# Seismic Behaviour of CFST Space Intersecting Nodes in an Oblique Mesh
## 6 Discussion
### Section Steel Content  
The curve diagrams of mechanical performance parameters for components with different steel contents are shown in Figure 11. As shown in Figure 11a, the steel content significantly influenced the hysteresis curve shape of the component. The hysteresis curve of the specimen with a steel content of 0.05 was shuttle-shaped, with a smaller area enclosed by the hysteresis loop, indicating a weaker energy dissipation capacity. Combined with Figure 11e, it can be seen that although the energy dissipation of the specimen with a steel content of 0.05 increased step by step, the increasing rate was relatively small compared to other components. This was mainly due to the smaller hoop coefficient, which cannot provide sufficient hoop restraint for the circumferential expansion of the steel tube on the core concrete, resulting in local bulging. The total energy dissipation of the specimens with steel contents of 0.20 and 0.25 was increased by 94.17% and 149.43%, respectively, compared to the specimen with a steel content of 0.05. This was partly due to the significant improvement of the tensile and compressive properties of the component with the increase in steel content and partly due to the nonlinear increase in the out-of-plane displacement of the specimens with steel contents of 0.20 and 0.25 in the last cycle, increasing in the vertical displacement of the nodes. As shown in Figure 11b, the steel content was positively correlated with the positive and negative ultimate loads and the overall stiffness of the component. The positive ultimate load of the specimen with a steel content of 0.25 was increased by 87.33% compared to that of the specimen with a steel content of 0.05, which was a significant improvement. However, it only increased by 3.21% compared to the specimen with a steel content of 0.20. This was partly due to the insufficient compressive strength of the core concrete and partly due to the insufficient out-of-plane restraint of the specimen with a steel content of 0.25, resulting in a sharp drop in the skeleton curve in the later stage of the loading cycle. As shown in Figure 11c, the ratio of residual stiffness to initial stiffness of the specimens with steel contents of 0.20 and 0.25 in the positive direction was 0.2712 and 0.2406, respectively, indicating that the larger the steel content, the greater the rate of stiffness decay in the later stage due to the sharp increase in the out-of-plane bending moment and displacement. Therefore, when aiming to improve the mechanical performance of the component by increasing the steel content in the structural design, larger out-of-plane restraints should be set to utilize the material properties fully. Figure 10: Mises stress nephogram and overall displacement nephogram of specimens with different plane angles. (**a**) \(2\alpha=20^{\circ}\). (**b**) \(2\alpha=70^{\circ}\). (**c**) \(2\alpha=90^{\circ}\). Figure 12 shows the Mises stress and compressive damage cloud maps of components with different steel contents. It can be observed that the specimen with a steel content of 0.05 exhibited a bulging and tearing failure at the end of the lower limb steel tube, while the specimens with steel contents of 0.20 and 0.25 failed due to instability caused by out-of-plane constraint failure. In the former case, the maximum stress occurred in the lower limb steel tube's middle and end regions, and the concrete's compressive damage reached its maximum value in these regions. This was mainly because, under axial compression loading, the lateral expansion of the core concrete at the end was restrained by the hoop effect, resulting in a multi-directional high-stress state in which the steel tube is subjected to axial compression and hoop tension, ultimately leading to the crushing of the core concrete at the end and the tearing failure of the outer steel tube. In the latter two cases, the maximum stress occurs on the concave side of the elliptical connecting plate, mainly caused by the failure of out-of-plane constraint during the later loading stage.