## 2 Prototype Experiment
### Test Setup  
As shown in Figure 5, a counterforce steel frame and a steel strand tensioning system are utilized in the experiment to provide the test load [29,30]. The tension-only steel strands connect the counterforce steel frame and hydraulic jacks. The hydraulic jacks exert test loads on the test specimen by pushing loading beams. The counterforce steel frame provides a reaction to the jacks via tensioned steel strands. In order to facilitate loading, the  
Figure 4: Diagram of joint surfaces (mm): (**a**) Schematic diagram of positive moment joint; (**b**) Schematic diagram of negative moment joint; (**c**) Photo of positive moment joint; (**d**) Photo of negative moment joint.  
Figure 3: Schematic diagram of column-free QRST (mm).  
counterforce steel frame is designed as a combination of curved components (corresponding to the small arcs on the left and right waists of the structure) and straight components (corresponding to the large arcs at the top and bottom of the structure). The curved components of the counterforce frame intersect with the steel strands perpendicularly, and the straight components of the counterforce frame intersect with the steel strands obliquely.  
To accurately simulate the soil and water pressure exerted on column-free QRST structures, thirty loading points are positioned around the test specimen [31], with each loading point exerting a force perpendicular to the tangent direction of the structure at that corresponding point. Each loading point consists of two hydraulic jacks and a loading beam. The maximum load at a single point is 2000 kN and the maximum deformation of a single jack is 300 mm. These 30 loading points are divided into three groups: P1, P2, and P3. The distribution and grouping of these loading points are shown in Figure 6a.  
In accordance with project backgrounds and design requirements, the test replicates the structural bearing states under the service stage with buried depths ranging from 9 m to 13 m, as illustrated in Figure 6b. The determination of test load from design load adheres to the following principles [32]:  
* All test loads must fulfill a self-balancing condition [33] to ensure the safety and stability of the prototype test platform during the test loading process. If the design  
Figure 5: Prototype test platform for column-free QRST structures.  
Figure 6: Loading states of QRST: (**a**) Test loading groups; (**b**) Design load at the service stage.  
of the test load is unbalanced, it will cause unexpected relative motion between the centers of the specimen and the counterforce frame, thereby affecting the stability of the platform;
* It is necessary to ensure that the deviation of target structural responses (deformation or internal force) between the design loading state and the test loading state remains within an acceptable range. Considering that the design of segments and joints is mainly governed by bending moment, greater emphasis should be placed on bending moment rather than axial force and shear force when determining the test loads.  
Based on the above principles, the equivalent stiffness homogeneous ring model (ESHR model) is used for the determination of test loads, with the obtained test loads outlined in Table 2. As intended, the test structure sequentially experiences design states with buried depths of 9 m, 10 m, and 13 m. When the loading step reaches step 17, the corresponding loading condition is the design condition during the service stage with a design buried depth of 13 m.  
In the prototype test of the column-free QRST structure, the structural responses that need to be measured and observed include:  
* Structural deformation;
* Joint deformation and bolt axial force.