PSI - Issue 44

Fabrizio Comodini et al. / Procedia Structural Integrity 44 (2023) 1076–1083 Author name / Structural Integrity Procedia 00 (2022) 000–000

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The connector element was modeled using beam elements with non-linear behavior, with a perfectly plastic elastic constitutive bond. An important characteristic of the coupling device concerns the possibility of replacing the connector if its yield threshold is exceeded independently of the other components. In this sense, the connector is equivalent to a fuse that is damaged to protect the systems connected to it. The transfer of the seismic shear plane forces is concentrated in the areas where the exoskeletons are present. To evaluate the resisting capacity of existing floors for these concentrated shear forces, linear analyzes were performed on a typical building floor. The results of the analyses showed that the shear forces caused by the interactions between the building and the exoskeleton do not generate tangential stresses higher than the limits allowed by the floor. 5. Results of seismic improvement analyses Preliminarily, a modal analysis was developed to identify the system's modal shapes and relative periods of vibrations within the new configuration. The main periods of vibration are: T1 = 1.01 s X direction, T2 = 0.81 s Y direction and T3 = 0.63 s Z direction. The coupling between the existing structure and external bracings has increased the composite system's stiffness and therefore led to a decrease in the oscillation periods. The increase in the stiffness of the coupled system has resulted in an increase in the shear force at the base. The global elastic stiffness ranges from 70,000 kN/m to 140,000 kN/m in the Y direction and from 30,000 kN/m to 95,000 kN/m in the X direction, while a reduction in eccentricity is observed between the central point of the stiffnesses and the center of the masses, which causes a decrease in the torque effects acting on the building. To evaluate the effects of the seismic improvement intervention on the seismic capacity, non-linear static analyzes were performed in both main directions of the building according to the indications of the technical code (NTC2018).

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Seismic Demand 100% Seismic Demand 60% Capacity building + exoskeletons Capacity building

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Fig. 9. Seismic demand Vs capacity in space Acceleration Displacement Response Spectrum (ADRS)

The results of the numerical analysis show (figure 9) that seismic improvement interventions increase the seismic capacity of the building. The increase in capacity is due to the increase in stiffness given by external bracing, these mitigate the seismic actions acting on the existing structure and delay the activation of the collapse mechanisms. The activation of the first collapse mechanisms occurs for greater global displacements, so the performance point (Fajfar 1999) between demand and capacity is obtained in correspondence with greater seismic demand. Furthermore, the elimination of local vulnerabilities and the reduction of torsional effects allow the building to achieve maximum elastic displacement. The seismic improvement interventions adopted make it possible to reach the level of seismic improvement corresponding to a Capacity / Demand ratio of 0.60 (table 3). The value is conditioned by the shear mechanisms which are activated in the transverse direction. The bending mechanisms are activated for similar seismic acceleration values.