the reasons mentioned above, FP devices have been used in many situations in practice to isolate both new buildings and existing ones [15; 16; 17; 18; 19]. In this framework, with the purpose of assessing the compliance of the isolation system with respect to the reliability requirements of current design codes, the performance-based seismic design (PBSD) methodology can be adopted, as widely discussed by [12; 13]. Contextually, the influence of the masonry infills on the response of RC-framed structures has been widely investigated and recognized for its relevance to overall structural behavior [20; 21; 22]. However, few studies have been devoted to understanding their interaction with the adoption of seismic isolation systems as FP devices [23; 24] with particular reference to buildings characterized by significant irregularities (in-plane, in-elevation, and related to the disposition and configuration of infills). The purpose of this paper is to explore, in mono- and bi-variate probabilistic terms, the advantages of the refurbishment using FPS with single concavity devices of one irregular RC-framed building placed in a highly seismic area of central Italy, while also investigating the interaction between the masonry infills and the FP isolation system. After the identification of the material and geometric characteristics in line with the \"knowledge levels\" philosophy [3], a suitable numerical model based on the fiber-modeling approach has been established in the SAP2000 software platform [25]. 3D non-linear dynamic analyses have been performed considering 21 natural ground motions [26; 27] with the three accelerometric components to evaluate the performance of the RC building, inclusive and not inclusive of the FP isolators. As mentioned above, the influence of the masonry infills on seismic performance has been investigated [23; 24]. In detail, the structure with the base fixed to the foundations (FB) and with the isolation system (BI) has been analyzed with and without the presence of the masonry infills. The local influence due to the interaction between the RC frame and the full and partial infill walls has been examined and accounted for during the definition of the numerical models. The outcomes of the 3D NL dynamic numerical simulations permit assessing the efficiency of the isolator devices to improve the seismic performance of the RC-framed building having both in-plane and in-elevation irregularities.