PSI - Issue 78

Roberto Nascimbene et al. / Procedia Structural Integrity 78 (2026) 193–198

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The threaded bars are designed to resist axial forces, maximizing their structural efficiency. To mitigate the risk of cracking in the grout due to tension-induced strain, each bar is encased in a 3 mm-thick rubber sheath within the grout zone, which reduces the mechanical interlock and limits stress concentrations. The anchoring configuration varies based on the location of the joint. For internal joints (cruciform connections), the bars are anchored either by overlapping with the continuation bars or through mechanical couplers. In contrast, for external joints (T-type configurations), the bars are secured using bearing washers or by direct bond to the surrounding concrete. The beam itself is supported on a standard reinforced concrete corbel, which includes a 1 cm neoprene pad to accommodate relative displacements and reduce local stresses. Importantly, the corbel is designed to resist shear only in one direction. In scenarios involving reversal of support reactions (i.e., during seismic loading), the shear resistance is deliberately transferred to the reinforced concrete section of the beam rather than relying on the corbel. This conservative design approach minimizes reliance on friction and enhances structural safety. The adopted structural module consists of a three-span frame, with hinged connections at the outer columns and partially restrained joints at the internal columns. This configuration significantly enhances global stiffness and offers better control over second-order (P-Δ) effects and interstorey drifts at the damage limit state. Compared to traditional fully pinned frames, this approach allows the use of smaller column sections, optimizing both material usage and construction effort. Furthermore, the system is compatible with the integration of additional damping devices, such as friction or viscous dissipators, placed at hinged joints to enhance seismic energy dissipation. To investigate the structural response of this configuration, a case study was carried out on a three-storey frame with three spans of 12 m, 10 m, and 8 m, and storey heights of 4.0 m (ground floor), 3.5 m (first floor), and 3.5 m (second floor). The structural model was developed in MIDAS GEN, using nonlinear beam elements with distributed plasticity. Given the specific nature of the partial fixity at internal joints, the composite beam-column connection was simulated using a system of vertical rigid links that connect parallel nonlinear springs, each representing the combined behavior of the embedded threaded bars and the grouted interface (see Figure 1). A unilateral constraint at the corbel was also included to simulate realistic flexural interaction at the interface.

Fig. 1. Three-span frame with hinged external connections and fixed internal joints.

The design was tailored for a site in Gemona del Friuli (Udine Province), characterized by soil type C and a peak ground acceleration (PGA) of 0.25g. The original design employed column sections of 70×50 cm at the ground floor and 50×50 cm at the upper floors. However, the P-Δ verification revealed that, under seismic loading, the second and third-storey columns do not satisfy stability requirements, while the ground floor columns require amplified action checks. Due to the necessity of applying a uniform amplification factor (θ) across the entire structure to maintain equilibrium, the assessment demonstrated the need to redesign the entire column system. The final verification indicated that compliance with code requirements could be achieved by increasing the column dimensions to 80×80 cm at all levels.