## 1 Introduction
Moreover, because of the strong elastic resilience the double-layer cable system can ensure its lateral stability, thus the stiffness of the main-cable is far greater than that of the load-bearing cable of an ordinary rail transit and has a far greater load-carrying capability, e.g., the shifting-girder trolley of the Aizhai suspension bridge during construction can bear a 240 t load. Therefore, our research focused on the RCSG technique are new and many related mechanical problems need to be solved. For the RCSG technique, the sling is the link between the main-cable and the rail-cable. The sling elongation is limited under the no-load condition, while the sling is undergoing alternating load in  
Figure 1: The shifting-girder system in the rail-cable-shifting-girder (RCSG) technique of the Aizhai suspension bridge during construction. the direction process of girder segments, and the vertical interaction force between the main-cable and rail-cable is discontinuous. Once the girder segment is installed, the sling force at this place reaches its maximum with a relatively stable value in the shifting-girder process. The sling is flexible and it extends under loads. This elongation must be considered in the refined analysis of the deformation coordination relation. Especially, the undergoing load of the sling is changing in the shifting-girder process, and its elongation cannot be ignored. The rail-cable is one part of the flexible system composed of the main-cable and sling, the large deformation must be considered, which brings the difficulty and complexity to the pseudo-static analysis and solution. The shifting-girder trolley, as shown in Figure 2, is an important part of the shifting-girder system, which is composed of two pairs of tackle-block and moves on the rail-cable. The systematical responses are different when the shifting-girder trolley locates in different position with various inter-segmental lengths and dragging methods. In the process of the first pair tackle-block transiting from the flexible rail-cable to the rigid hanging saddle and then sliding from the hanging saddle to the rail-cable, the locally relative deformation of the rail-cable slowly increases. With the moving forward of the tackle-block by the dragging force, the relative deformation of the rail-cable is becoming bigger, and the tackle-block has the trend of declining quickly under the effect of its gravity. While the subsequent pair tackle-block start to climb the hanging saddle through the rail-cable, to slow down the movement of the tackle-block. In the design of longitudinal size of the shifting-girder trolley, the span of the two gravity centers of the shifting-girder trolley adopts half of the hanging sling spacing, to reduce the shock to the shifting-girder trolley. When the first pair tackle-block is climbing, the second one will move downhill. Conversely, when the first pair is downhill, the second pair will move uphill, ensuring the consistency of the drag force of the system and preventing a crash. The global transportation of the girder segment is realized in the process of the RCSG technique, which greatly reduces the air operating works and erecting time. For instance, the installation period of the 1000.5 m girders of the Aizhai suspension bridge took only 2.5 months, saving about 2000 t of steel used for the steel truss girder. With this new technology, the construction cost is lower, and the construction qualities of the girder are improved. Therefore, this technique provides a new, economical, and efficient erection method for the girder of the long span suspension bridge in rural mountainous areas with poor transportation and no navigable rivers. The stiffness of the shifting-girder system increases because of the pre-tension of the rail-cable. When the force of the rail-cable is larger, the vertical stiffness is greater, the up/down angle of the shifting-girder trolley on the rail-cable is smaller, and the moving of the girders is smoother.