Numerical Seismic Performance of a Beam–Column Connection in Hospital RC Buildings Retrofitted with Dissipative Bracing: Implications for Behaviour Factor Evaluation

The seismic retrofit of existing reinforced concrete (RC) buildings equipped with dissipative bracing systems requires not only a global performance-based assessment, but also a rigorous verification of the local behavior of critical structural connections. In this context, the present study focuses on the numerical seismic performance of a beam–column connection extracted from a retrofitted RC hospital building located in Italy. The investigated joint represents a central node where two orthogonal steel bracing systems converge and transfer seismic forces to an RC column strengthened with heavy steel jacketing and anchorage devices. A detailed three-dimensional finite element model of the connection is developed using solid elements for concrete and steel components, explicit modeling of reinforcement bars, bolts, and anchor rods, and advanced nonlinear constitutive laws for both materials. Two modeling strategies are considered, including the explicit simulation of contact interfaces between steel components, in order to capture local stress redistribution and potential interaction effects. The connection is subject to seismic demand derived from the global structural analysis, corresponding to different values of the behavior factor, thus ensuring consistency between global design assumptions and local verification. The results highlight the progressive activation of nonlinear mechanisms within the steel components, the development of cracking and compression damage in the concrete core, and the preservation of a clear hierarchy of resistances under design-level seismic actions. The numerical outcomes allow a critical discussion on the role of local connection behavior in supporting the global dissipative strategy and provide quantitative insights into the evaluation of the behavior factor from a local-response perspective. The study emphasizes the importance of detailed connection-level analyses in the seismic retrofit of strategic facilities and supports a more consistent integration between global performance objectives and local structural design.