PSI - Issue 78

Paolo Ielpo et al. / Procedia Structural Integrity 78 (2026) 1024–1031

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5.1. Results with the retrofit system on the calibrated model The SPEAD strengthening system was integrated into the calibrated numerical model of the T4 joint, adhering to the specified application conditions. The same numerical analyses performed on the unreinforced joint were replicated, maintaining identical loading steps and boundary conditions. The results indicate an increase in maximum strength of 35.55%, while the initial stiffness and overall ductility remain substantially unchanged compared to the original configuration. Specifically, the key parameters of the strengthened joint show a maximum force (Fmax) of 58.26 kN, a yield drift of 1.59%, and an ultimate drift of 5.02%, defined as the point at which the strength decreases by 20% relative to the maximum value (Fig. 6).

Fig. 6. (a) retrofitted FEM model and (b) comparison of load – drift curves: SPEAD retrofit system versus calibrated numerical model and as-built configuration. 6. Conclusions The numerical analyses conducted in this study aim to develop a local strengthening technique that integrates the enhancement of seismic capacity in existing buildings with the adoption of sustainable solutions. The objective is to improve the seismic performance of reinforced concrete structures while containing economic, environmental, and social costs. The SPEAD (Steel Plate Energy Absorption Device) retrofit system is a low-impact solution involving the external application of a steel plate to beam – column joints in reinforced concrete structures. This installation does not interfere with slabs, infills, or transverse beams. A key strength of the solution lies in its minimization of invasiveness and intrusiveness, thereby preserving the daily activities and social functions of building occupants. The study is based on the development of a nonlinear 3D finite element model, calibrated using cyclic experimental tests performed on a beam – column joint (T4) at the University of Basilicata laboratories. The seismic performance of the beam – column joint was assessed by comparing the numerical model before and after the local retrofit intervention, with particular attention to changes in strength, stiffness, and ductility. The results are promising: the T4 specimen exhibited approximately a 36% increase in strength, while ductility and stiffness values remained substantially unchanged. Another fundamental aspect concerns the effects of the device on structural