## 3 Results and Discussion  
### Cracking Patterns  
The failure modes of the nine tested specimens under reverse cyclic loading were found to be similar. Firstly, a flexural crack appeared in the beam adjacent to the joint region; then, diagonal cracks appeared in the joint core; finally, shear failure occurred in the joint core. The failure processes consisted of four stages: initial crack, penetrating crack, ultimate carrying capacity, and failure stage. At the beginning of the first stage, several flexural cracks appeared in the beam. When the cyclic load reached 50% to 75% of the maximum peak load, the first diagonal crack with a small width and length appeared in the diagonal direction of the joint regions. The UHPFRC strains in the joint core were 282 \(\times\) 10\({}^{-6}\)-396 \(\times\) 10\({}^{-6}\) at the initial crack moment. In the reverse direction loading phase, a diagonal crack was observed along another diagonal. Compared to reinforced high-strength and NC beam-column joints, reinforced UHPFRC beam-column joints exhibit a higher initial cracking strength. Average shear stress value (\(V_{j\mathrm{cr}}\)/(\(b_{j}\)\(h_{j}\))) of UHPFPC joint is 3.84 MPa when cracks appear in the joint area. The average crack shear stress value of total 22 NC joints from literature [5; 18; 19] is 2.60, which is 32.3% smaller than that of UHPFPC joint. This may be explained by the fact that UHPFRC has a higher tensile strength and ultimate tensile strain than high-strength concrete and NC without steel fiber. The tests demonstrated that the stirrup strains in the joint core were small, indicating that its  
Figure 3: Test setup: (**a**) exterior joint and (**b**) interior joint. function during the stage was smaller, which means that the joint shear force was primarily resisted by UHPFRC. The initial crack shear strengths of all of the same joint types were similar, indicating that the initial crack shear strength of the joint depends primarily on the UHPFRC tensile strength, rather than the stirrups in the joint core. Following cracking, with an increase in displacement and cycle times, the initial crack along the diagonal direction continued to elongate. A number of small cracks were observed along two diagonal directions of the joint core with an increase in loading, which divided the UHPFRC joint core into several diamond-shaped sections. The width of the two main diagonal cracks in the orthogonal directions continued to increase, with the largest crack reaching a 0.3 mm to 0.5 mm width. The two main diagonal cracks propagated towards the lower and upper corners of the joint and eventually passed through the entire joint regions, and this stage is referred to as \"penetrating crack\". With a further increase in displacement, the load-carrying capacity of the joint increased within a small range, up to the maximum load, and the specimens entered the third stage. During this stage, the joint cracks became unstable and propagated up and down into column. The crack width increased significantly, and the shear deformation of the joint regions multiplied. Some steel fibers in the main cracks were pulled out, followed by some UHPFRC spalling at the intersection of the two orthogonal diagonal directions. The stirrup strains increased rapidly, and the stirrups in the joint core reached their yield before the load reached its maximum. Therefore, the function of stirrup is greater during the second stage than the first stage. During this stage, the UHPFRC and stirrups resisted diagonal tensile stresses, while the UHPFRC resisted diagonal compressive stresses. In the final stage, the cracks continued to propagate and were accompanied by joint bulging in both the in-plane and out-of-plane directions, while the shear deformation of the joint regions increased further. The bulging heights were approximately 2 to 5 mm in the in-plane direction. The joint bulging can be attributed to an increase in volumetric strain, resulting from a lack of confinement of the joint core.