PSI - Issue 78

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

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reinforcement was provided within the joint core. A constant axial load corresponding to 15% of the column’s axial load capacity was applied throughout the test. During cyclic loading, the specimen exhibited two distinct failure mechanisms (Fig. 1a). The first was yielding of the beam longitudinal reinforcement, evidenced by the formation of flexural cracks in the beam. Subsequently, diagonal shear cracking developed in the joint core. The initiation of these shear cracks acted as a structural fuse due to their brittle nature relative to other failure modes. As the diagonal cracks propagated, a progressive closure of the previously formed beam flexural cracks was observed. Concurrently, the widening of the diagonal cracks activated a secondary, low-strength failure mechanism governed by bond slip of the column longitudinal reinforcement. This mechanism is illustrated in Fig. 1b, for positive (left) and negative (right) displacement cycles.

Fig. 1. (a) Cracks formation sequence; (b) joint failure mechanisms. Braga et al. (2009).

The failure mechanisms illustrated in Fig. 1b can be described as rigid-body rotations of portions of the joint around the compressed zone of the column, oriented toward the beam side. Under positive lateral displacements, the rotation occurred about point A, whereas under negative displacements, it occurred about point B. These rotational mechanisms promoted the extension of cracking into the column regions and induced tensile bending (traction inflection) in the external longitudinal bars of the column that passed through the joint panel. Due to the absence of transverse reinforcement within the joint, these column bars were insufficiently confined, leading to the outward ejection of the concrete triangular wedge from the joint core. Notably, the hooked beam bars did not exhibit any signs of slippage and remained well anchored within the internal concrete region. Moreover, the authors emphasized that the bond slip of the column’s smooth bars contributed to the stability of the cyclic response. The low bond stress between the plain bars and the surrounding concrete limited stress transfer within the joint core, thereby preventing a significant increase in joint panel damage. 1.4. Cosgun et al. (2020) The experimental behavior of four exterior beam-column joints constructed with plain reinforcing bars, low strength concrete was investigated. The study focused on the influence of three types of longitudinal beam bar anchorages within the joint core, in the absence of joint transverse reinforcement: specimen J2 with 90-degree bent bars, specimen J3 with 180-degree hooked bars, and specimen J4 with straight bars. Additionally, a control specimen, J1-REF, was included. This specimen was designed to comply with the provisions of the Turkish Seismic Code (TSC 2007) and the Turkish Building Code (TS500) to prevent shear failure. J1-REF was also constructed using plain bars and low-strength concrete, but included ribbed stirrups as joint transverse reinforcement. A constant axial load equivalent to 10% of the column’s axial load capacity was maintained throughout the tests.