## 2 Torque Transmission Mechanism and Experimental Scheme  
### Torque Load Source and Transmission Process  
When the turbine is in operation, the wind load on its blades and the load caused by the wind turbine yaw will act on the top of the tower through the structure and the transmission mechanism and will be further transferred to the top of the foundation structure [29; 30; 31; 32]. The torque load on the horizontal plane, which the foundation structure bears, is mainly caused by fan yaw load, as shown in Figure 1. Due to the gyroscopic effect, the yaw operation will cause the gyroscopic force to act on the top of the tower when the fan is yawing, i.e., the yaw load. At different stages of yaw movement, the load is divided into the starting load and uniform rotating load.  
1. Start deflection When a fan is yawing, the yaw activation torque on the tower is \[M_{ZT}=P_{H}\times A\times e_{w}\] (1) where \(A\) is the swept area of the wind blades; \(P_{H}\) is the average pressure on the swept area of the wind blades; \(e_{w}\) is the eccentricity between the aerodynamic force and the center of the wind blades. At the beginning, in addition to torque, there is also a lateral force acting on the top of the tower, i.e., \[F_{\gamma T}=m_{M}\cdot e_{M}\cdot\dot{\Omega}\] (2) where \(m_{M}\) is the total mass of the generator and blades; \(e_{M}\)s the distance from the centroid of total mass to the tower; \(\dot{\Omega}\) is the yaw acceleration.
2. Uniform deflection movement If reasonable acceleration time data are not available for a wind turbine with a passive yaw system, then the acceleration duration is assumed to be 1 s, and the system's yaw rate is adopted. For a wind turbine equipped with an active yaw system, its angular velocity is usually small and can be ignored accordingly. In this case, the following loads will be applied: \[F_{XT}=-Z\cdot m_{M}\cdot e_{M}\cdot\Omega^{2}\] (3) \[M_{YT}=-Z\cdot I_{B}\cdot\omega\cdot\dot{\Omega}\] (4) where \(Z\) is the number of blades; \(I_{B}\) is the moment of inertia of blades relative to the rotor shaft; \(\Omega\) is the yaw rate. In this paper, the scale model test was based on a 70-m tower foundation structure with a 1.5 MW wind turbine in a real project. Based on the above calculation method, the horizontal torque load was obtained, and the torsion test on the scale model was carried out using a graded method. The upper structure transfers the torque load to the top of the foundation ring, then to the internal components of the foundation through the foundation ring, and finally to the base. The foundation resists the upper torsional load by its own frictional resistance to the underlying soil. As shown in Figure 2, the torque load is transferred to the base structure as follows: top flange of foundation ring \(\rightarrow\) top radial reinforcement (with the surrounding concrete) \(\rightarrow\) sidewalls on the foundation ring (with the surrounding concrete) \(\rightarrow\) foundation ring flange (with the surrounding concrete). In this process, the main component responsible for the transfer process is the foundation ring: one part of the load is shared by the top radial reinforcement, while the other part is further passed downwards.  
Figure 1: Transfer process of unit yaw torque load.  
In the design process, the effect of top radial reinforcement on the antipull capacity is mainly considered, excluding its effect on the antitorsional bearing capacity of the foundation. This is mainly due to the fact that there is currently no definite conclusion about the torque load sharing ratio of antipull bars, and the torque load sustained by the foundation is relatively smaller. However, the load during the actual operation of the foundation ring infrastructure is a composite load, i.e., \(T\)-\(V\)-\(H\)-\(M\), of which the torque load also has a great influence on the stress and force transfer mechanism.