## 3 Results and Discussion
### Load Deflection Behavior of UHPFRC Joints
Therefore, the favorable effects of the axial compression load level on the shear strength should be reasonably considered. When it exceeds a certain range, any increase in the column compression load will extend the compression zone further along the joint boundaries. As a result, the inclination of the diagonal compression strut will decrease, as will its contribution in resisting the shear force [21]. The maximum shear strengths for specimens J-1 and J-3 were similar, but the shear bearing capacity of J-4 was reduced by 7.5% compared to J-2, the main reason being that the UHPFRC strength of specimen J-4 was lower than that of specimen J-2. [PERSON] et al. [22] and [PERSON] et al. [23] investigated the influence factors for the shear behavior of RC beam-column joints, and the results demonstrated that the concrete compressive strength was the main governing parameter for the joint shear strength. Furthermore, the shear bearing capacity of the UHPFRC joint was mainly dependent on the UHPFRC strength as well. However, with fewer stirrups configured in the joint core, the stirrups yielded before the maximum shear strength. The shear force in the joint was mainly carried by the UHPFRC, rather than the stirrup, at the peak shear force moment. The joint type is another factor affecting the shear carrying capacity of beam-column joints. It can be seen from the figure that the shear strength of the interior joints was higher than that of the exterior joints, which can be attributed to changes in the UHPFRC diagonal strut. Because a vertical anchoring portion of the beam bar was present in the joint, the diagonal compression strut in the exterior joints had a smaller cross-section and larger inclination angle than that of the interior joints. In this research, the shear strength ratio of exterior to interior joints is approximately 80%, and similar behavior has been observed in normal and high-strength concrete beam-column joints [21; 24; 25]. A similar result for NC joints was reported by other researchers, in which the shear strengths of exterior joints are approximately 80-90% of those of interior joints [24]. Figure 7 illustrates the typical curve of the stirrup strain versus beam tip load. The figure indicates that, prior to UHPFRC cracking in the joint regions, the stirrup strains of were small, generally less than 800 \(\times\) 10\({}^{-6}\). The stirrup strains can be restored during unloading. Following cracking, the stirrup strains in the joint core increased rapidly and maintained residual deformation when unloading. During the first cycle of 2\(\Delta_{y}\), the stirrup strains in the joint core rapidly increased to 2100 \(\times\) 10\({}^{-6}\) to 2600 \(\times\) 10\({}^{-6}\), indicating that the stirrup was close to yield, and the strains could recover partially after unloading. In the reverse direction loading, the stirrup yielded when the load approached the peak. The results indicate that the UHPFRC exhibited a higher deformation capacity under tensile stress owing to the use of steel fiber, which can enhance ductility and improve the deformation capacity of beam-column joints. Figure 6: Envelope curves of shear force versus shear deformation angle of joint cores: (**a**) exterior joint and (**b**) interior joint. When the load neared the peak load, the stirrup in the joint core yielded owing to the low stirrup ratio, so the effect of resisting the stirrup shear force in the joint core was fully utilized. Furthermore, with the stirrup restraint effect in the joint core, shear cracks in the joint appeared late, and the shear carrying capacities of the joints were improved. However, when the stirrup ratio exceeds its upper limit value, most stirrups do not yield when UHPFRC crushing occurs in the joint core, and the effect of resisting the shear cannot be fully achieved, which would result in a higher value of the calculated shear carrying capacity.