## 2 Introduction of Annular Cable Supported Grid Structure and Education City Stadium  
The annular cable supported grid structure is a new type of large-span space structure that uses rigid grids to replace the flexible cables at the top chord of the cable truss spokes. It includes a peripheral support structure with an upper single-layer grid and lower cable strut system. The upper chord of the structure is a rigid single-layer grid with a large opening that usually does not have strong vertical stiffness and bearing capacity. Therefore, it is essential to establish the pre-tension load in the cable strut system to make the whole structure rigid, provide vertical support for the upper structure, and greatly improve the bearing capacity of the whole structure. Meanwhile, the horizontal component of the radial cable force can be balanced by the upper grid or compression ring, which means that the cable net, upper grid, and compression ring form a self-balancing system. This kind of structure uses a tension cable net as the main load-bearing system, which allows full play of the high-strength material characteristics of cables, greatly reducing the burden on the lower supporting system and increasing the span in an economic and effective manner. Existing annular cable supported grid structures are usually divided into three categories according to the pre-tension cable balancing mode: upper reticulated shell, outer compression ring, and inner compression ring balancing radial cable force, as shown in Figure 1.  
The roof of Education City Stadium is a typical annular cable supported grid structure; the length of the whole structure along the long axis is 225 m, the length along the short axis is 196 m, and the maximum cantilever span is about 55 m. The cable strut system is composed of 56 straight radial cables, spatial curve tension rings, and vertical struts. The elevation is higher at the 45\({}^{\circ}\) tension ring than at the long and short axes, and the plane is projected into a ring. A three-dimensional axonometric drawing of the stadium is shown in Figure 2.  
The composition of the cable-supported grid structure is clear (Figure 3), which can be divided into the following components: structural columns, outer compression ring (CR), cable net system, upper main steel members, and upper secondary steel members. The cable net system includes radial cables and tension rings, all of which adopt fully enclosed steel strands; the tensile strength grade of the steel wire is 1670 MPa, and all cable heads use hot-cast anchor sockets. The tension rings are composed of eight cables in parallel, and radial cables are divided into five specifications based on stress level. Figure 4 shows the layout of all structural cables, and Table 1 lists all cable parameters in detail.  
Figure 1: Types of cable supported grid structures: (**a**) upper reticulated shell, (**b**) outer compression ring, and (**c**) inner compression ring balancing radial cable force.  
Figure 3: Detailed 3D axonometric drawings: (**a**) upper main steel members (radial main beams, braces, etc.), (**b**) roof purlins, (**c**) outer compression ring, (**d**) cable net system (radial cables + tension rings), (**e**) structural columns and crossing braces between columns, (**f**) partial overall structure.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Specification** & **Cable Section (mm2)** & **Cable Weight (kg\(\cdot\)m\({}^{-1}\))** \\ \hline Tension rings & 8D110 & 67,680 & 544 \\ \hline  & D75 & 3890 & 31.3 \\  & D80 & 4420 & 35.5 \\ Radial cables & D90 & 5600 & 45 \\  & D115 & 9280 & 74.5 \\  & D135 & 12,920 & 104 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Cable materials and specifications.  
Figure 2: Integral 3D axonometric drawing.