GNGTS 2021 - Atti del 39° Convegno Nazionale

349 GNGTS 2021 S essione 2.2 columns is observed. In order to avoid the collapse of the hinges of the RC building, the solution of MOD_02 is then the most suitable and efficient. The overall resistance of the hybrid solution is significantly enhanced, compared to the un-retrofitted RC frame and it can be easily calibrated at the design stage of the exoskeleton, independently from the existing structure. Design procedure In order to draft a practical guideline for the design of the proposed hybrid solution (RC frame with sliding devices + exoskeleton) some basic steps to follow are herein summarized. The overall design method is based on non-linear static analysis (pushover) method, in order to account for the non-linear behaviour of both the existing RC frame and the newly introduced steel exoskeleton. In this context, the fundamental steps can be summarized as follows: 1. Analysis of the existing building. The capacity curve needs to be plotted in ADRS format, where the capacity of the building can be checked, based on the given response spectrum at Ultimate Limit State (ULS). 2. Fix the desired stiffness of the exoskeleton and its resistance. They are regulated by the BRB element at the ground floor. The stiffness of the upper storeys must guarantee very limited inter-storey drifts (max 0.5%) and a totally elastic behaviour of the braces. 3. The ultimate displacement capacity of the exoskeleton (based on the current code in the designer’s country) must be calculated. To optimize the intervention, this parameter should be approximately in the order of 5-10 times the elastic displacement. 4. The non-linear static (pushover) analysis is carried out on the hybrid structure (RC frame with sliding devices at the base + exoskeleton). The capacity of the system is positive as far as the structural elements of the upper floors of the exoskeleton remain elastic, while the RC building is nearly undamaged. 5. Final check (also with the use of ADRS format – Fig. 3 ). If all the requirements are satisfied, the design choices can be considered appropriate. On the contrary, the iterative procedure restarts from step #2. Fig. 3 - ADRS format with the Design response spectrum, three of the possible different options for the design of the exoskeleton and the capacity curve of the existing RC frame