1. Introduction
With the wide application of steel pipe concrete arch bridges, its forms of construction are becoming more diversified [
1]. The span of the bridge has developed from just over 100 m in the beginning to nearly 600 m at present [
2,
3]. The design of a concrete-filled steel tube (CFST) arch bridge is usually guided via finite element simulation, and the use of a scaled model test can not only verify the mechanical performance of the structure, but also preview and evaluate the actual control of the construction. The control of the accuracy of pylon deflection will affect the alignment of such long-span arch bridges [
4], but there is little research available into pylon control (especially as related to the middle pier during cable-stayed suspension). At present, some Chinese researchers have studied the linear control of CFST arch bridges in different forms. Gu, Y [
5] described arch rib construction technology to analyze arch rib installation control methods from the perspective of arch processing and the splicing of the steel tube based on the arch bridge characteristics and construction-control processes of CFST bridge types. Based on the grey prediction theory, Zhuo Xiaoli [
6] established a grey model for line-shape prediction, considering the residual correction during the construction of main beams in the context of mid-span steel box arch bridges, and predicted the construction pre-elevation value of main beam segments through tower deviation. It can be seen that tower deflection and arch rib alignment in arch bridges interact [
7,
8,
9]. The suspension assembly method is a common construction method for long-span arch bridges [
10,
11,
12]. However, the studies on the suspension assembly method mainly focus on improving the accuracy of the cable forces [
13], while the studies on tower deflection are few. At present, for a long-span steel truss arch bridge, suspension tower construction is the most difficult and risky part of the construction process. The steel truss arch erection side span of the Jianghan Seventh Bridge is installed by using the support method, and the middle span is installed via the cantilever method. The suspension tower plays an important role in the cantilever erection construction of the steel truss arch. The suspension tower is one of the key structures in the cantilever construction of the steel truss arch bridge, and it connects the cantilever end of the main arch with the balance end of the side beam using the suspension cable, so that the steel truss arch bridge can maintain overall stability during the construction of the main girder [
14]. The application of monitoring technology to realize the construction-line monitoring of the suspension tower can not only meet the installation accuracy requirements of the suspension tower members, but also improve the safety and accuracy of subsequent main beam closure [
15]. This issue has also been investigated by some foreign researchers in succession. In the large-span concrete arch bridge constructed via the cantilever pouring method [
16], path control is key to ensuring construction quality, and the deflection of the buckle tower often affects the arch elevation of each segment. The influence of the deflection of the buckle tower on the elevation of each section during construction was analyzed via a geometric analysis method, and the finite element model was established in Midas software to verify the results. The results show that the deflection of the buckle tower has little effect on the elevation of the arch foot, but it has a greater effect on the elevation of the section near the arch top, and the section at the arch top is more sensitive to the change in the buckle-tower height. According to Jiang Wei [
17], the structural state of the main cable of the hoisting system changes during the hoisting process. Based on the engineering background of a large bridge, this paper obtained a high-precision calculation method of the main cable sag and cable force during the hoisting process through theoretical deduction, and estimated the influence of vehicle traffic on the main cable force by numerical simulation. The results show that the proposed method for calculating the sag of the main cable has high accuracy and can be applied in practice. The main cable force increases gradually as the vehicle moves toward the mid-span and the main cable force at the pylon end reaches its maximum when the vehicle is within the span. Some scholars have also shown how to prevent the occurrence of tower deviations by monitoring via three-dimensional scanning, but they have not avoided the occurrence of tower deviations from the root.
In terms of the method of the tower deflection theory, Lin T.M. [
18] used the suspension cable element method to derive a formula giving the deflection of the hinged tower of the cable hoisting system where the pressure tower is set, so that the calculation of the cable hoisting construction stage and the tower deflection calculation were unified. A feasibility control system of intelligent tower deviation was verified by theoretical analysis, but it has not been applied in engineering practice. Deng JiangMing [
19] derived an analytical formula of the influence of the tower deflection with an integrated button-back on the linear arch rib by using a geometric analysis method. The feasibility of these proposed control systems as applied to intelligent tower deviation was validated by theoretical analysis, but the deformation of the tower is not considered.
In terms of calculation methods, Zhen-guo C et al. [
20] considered the influence of the deflection of the buckle tower during the calculation of the cable force, and ascertained the that optimal buckle cable force matrix can be used to reduce the influence of buckle-tower deflection on the alignment of the arch rib. Ke-Jian Y [
21] studied the sensitivity of each section of the buckle of the lower part of the buckle-tower to the vertical displacement of the closing hole, and found that too small a horizontal inclination would weaken the influence of the section buckle force on the vertical displacement of the closing hole, and they solved the problem of the low adjustment efficiency of the long buckle cable by calculating an optimal cable force via the formal installation iterative method. However, these two methods tend to be those viewed as lacking in innovation: if the number of iterations is excessive, the calculation will be slow. Xue-Tao D et al. [
22] studied the influence of the main cable slip on tower deflection in the design of the cable hoisting system, and found that the main cable slip had a significant influence on the cable hoisting system, which could significantly change the deviation of the top of the tower and the stress thereon. Wei S et al. [
23] found that the displacement of the top of the tower in the integrated construction of cables and buckle towers exerts a significant influence on the accuracy of the arch-rib splicing, and the key to construction control is to reduce the deflection of the cable and the buckle tower. Therefore, the initial tension of the wind rope of the top cable should be adjusted while observing the deviation of the tower during the construction of the arch rib assembly, thereby reducing the horizontal cable force difference and keeping the cable and buckle tower in a vertical state as far as possible. The method is novel and unique in its technical aspects, but this method lacks theoretical support.
The safety and construction accuracy of the tower has been studied widely; taking the Changshou Yangtze River Bridge as the background, Gao Z [
24] introduced the construction technology of a 60 m high, single-story, suspension tower from the design, construction, and to demolition. Taking the Nanjing Dashingguan Bridge as the background, Sheng Zhiping [
25] innovatively laid a suspension tower with three layers of cable stays on the outside of the two-span steel beams, with a height of 68.5 m, and adopted flat cable construction technology on the inside, which overcame the problems of the internal force and alignment adjustment difficulty during the erection of steel beams. Zhi-Hu Z et al. [
26], taking the Jianghan Seventh Bridge as the background, conducted the stress analysis of various working conditions of the 89.6-m high sling tower during the assembly process. Renbo F et al. [
27] studied the application of 39-m high suspension tower in flexible arch of a continuous steel girder in the Guanhe Super-Bridge on the Lianyan Railway. This long-span bridge has certain advantages and broad prospects for application. Due to its wide range of application, especially in long-span bridges, cable hoisting construction has become the main method of the erection of arch bridges [
28,
29,
30]. When cables are used to lift arch ribs, the processes of each span affect the deformation of the tower, induce deviations, and have linking effects on the arch ribs suspended on the tower. When the linkage effect gradually accumulates, assembly accuracy and tower deviation will be affected if no adjustment is made [
31,
32]. In the present work, a set of new methods and theories are used to control tower deviation, limiting the occurrence of unbalanced tower deviation and the corresponding torsion at the root, which is different from the methods of other scholars. The problem of tower deviations is solved, and the accuracy of tower deviations is controlled to within the theoretical value, thereby achieving the ideal control effect.
5. New Methods of Optimization Data Analysis
Using data comparison, it can be found that the transverse deviation of the tower is insignificant, and most of the deviation is within 0.5 mm, so there is little scope for the description and calculation of the transverse deviation therein. The working conditions of A2, B2, and Section 6 are given in
Figure 13, and the data pertaining to both the simulation and experiment are compared. To improve practicability, the maximum stress of the arch foot of the two methods corresponding to the section is listed (
Figure 13).
It can be seen above that the method of the back cable can reduce the stress on some arch feet, and that on the eighth section, it can be reduced by about 25%. The following is a comparison of the actual and theoretical data of each section, and the last two charts allow the comparison of the data of the post-cable and traditional methods (
Figure 14,
Figure 15,
Figure 16,
Figure 17,
Figure 18 and
Figure 19).
It can be seen that the traditional method induces asymmetric column deviation control and torsion problems. The actual data indicate that T3–T6 is the control point at the top of the buckle tower, and the actual data differ greatly, so the effect of lifting the unilateral rear cable is poor.
The following new method is applied to Section 8: after hoisting two symmetrical segments, both sides are tensioned at the same time, and the data are compared with that collected when hoisting one side segment at a time(
Figure 20 and
Figure 21).
The comparison of the pictures shows that this method can actively and timeously correct the displacement deviation of the tower top and the torsion on the symmetric segment during the cable hoisting process, reducing the deviation. The maximum deviation of the tower pier can be controlled, in real time, to within the range of ± 0.5 mm.