PSI - Issue 78

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

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1.9. Russo et al. (2012) The seismic behavior of five reinforced concrete exterior beam – column joints, representative of typical construction practices in Italy during the 1950s and 1960s, was experimentally investigated. The test program included four 2:3 scale specimens with straight beam bar anchorages, in which the amount of beam longitudinal and transverse reinforcement was varied, and one full-scale specimen with 180-degree hooked beam bars. All specimens were constructed using plain round bars and without transverse reinforcement in the joint core. The column axial load was not maintained constant but was varied during testing as a function of the applied lateral load. Specimens with straight beam bar anchorages experienced bond failure of the beam longitudinal reinforcement, resulting in significant strength and stiffness degradation under cyclic loading. In contrast, the specimen with 180 degree hooked beam bars exhibited beam flexural failure, with no evidence of bar slippage. Remarkably, no diagonal shear cracking was observed in any of the specimens at the end of testing, despite the absence of joint transverse reinforcement. This suggests that joint shear failure was prevented, even in the specimen with hooked beam bars, likely due to the relatively low level of longitudinal reinforcement in the beam. 2. Conclusive remarks Based on the reviewed experimental studies on the seismic behavior of exterior beam-column joints reinforced with plain bars, the following critical observations can be drawn.  Compared to joints reinforced with deformed bars, joints with plain bars subjected to cyclic loading consistently demonstrate lower energy dissipation capacity, reduced stiffness, and lower equivalent damping.  Joints with horizontal transverse reinforcement demonstrate superior seismic performance compared to unconfined joints. Specifically, they exhibit increased load-carrying capacity, higher strains in beam reinforcement, and improved energy dissipation capacity.  An increase in joint aspect ratio leads to greater energy dissipation and joint shear deformation at peak load. However, it is associated with reduced joint shear strength.  An increase in the amount of beam longitudinal reinforcement leads to a higher joint load-carrying capacity.  Beam anchorage detailing plays a fundamental role in the seismic performance of exterior joints. Inadequate anchorage significantly reduces the load-carrying capacity and alters the failure mechanism, primarily due to premature bond slip of the beam longitudinal reinforcement. In particular: - Straight beam bar anchorages consistently lead to bond failure, resulting in severe degradation of both strength and stiffness under cyclic loading. - Anchorage with 180-degree hooks within the joint core does not fully prevent bond slip in plain bars. Experimental results indicate that this anchorage detail may still result in a hybrid failure mechanism involving the formation of diagonal shear cracks, expulsion of a concrete wedge from the joint core, and buckling of column longitudinal bars and/or pull-out forces generated by the beam bar hooks. - Anchorage with 90-degree hooks is similarly ineffective in fully restraining plain bars. When beam bars are bent into the joint core, and joint transverse reinforcement is absent, tensile forces tend to straighten the hooks, causing concrete spalling from the joint. When hooks are bent outside the joint, straightening under tension induces tensile cracking in the column and buckling of column bars. These detrimental effects can be mitigated by providing sufficient transverse reinforcement in the column near the joint. Among these configurations, joints with beam bars bent into the joint core generally exhibit higher load-carrying capacity than those with bars bent into the upper columnn.  The presence of a compressive axial load on the column involves the following effects: - Axial load imp roves the joint’s ability to resist horizontal loads by enhancing the bond between the beam reinforcement and the surrounding concrete. This results in more ductile behavior, greater energy dissipation capacity, and suppression of shear cracking due to improving of the diagonal strut mechanism.