## 5 Conclusions  
In this study, a theoretical investigation of the mechanical behavior of column-free QRST structures based on prototype experiments and numerical models is conducted. The structural performance, deformation characteristics, and joint behavior under service stage conditions are fully explored. The applicability of different simulation models is compared. Additionally, design studies that consider the construction process are conducted. Through the study, the deformation mechanism and weak parts of the structure are revealed, providing suggestions for the engineering application for this new type of structure. The following conclusions are drawn:  
1. At the service stage with a designed buried depth of 13 m, the inward vertical convergence deformation along the short axis of the structure is 108.64 mm, while the outward horizontal convergence deformation along the long axis is 49.03 mm. The column-free QRST structure predominantly experiences inward deformation;
2. The development of joint deformations and bolt axial forces indicates that as the load level increases, among the positive moment joints, deformations and internal forces of joints 1 and 5 develop faster than those of joints 6 and 10. For negative moment joints, joints 3 and 8 exhibit faster development compared to joints 2 and 7, with joints 4 and 9 exhibiting only rapid initial development. Based on the experimental results, it is recommended that the bolt dimension and bolt position for negative moment joints be optimized, and the rows of M48 and M30 bolts be repositioned for positive moment joints, with the help of sophisticated finite element models;
3. The experimental results indicate that the deformation and internal force distribution of the structure are mainly determined by the shape of the tunnel cross-section and joint distribution. The span of the structure in the horizontal direction results in significant convergence deformation along the short axis at the top and bottom. Basedon the given distribution of the joints, the structural deformations at the top and bottom exhibit an axis symmetricity with respect to the long axis of the structure, while those at the waists exhibit a central symmetry with respect to the center of the structure;
4. The ESHR model, BS model, and MBS model are capable of reflecting the mechanical performance of column-free QRST structures. The BS and MBS models exhibit better simulation effects, particularly for local deformations near joints, while the ESHR model can serve as a design model for structures with appropriate calculation parameters;
5. The deformations contributed by joints in column-free QRST structures account for approximately 65-70%, indicating optimization potentials for joints;
6. When using the ESHR model to design column-free QRST structures subjected to the construction process, the results calculated by the three-stage calculation method are more reliable. For ease of engineering applications, structural responses calculated by the direct calculation method, with the response magnified by 10%, is sufficient to ensure structural safety;
7. The influence of structural buried depth on structural response is generally proportional. In cases of changes in offset load and lateral pressure coefficient, the negative bending moment regions at the waists of the structure are more sensitive. Considering unexpected conditions such as ground surface surcharge or adjacent construction activities during the service period, special attention should be paid to the structural safety reserves of the negative bending moment regions at the waists. Currently, the research on the structural performance of column-free QRST structures in this paper is limited to laboratory experiments. Only the performance of a single-ring structure has been studied, and the influence of adjacent rings in actual tunnels has not been considered.