# Seismic Behaviour of CFST Space Intersecting Nodes in an Oblique Mesh
## 6 Discussion
### Concrete Strength  
Figure 15 shows the parameter curves of specimens with different concrete strength groups. As shown in Figure 15a, the concrete strength was proportional to the positive ultimate load. It had little effect on the negative ultimate load and the evolution law of the hysteresis curve. Combined with Figure 15b, it can be seen that the axial compression skeleton curve presented different evolution laws with the change in concrete strength. The peak point and the descending section of the curve of the specimen with concrete strength \(\leq\) 50 MPa were not obvious, while the curve of the specimen with concrete strength \(>\) 50 MPa had a clear inflection point, peak, and descending section, which was mainly related to the brittleness of the specimen with higher concrete strength. All components' axial tension skeleton curves of all components had little difference, and the  
Figure 14: Mises stress nephogram of components with different out of plane constraint sizes. (a) \(\dot{\omega}\) = 0.00. (b) \(\dot{\omega}\) = 0.20. Figure 13: Relevant mechanical property parameter curves of members with different out of plane constraint sizes. (a) Displacement load curve. (b) Skeleton curve. (c) Stiffness degradation curve. (d) Axial displacement-out of plane displacement curve. (e) Energy consumption curve. curves basically coincided. This was mainly because the tensile strength of concrete was weak. In the process of axial tension, almost only the outer steel tube bore the axial tension. The role of concrete is to delay the necking phenomenon of the external steel tube and improve the proportional limit and yield point of the steel tube, which is also the critical factor for the initial axial stiffness of components with higher concrete strengths to be slightly larger. According to Figure 15e, the energy consumption capacity of each specimen stage increased with the increase in concrete strength. The total energy consumption of the component with a concrete strength of 120 MPa was 31.70% higher than that of the component with a concrete strength of 30 MPa, but the improvement was not very large. After analysis, it was found that this was mainly because when the concrete strength is higher, the core concrete needs a larger constraint, i.e., the hoop coefficient is larger. For stiffness degradation, the rate of stiffness decay in the later stage of the component with higher concrete strength increases due to its high brittleness, and the residual stiffness is not much different. For the out-of-plane displacement evolution, all specimens' vertical displacement-out-of-plane displacement curves have the same shape and evolution law without apparent fluctuation, indicating that the out-of-plane constraint condition is sufficient. Except for the curve of the component with a concrete strength of 120 MPa, all other curves evolve cyclically around the coordinate (0, 0). After analyzing the results of the finite element model, it was found that this was mainly because the non-node area steel tube concrete column of the component with a strength of 120 MPa undergoes slight bending in the early loading stage, resulting in out-of-plane residual deformation at the node. However, the maximum out-of-plane displacement of all specimens was not greater than 2.5 mm, which is still within the controllable range. The strength enhancement coefficient in Table 3 was inversely  
Figure 15: Relevant mechanical property parameter curves of members with different concrete strength. (**a**) Displacement load curve. (**b**) Skeleton curve. (**c**) Stiffness degradation curve. (**d**) Axial displacement-out of plane displacement curve. (**e**) Energy consumption curve. proportional to the concrete strength, indicating that the constraint effect of the external steel tube on the core concrete gradually decreased with the increase in concrete strength. Figure 16 shows the Mises stress cloud maps of components with different concrete strengths.