GNGTS 2021 - Atti del 39° Convegno Nazionale

347 GNGTS 2021 S essione 2.2 Selected SDOF configurations were assessed with the support of nonlinear static analyses (push-over analyses), that were carried out in SAP2000 software. The results are analysed in terms of base-shear R / top displacement δ . For sake of clarity, all the parametric estimates are proposed in non-dimensional form. The measured base shear R at any instant of the analysis is normalized to the RC frame shear resistance, R 2 , while the corresponding top displacement δ is normalized to the lateral displacement leading the RC columns to the first yielding configuration, δ y,2 . The attention is thus focused on the response of the RC frame, the steel exoskeleton and the hybrid system. In Fig. 1 (b) it can be seen that the un-retrofitted BLD system has a brittle behaviour with no residual capacity. The EXO-1 solution represents the typical application of a steel bracing system that is expected to optimally react to the input seismic loads, given its limited stiffness K 1 and relatively high resistance. The stiffness of the exoskeleton alone can be clearly perceived from the 0-P 3 segment in Fig. 1 (c) . A strong dissipative contribution would be expected from the exoskeleton itself, but due to the limited stiffness, the potential plastic deformations and dissipative phenomena of the steel bracing system are activated only once the BLD structure is already collapsed. From a seismic design point of view, the EXO-1 retrofit intervention is thus not successful and the hybrid assembly is still strongly sensitive to the initial BLD features. Moreover, the higher stiffness of the system leads to increased input seismic loads. In EXO-2 system ( Fig. 1 (d) ) it is possible to perceive that the hybrid solution can take strong benefit of the exoskeleton features, thus achieving a relatively higher stiffness (segment 0-P 0 ) and maximum resistance. It is however possible to see that this solution does not always preserve the RC building from a potential brittle collapse, thus vanishing the benefits due to the elasto-plastic steel members. The hybrid structure can work efficiently for limited deformations only (segment P 1 -P 2 ). Fig. 1 (e) shows the EXO-3 solution with sliding devices at the base of RC columns. In this system, the stiffness (segment 0-P 1 ) and resistance (P 1 ) parameters of the total hybrid assembly fully reflect the behaviour of the steel exoskeleton alone. In other words, the hybrid solution assumes the structural features (and thus benefits) of the steel exoskeleton. Analysis of a 2D multi-storey RC frame In support of the outcomes of SDOF investigations, the design concept is also applied to a plane multi-storey RC frame (MDOF) simulating an existing building. The RC frame is a two- bay, four-storey frame, with 5m wide spans and 3m high floors (R ck 300 the resistance class for concrete and FeB44k for the steel reinforcement). The beams (30 x 45 cm 2 ) are loaded with dead load and additional permanent and imposed loads. The columns have section 65 x 45 cm 2 at the 1 st and 2 nd floor, and section 50 x 45 cm 2 at the 3 rd and 4 th floor. Plastic hinges are assigned to the ends of beams (M3 hinges) and columns (interacting PMM hinges; base columns have an additional flexural hinge at the base section). The frame is analysed with SAP2000 software. Similarly to the case of the SDOF system earlier investigated, three different FE numerical models are developed: • MOD_00: existing RC frame; • MOD_01: RC frame connected to a traditional exoskeleton system (with steel members for the bracing system that yield simultaneously at different storey levels, considering only the tensile braces active); • MOD_02: RC frame, with a cut at the base of the columns (for the installation of the sliding devices) and connected to a steel braced exoskeleton (in practice the sliding devices can be placed more easily in the middle of the base columns). Just the bracing members of the 1 st floor are expected to yield and a BRB (Buckling Restrained Brace) is proposed, while all the other steel members of the exoskeleton are expected to remain elastic and stiff under the imposed seismic loads. The plasticization of upper-floor diagonals does not