PSI - Issue 78

Gaetano Della Corte et al. / Procedia Structural Integrity 78 (2026) 199–206

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1. Introduction Steel bracing has long been used for the retrofit/upgrading of existing reinforced concrete (RC) moment frame building structures. Braces are mainly used in cases where the need for upgrading of the existing RC structure requires an increase of the system lateral stiffness and resistance. In fact, the need for a reduction of the displacement demand to the original structure implies the need for an increase of the system initial (elastic) stiffness, and the need for a reduction of the ductility demand implies the need for an increase of the system resistance. There are two forms of bracing systems for moment-resisting RC frames: 1) internal bracing and 2) external bracing. In the first case, the new braces are located inside the existing frame. In the second case, the new braces are located externally to the existing building, as a completely new structural system. The first form of bracing is sometimes referred to as “endoskeleton”, while in the second case the system is sometimes called “exoskeleton”. In both cases, one main aspect to consider in the design is the connection between the new bracing system and the old RC structure. In the case of braces located inside the existing RC frames, a typical solution consists in connecting the brace members to a steel moment frame, which is in turn connected to the existing RC beams and columns (Lee et al . (2020)). Classical steel connectors are welded on the steel side, while anchors are post-installed into the existing RC structure. A simpler solution was also investigated, with the new bracing system connected only to the RC frame joints at brace ends (Della Corte et al. (2015)). In the case of braces external to the existing frame, the connection depends on the type of bracing (parallel or perpendicular to the existing façades). In general, as for the internal bracing system, steel connectors can be attached to the steel members and post-installed connectors can be made onto the existing RC frame, while the gap between the two structures can be finally filled with high-strength mortar. Such a system was investigated by Lee et al. (2023), who proposed a steel moment frame without braces for minimizing the forces transmitted by the new steel frame onto the old RC beam-column joints and foundation. The fundamental role of the existing RC beam-column joints and foundations in selecting an upgrading solution by means of steel braces was also highlighted by Nigro et al. (2024a and 2024b), especially referring to construction and ecologic costs. For design purposes, in the framework of the Eurocodes, connections between steel and concrete should satisfy the requirements provided by EN 1992-4 and EN 1993-1-8, besides to the general rules and criteria for seismic actions provided by EN 1998-1-1. Particularly, Annex C of EN 1992-4 provides specific rules for the design of steel concrete fasteners in case of seismic actions. EN 1993-1-8 provides general rules to calculate the resistance, the rotational stiffness and an empirical approximation of the entire moment-rotation curve of steel connections, based on more than forty years of research on the “component method”. However, EN 1993-1-8 provides rules only for connections with unstiffened plates, though stiffeners were considered by Wald et al. (2014) and Khulman et al. (2014) in research investigations addressed to extend the component method to steel-concrete connections. On the other hand, EN 1992-4 does not provide rules to predict the entire moment-rotation response of a connection. Therefore, within the context described, this paper presents a theoretical study on the prediction of the moment rotation-axial force response of connections with a stiffened plate, with a focus on the extension and application of the component method of EN 1993-1-8. The study is especially devoted to consider the effect of plate stiffeners and prying forces on both the stiffness and the resistance of the connection. Within the context of the component method, the requirements of EN 1992-4 are also considered, for the calculation of the resistance of tension anchors involving any of the potential concrete failure modes. 2. The proposed analytical model for steel-to-concrete connections with stiffened plates 2.1. The background “component method” and the needs for extensions to hybrid steel-concrete connections The “component method” is stipulated by EN 1993-1-8 for predicting the resistance and the stiffness of steel connections. Empirical rules are also provided for the connection rotation capacity. The method assumes a connection model made of rigid bars and springs. The springs are characterized by elastic-perfectly plastic constitutive laws, with each spring representing a “component”, i.e. a source of deformation for the connection. Column base plate connections are the only type of hybrid steel-concrete connection explicitly covered by EN 1993-1-8. With reference to the general topological features of the connection, the main limiting assumption for the application of the currently codified method is that the base plate should be unstiffened. It can be easily foreseen that