# Seismic Behaviour of CFST Space Intersecting Nodes in an Oblique Mesh
## 3 Finite Element Model  
### Model Building and Boundary Condition Determination  
The existing finite element models for the OMIN have certain limitations. The steel components, such as steel tubes, reinforcing rings, and elliptical connecting plates, are all modelled using shell elements. In Abaqus, shell elements are two-dimensional finite elements commonly used to analyze thin shell structures, such as metal sheets and plastic films. As the name suggests, shell elements are shell-shaped with a thin thickness defined by two nodes. The applicability of shell elements depends on their geometric shape. Generally, shell elements are suitable for the following situations: (1) structures with minimal thickness. Very thin structures such as wall panels, roofs, and panels; (2) a structure with a specific curvature. Shell elements are suitable for structures with large curvature; (3) the shell element is one of the best choices for planar stress state structures; The direction of action of the load acting on the structure is usually in one of the two principal planes. It should be noted that the geometric shape and thickness of shell elements should match the actual situation of the structure; otherwise, the calculation results will be inaccurate. In summary, the applicable conditions for shell elements are that the one-dimensional size (thickness direction) is much smaller than the other two-dimensional sizes (height and  
Figure 1: Oblique Mesh Intersecting Nodes Structure (\(2\alpha\) is plane oblique angle; \(\beta\) is space oblique angle). (**a**) Front view. (**b**) Lateral view.  
width directions), and the deformation perpendicular to the thickness direction can be ignored. However, the steel components such as reinforcing rings, elliptical connecting plates, and stiffening ribs do not meet the above conditions, and the deformation in the thickness direction is also a research focus. Therefore, this paper establishes a three-dimensional solid model (C3D8R) using ABAQUS_2022 finite element analysis software. To ensure the accuracy of the model calculation results, the model was divided into structured meshes using meshing techniques, as shown in Figure 2. Considering that the specimens are prone to bulging and failure in the node area during the experiment, the mesh in the node area was refined.  
Due to the tangential bond-slip between steel tubes and concrete in practical situations, cohesive elements are used to simulate the bond-slip effect in the tangential direction, while hard contact is used in the normal direction to prevent element penetration. In the actual situation, two steel columns on the same side of the connecting plate are connected by groove welding, and the connection between the node and the upper and lower limb steel tube concrete columns is achieved by inner lining pipe welding. Therefore, the contact between all steel components is set as a tie constraint in the model. Axial displacement is applied along the axis of the steel tube at the top of the steel tube to extract the load data for the steel tube and the core concrete by establishing a common reference point. To ensure that the steel tube is subjected to zero bending moment at the bottom during the experiment, a ball joint is used to connect the bottom, as shown in Figure 2e. When the component undergoes local bulging or significant lateral deformation, it can be considered a failure.  
Figure 2: Finite Element Model. (**a**) Overall model. (**b**) Mesh division of external steel pipes. (**c**) Mesh division of core concrete. (**d**) Circumferential constraints. (**e**) Backing plate and spherical hinge.