PSI - Issue 78

Giada Frappa et al. / Procedia Structural Integrity 78 (2026) 89–97

90

1. Experimental tests on beam-column joints available in the literature This section presents the largest possible collection of experimental tests available in the literature on exterior RC beam-column joints reinforced with plain bars, as made for interior RC joints by Di Marco et al. (2023). The following subsections summarize the main research findings in order to provide a comprehensive understanding of the behavior of such joints under seismic actions. These findings are useful for developing effective retrofit solutions for RC buildings constructed with smooth bars, as in Frappa et al. (2022). 1.1. Adibi et al. (2018) In this study, the experimental behavior of two exterior joints differing only in the column axial load ratio, which was 7% for specimen SC1 and 15% for specimen SC1-1, was investigated. The joint subassemblies reflected the typical deficiencies of non-seismically designed RC structures, with plain round bars, no hoops in the joint core and beam longitudinal bars anchored with 180-degree hooks. A deep vertical crack developed at the beam – joint interface in both specimens. However, the higher axial load in SC1-1 inhibited the formation of diagonal shear cracks in the joint core. Conversely, diagonal shear cracking occurred in SC1, which, at failure, exhibited concrete wedge spalling and crushing. Specimen SC1-1 demonstrated a higher load-carrying capacity compared to SC1. Nevertheless, its behavior was also governed by slippage of the beam bars, as it did not reach its nominal flexural capacity. Regarding the hysteretic behavior, specimen SC1 (with lower axial load) exhibited significant pinching and rapid strength degradation. The authors attributed the pinching to bar slippage and shear failure in the joint region, while the strength degradation was likely due to the P – Δ effect . 1.2. Bedirhanoglu et al. (2010) Nine exterior beam-column joints constructed with plain reinforcing bars and low-strength concrete were tested under cyclic loading. All specimens featured 90-degree anchorages of the beam longitudinal reinforcement within the joint core. In two of the specimens, the hooks of the top and bottom beam bars were welded together to assess the effectiveness of a potential strengthening technique. The test variables included the column axial load (ranging from 0% to 50% of the axial capacity) and the joint transverse reinforcement ratio (ranging from 0.00% to 0.32%). Under cyclic quasi-static loading, the specimens with welded bars demonstrated an average peak strength 33% higher than those with conventional 90-degree bent anchorages. In specimens with standard 90-degree hooks, performance was primarily limited by bond slip of the beam longitudinal reinforcement within the anchorage region. All specimens exhibited diagonal shear cracking within the joint core. For drift ratios exceeding 4%, joints with welded bars experienced more severe damage, attributed to increased shear demand resulting from enhanced anchorage performance. Despite this, none of the specimens exhibited complete bond failure. In all cases, the beam and column elements failed to reach their nominal flexural capacities, due to either bond slip or, in the case of welded anchorage specimens, premature joint shear failure. Specimens provided with horizontal joint hoops developed narrower diagonal cracks, exhibited higher load carrying capacity, experienced larger strains in the beam reinforcement, and demonstrated enhanced energy dissipation capacity compared to those without joint reinforcement. Regarding the influence of column axial load, an increase in energy dissipation was observed with increasing axial load. While strength also improved with higher axial load, it was found to be a less significant factor than the joint transverse reinforcement ratio. 1.3. Braga et al. (2009) The experimental behavior of an exterior beam-column joint, designated T23-1, designed for gravity loads and subjected to cyclic lateral loading, was investigated. Specimen T23-1 was constructed using low-strength concrete and plain reinforcing bars, with beam longitudinal reinforcement anchored using 180-degree hooks. No transverse