# Form-Finding Analysis of the Rail Cable Shifting System of Long-Span Suspension Bridges
## 3 Global Model Test of the Moving Grider by Rail Cable  
The Aizhai suspension bridge (shown in Figure 6) is a super-long steel girder suspension bridge with a main cable span of \(242+1176+116\) m, which built in the mountainous area of Hunan province, China. The bridge spans the u-shaped Dehang Grand Canyon, more than 500 m deep. The middle span main cable adopts the vertical span ratio of 1/9.6, the central distance of main cable is 27 m, and the plane cable layout is adopted. The bridge adopts 71 pairs of slings with the standard spacing of slings being 14.5 m. The two main cable towers adopt the reinforced concrete tower structures. The gravity anchor is used for the anchor at the Jishou side, while the tunnel anchor is used for the anchor at the Chadong side. There are 69 main girder sections for the whole bridge, which are 7.5 m in height, 14.5 m in length, 27 m in width, and 124.5 t in weight. Figure 5: The flow diagram of calculations. The rail cable launching method is the first used in the Aizhai suspension bridge to realize the horizontal movement of the girder segment. The horizontal cableway utilizes eight relatively independent 60-ZZZ-1570-type sealed steel wire ropes as rail cable. One side of the rail cable is anchored by the OVM-CPS shear force dispersing the type rock anchor system, while the other side is anchored in balance weight. The pretension force of a single rail cable is 105 t. The global laboratory experimental model was designed and manufactured with a geometrical reduced-scale ratio of 1:33. The span size of the main cable of the model is \(7.333+35.636+3.515\) m, its center spacing is 818 mm, and the longitudinal spacing of the sling is 439.4 mm, as shown in Figure 7.  
The test model [39] mainly consists of 2 anchorage systems, 2 main cables, 68 pairs of slings and cable clamps, 3 pairs of rock anchor cables, 2 cable towers and cable towers buttresses, rail cables, rail tensioning devices, and test systems. According to the principle of stress equivalence and similar axial stiffness, the parameters of the rail cable are designed. Four 2.5-mm-diameter high-strength galvanized steel wire ropes are selected to replace rail cables. The rail cable tensioning device is designed to adjust the relative movement of the screws and nuts to achieve the tensioning of rail cables. Due to the fact that in suspended and cable-stayed bridges the cable forces are responsible for the strong variations in the static and dynamic performance of the system [40], a pressure sensor is used to monitor the changes of their cable forces in real time. On the basis of the global model test design, the finite element software MIDAS Civil is used to establish the full bridge numerical modeling on the initial state. In the test process, the no-load stage of the cable is ensured to be consistent with the finite element analysis model by adjusting the no-load stage of the test model. After the cable clamps, slings and saddles are installed, four rail cables are tensioned, and the slings are in a vertical working condition. Then, the main cable force and line shape, rock anchor cable force, rail cable line shape, and sling cable under the tension working condition are tested to find out the changes. Figure 6: The schematic views of Aizhai long-span suspension bridge. Figure 7: The global model of the Aizhai suspension bridge (unit: mm). The detailed contents of the MIDAS Civil modeling of the bridge are as follows. Firstly, the towers on both sides are simulated by using variable section beam element. Secondly, the main cable adopts the simulation of segmental catenary cable element, the cable clip is taken as the segmented node, the main cable between each adjacent node adopts the simulation of one catenary cable element, each sling adopts the simulation of one two-node direct cable element, and the sling connects with the main beam through the simulation of the beam element. Thirdly, the track rope is simulated by segmental straight cable element, and the saddle is taken as the segmentation node.