## 2 Key Segment Closing Method
### New Key Segment Closing Method  
The process of set back and reset back is widely used for key segment closure of a cable-stayed bridge constructed with the FCM method. As shown in Figure 3a, after a predetermined spacing between the key segment and the adjacent segments is obtained, the key segment is lifted for connection. One side of the key segment is connected temporarily, and alignment of the girders is checked vertically and transversely. Finally, the girder is reset back into position and the connection is made. This method needs additional reinforcement of girders located near the pylons because it involves jacking work at the pylons. In addition, changes in structural behavior must not occur during the process.  
Figure 2: Types of thermal prestressing methods. (**a**) Uniform temperature; (**b**) Temperature gradient; (**c**) Thermally prestressed cover plate.  
However, it is difficult to apply such a process for a partially earth-anchored cable system because the presence of earth-anchored cables requires tremendous forces for key segment closure, a large number of hydraulic jacks, and substantial reinforcement of the girders for jacking. Unlike the self-anchored cable-stayed bridges, the partially earth-anchored cable-stayed bridges show relatively large reactions in the longitudinal restraints. Owing to the difference of axial forces between the left and right side girders of the restraints, which are installed between the girders and the pylons to guarantee stability during the construction sequence, large reactions occur in the restraints. The release of the reaction forces, which should be done before the set back process, causes large movements in bridges that use a partially earth-anchored cable system. Thus, the set back process cannot be applied to partially earth-anchored cable-stayed bridges.  
A new key segment closing method using thermal expansion with uniform temperature (Figure 2a) is applied as an alternative method; it was verified in a partially earth-anchored cable-stayed bridge with a medium span length of 344 m in the authors' previous research [6]. A detailed construction sequence for the new closing method is presented in Figure 3b. While a derrick crane holds a key segment in position, certain girders are heated. After the key segment and the adjacent segments make contact due to the elongation of the heated girder, the connection between the segments is made. The longitudinal restraints that restrict the longitudinal movement at the pylon in the cantilever state are then removed. Finally, the artificial heating is removed.  
Figure 3: Comparison of key segment closing methods. (**a**) Conventional set back and reset back method; (**b**) new key segment closing method based on thermal prestressing.  
A feasible way of heating girders with large cross sections is to use large-capacity industrial fan heaters. By blocking one end of the diaphragm inside of a girder and blowing hot air from the other end, the whole segment can be heated uniformly with ease. This kind of heating process by convective heating ensures a uniform temperature distribution within the girder section. In addition, the authors' previous research for the temperature distribution of a steel box girder in a cable-stayed bridge showed that the temperature of the inner steel box is even at dawn [9]. From the temperature distribution results, a heating schedule can be determined. The objective of this study is to examine the structural behavior of a partially earth-anchored cable-stayed bridge with a long main span during the new key segment closing process. Thus, construction details such as operational details of a heating device, the possible heating time during connection process, economic considerations, etc., will be studied further.