PSI - Issue 78

Francesco Nigro et al. / Procedia Structural Integrity 78 (2026) 1537–1544

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1. Introduction The European Commission (2013) has recently stated that Europe is characterised by the highest land use proportion among all the continents. Hence, civil engineers are increasingly required to deal with the rehabilitation and reuse of existing buildings, especially reinforced concrete (RC) buildings which represent a large proportion of the built environment. In principle, rehabilitating existing buildings to make them adequate to modern standards is a complex task which can be addressed by adopting various types of structural intervention techniques, as extensively described in the fib bulletin 24 by Fardis et al. (2003). More specifically, when the seismic displacement demand to the existing structure is exceedingly large, the installation of a new concentrically braced steel structure is an effective system to improve the seismic performance. On the other hand, limiting the force demand could also be important because of the limited force capacity of the existing members and joints. In such cases, the new bracing system can incorporate a yielding sacrificial element, thus becoming effectively a system adding both stiffness and (hysteretic) damping while limiting the maximum forces transferred to the existing structure. For instance, Mazzolani et al. (2009) and Della Corte et al. (2013, 2015) have tested experimentally and studied analytically the use of both buckling-restrained braces and eccentric braces with short links on real existing RC structures, highlighting the potential failure modes and outlining possible solutions. Mazzolani et al. (2023) have also shown an application of the eccentric bracing system with short links to a real-world case. As an alternative to the braces installed inside the existing frame structure, the installation of a new bracing system can be carried out from outside of the existing structure in such a way that the new steel bracing structure can be efficiently combined with an energy retrofit of the building, as recently outlined by Marini et al. (2022) and D’Agostino et al. (2024) . Usually, the design of the new steel bracing system is carried out by means of a structural analysis model including the bare RC frame. However, existing RC structures are often characterized by relatively stiff masonry cladding and partitions. It is known that such masonry panels might have a non-negligible effect on the structural response, being the initial stiffness of the existing frame certainly affected to a significant extent by the masonry cladding. Besides, uneven failure of the masonry panels belonging to different stories and different spans might also lead to significant effects on the inelastic response, as demonstrated by Chelapramkandy et al. (2025). Therefore, it is of interest to investigate how the presence of the panels might affect the structural performance of existing RC frames with the addition of new steel braces. Nevertheless, most of the existing studies assume that there is no cladding contribution into the structural model. Hence, an archetype RC structure presented by Di Domenico et al. (2023) was adopted as a representative study case. After seismic retrofitting with steel exoskeleton, the retrofitted structure was analysed by means of static non linear analysis to assess the seismic performance including the effects of masonry infill panels. 2. The archetype RC frame and its seismic performance For the scope of the present study, the archetype 6-storey residential RC building described by Di Domenico et al. (2023) was adopted. The building is ideally located in a high seismic hazard city in central Italy ( L’Aquila ). The building structure is characterized by a rectangular blueprint (Fig. 1), with five spans in the X direction and three spans in the Y direction, and with an interstorey height equal to 3.40 m for the first storey and 3.05 m for the upper stories. The simulated design of this structure was carried out according to the technical code adopted in Italy at the construction time (R.D. 1939). As reported by Del Gaudio et al., (2015), the region was classified as “seismic category II”, corresponding to a “moderate” seismic design level at the time of the construction (applying to each frame an equivalent storey force equal to the 5% of the weight). A detailed description of the structure and its simulated design are provided by De Risi et al. (2023) and Di Domenico et al. (2023). The nonlinear model of the existing structure was developed using a lumped plasticity approach, having independent zero-length elements for each plane of flexure, to represent the moment-chord rotation behaviour at member ends. More specifically, each zero-length element follows an empirical predictive model proposed by