## 3 Case Study
### Old Hanger Demolition Process  
The removal process of hanger #10 was calculated by the equations in Section 2.2. The pocket hanging and cutting processes were divided into five steps with the same step size, that is to say, the pocket hanging force of each stage increased by 20% of the internal force of the hanger to be replaced, and the cutting area of each stage was 20% of the area of the hanger to be replaced.  
The displacement of the bridge deck at the lower end of the hanger and the arch rib displacement at the corresponding part of the hanger were monitored in the process of the hanger removal, in which the displacement of the lower end of the hanger was measured  
Figure 8: The hanger replacement process by the pocket hanging method: (**a**) displacement measurement and (**b**) cable force test.  
Figure 7: The displacement of the bridge deck at the lower end of hanger #10 varying with the number of reserved hangers on both sides of hanger #10.  
by a high precision Leica electronic level LS15 with a precision of 0.2 mm per kilometer round trip. The displacement of the arch rib at the corresponding point of the hanger was measured by a TS15A total station, with a test accuracy of 0.5 s, as shown in Figure 8a.  
It was found that the displacement difference of the arch rib at the corresponding place of the hanger to be replaced was very small before and after the hanger was removed, less than 0.2 mm according to the displacement test results, which also verifies the correctness of the assumption that the arch rib has no deformation in the theoretical model in this paper. The test result of the bridge deck displacement at the lower end of the hanger to be replaced is exhibited in Figure 9. As can be observed in Figure 9, the displacement calculated in this paper is very close to the measured value, which verifies the correctness of the theoretical calculating method.  
It can be seen from Table 1 that (1) the deformation of the main beam shares part of the pocket hanging force in the pocket hanging process, so that the change of the internal force of the hanger to be replaced was less than that of the pocket hanging force. (2) The internal force of the pocket hanging hanger was gradually increased, while the internal force of the hanger to be replaced was gradually reduced whether it was pocket hanging or cutting (see Figure 10), which is also the reason why the old hanger could be removed after only a few gradings by the pocket hanging method. (3) The displacement was upward when the pocket hanging was carried out, while it was downward when the cutting was carried out. Therefore, the cumulative displacement was small after a pocket hanging and cutting, and it can be seen that the maximum cumulative displacement after all levels of cutting was 0.77 mm. (4) Since the internal force of the cut wire is shared by the remaining wire when cutting, the downward displacement during cutting will always be less than the upward displacement during pocket hanging; as a result, the accumulated displacement after each cutting stage increased gradually, and the maximum cumulative displacement was 2.60 mm, which was less than that of threshold \([D]=min(10\,\text{mm},\,S/1000)=5.1\) mm.  
Figure 9: Test results of bridge deck displacement at the lower end of the hanger to be replaced in different cases.