## 1 Introduction  
The spatial grid structure's (Figure 1) characteristics of large space, lightweight, and firmness enable designers to apply it in continuous innovation and exploration [1, 2]. It can be traced back to the early 20 th century when a prototype of a small space grid structure formed by the combination of several triangular and quadrangular pyramid elements was developed by the American inventor [PERSON]. As a highly modular statically indeterminate structure, the spatial grid is beautiful in shape, reasonable in strength, and variable in form, which fully meets the professional requirements of architects and structural engineers. Therefore, the spatial grid is widely used in public and industrial buildings, such as convention and exhibition centers, gymasiums, waiting halls, hangars, and dry coal sheds [3]. As shown in Figure 2, due to the abnormal design [4], construction [4, 5, 6], use [7, 8], and the need for subsequent functional upgrading and transformation, the spatial grid structure contains a large number of congenital defects, which can result in long-term cumulative damage and overload. If the bearing capacity does not meet the safety and use requirements of the current regulations, accidents involving the collapse of the structure are not uncommon [9, 10, 11]. Therefore, reinforcing the bearing capacity of an in-service spatial grid structure has become an important discussion topic in academic and engineering circles. As an axial force system, the members of a space grid structure are divided into axial compression and axial tension, characterized by a large aspect ratio, small steel pipe section size, and high-frequency-welded pipe applications. The method put forward in this paper of strengthening the compression bars of a grid structure is based on steel tube strengthening with filled concrete from \"Steel Structure Reinforcement Design Standard\" GB 51367-2019 [12]. This standard technique requires pouring a lightweight and high-strength cement-based material into the steel pipe through a grouting hole bored in the tube. The research on the mechanical properties of concrete-filled steel tubes [13,14] has matured, and the selection of infill concrete has gradually shifted from ordinary concrete to ultra-high strength concrete [14,15]. Considering the size of the grouting hole, reactive powder concrete (RPC) [16] with ultra-high strength, micro-expansion, no coarse aggregate, and excellent durability was selected as the filling material in this paper. Research on RPC-filled steel pipes primarily includes axial compression [17,18], axial tension [19], bending resistance [20], compression bending [20,21], and stability [22]; and several design methods have been proposed. Existing research on concrete-filled steel tubular (CFST) grid structures [23,24,25,26] show that as the self-weight of the CFST grid increases, the stiffness and ultimate bearing capacity of the grid frame improves significantly, and it exhibits good seismic performance compared to a non-reinforced grid structure of the same design. Using CFST for grid design reduces the amount of steel required. The overall cost can be reduced by more than 30%, which demonstrates its excellent economic benefits  
Figure 1: Space frame structures. (**a**) Space truss; (**b**) Latticed shell; (**c**) Spatial truss. and mechanical properties and provides strong evidence for the application of the internal pouring of concrete reinforcement method in the grid structure. Seamless steel pipes are mostly used in axial compression performance tests of RPC-filled steel tubes. Additionally, the specimen size is not commonly used in the grid structure, being significantly shorter than the length in the grid structure. [PERSON] and [PERSON] [27] showed that the 10 Chinese codes involving CFST stipulate that the minimum diameter of the circular pipe should be no less than 168 mm, and the length-to-diameter ratio should not be greater than 20.