PSI - Issue 78

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

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Experimental results showed that, with the exception of specimen J1-REF, none of the test specimens reached the nominal flexural capacity of the beam section. Instead, reductions in load-carrying capacity were observed, amounting to 31.3% in J2, 41.7% in J3, and 54.5% in J4. The cyclic response of specimens J2 and J3 was governed by a combination of bond slip of the beam longitudinal reinforcement and joint shear cracking, whereas the response of specimen J4 was dominated entirely by bond slip. All three specimens reached their ultimate strength at approximately 4% drift. In contrast, specimen J1-REF sustained stable performance up to 7% drift. The significantly different responses of specimens J2, J3, and J4 underline the critical influence of beam anchorage detailing on the seismic performance of exterior joints, as they can substantially reduce the load-carrying capacity and alter the failure mode, primarily due to bond slip of the beam longitudinal reinforcement. Four exterior beam-column joint specimens were tested under cyclic lateral loading to evaluate the influence of joint aspect ratio and beam longitudinal reinforcement on seismic performance. The joint aspect ratio was varied by altering the beam depth. All specimens were constructed without horizontal transverse reinforcement in the joint core and employed plain round bars for the beam and column longitudinal reinforcement. The beam bars were anchored into the joint using 180-degree hooks. A const ant axial load equivalent to 10% of the column’s axial load capacity was maintained throughout the tests. All specimens exhibited joint shear failure, accompanied by significant concrete spalling from the exterior face of the joint and adjacent column. The authors attributed this spalling to pull-out forces generated by the beam bars hook anchorages. Analysis of the lateral load-drift response showed that specimens with larger joint aspect ratios exhibited greater energy dissipation. In terms of deformability, an increased joint aspect ratio led to larger joint shear strains at peak load. With respect to strength, a higher amount of beam longitudinal reinforcement, for a constant joint aspect ratio, resulted in greater joint shear capacity. Conversely, specimens with increased joint aspect ratios but identical beam reinforcement demonstrated reduced joint shear strength. 1.6. Liu et al. (2001) Four exterior beam-column joint specimens, reinforced with plain round bars and beam longitudinal reinforcement anchored using 90-degree hooks, were tested under cyclic lateral loading. The specimens differed in the beam bar anchorage configuration and the level of axial load applied to the column. In specimen EJ1, the beam bar hooks were bent away from the joint core, with an anchorage extension of four times the bar diameter beyond the hook. In contrast, specimen EJ2 featured beam bars bent into the joint core, with a more extended anchorage length of twelve times the bar diameter beyond the hook. Specimens EJ3 and EJ4 were identical in geometry and detailing to EJ1 and EJ2, respectively, but were tested under a column axial load corresponding to 25% of the column ’s axial load capacity. The behavior of all specimens was governed by degrading flexural performance of the beams and columns, primarily due to premature bond failure along the longitudinal reinforcement. Despite this, the joint cores of all specimens remained intact and exhibited good structural integrity throughout the tests. The seismic response of specimens EJ1 and EJ2 was dominated by concrete tensile cracking in the region surrounding the beam bar hooks, resulting from the opening action of the hooks under tension (Fig. 2a and 2b). This degradation mechanism was further exacerbated by buckling of the column longitudinal reinforcement adjacent to the joint core. Conversely, the behavior of specimens EJ3 and EJ4 was governed by tensile cracking of the concrete along the outer layer of the column longitudinal reinforcement adjacent to and within the joint core. This damage mechanism was again attributed to the combined effects of beam bar hook straightening under tension and buckling of the column longitudinal reinforcement. However, the compressive axial load in the columns delayed the onset of such premature tensile cracking, thereby resulting in significantly improved seismic performance. The maximum shear strengths attained by specimens EJ1 and EJ2, tested without axial column load, were 57% and 75%, respectively, of their corresponding theoretical flexural strengths. These results clearly demonstrate the substantial influence of beam bar hook arrangement on the strength of the joint under zero axial load. In specimen 1.5. De Risi et al. (2017)