# Review of the Main Cable Shape Control of the Suspension Bridge
## 3 Construction Control Technology of the Main Cable
### Sag Control of Cable Strands
[72] reported that when adjusting the general strands, the three rows of strands at the bottom took the No. 1 strand as the reference, and when adjusting the sag of the remaining strands, a strand that had been adjusted at the same row or at the outermost of the next row was chosen as the reference. Then, the same approach was also exploited on Xi Houmen Bridge [73]. This was conducted to solve the problem where the reference strand was pressed by general strands in the later stage and could not be employed as a reference for general strand erection. A ruler strand [74] in Figure 10, separated from the main strand, was proposed as a reference for the alignment control of the main strand. [PERSON] et al. [75] invented a strands' section methodology based on multi-standard wires, which can locate strands according to their marking points. Since the production accuracy of cable strands was enhanced by setting multiple standard wires, it was only necessary to align the cable strand mark points with the corresponding marks on the saddle during erection, and the strand sag did not need to be adjusted. 2. Environmental conditions for the sag adjustment  
The precision of the main cable erection is greatly influenced by temperature [76]. According to the construction specifications [66] in China, the sag adjustment of cable strand alignment should be carried out when the temperature becomes stable at night, and when the wind force is less than level 5. The temperature stability condition is that the temperature differences in the length direction and cross-section should not be greater than 2 \({}^{\circ}\)C and 1 \({}^{\circ}\)C, respectively. In the sag measurement process of the cable strand, environmental factors such as temperature, wind, and rain lessen the measurement accuracy and efficiency, which directly affect the erection quality of the cable strand. Although a variety of advanced measurement techniques have been developed to enhance the measurement efficacy and accuracy, most of them were first proposed for the structural health monitoring of the finished bridges [77; 78]. Although advanced measurement techniques in the construction process are not common, [PERSON] et al. [74] proposed a general strands alignment measurement approach based on the vision, and developed a vision-based sag measurement system, as demonstrated in Figure 11. The actual height difference between the general strand and the reference strand was obtained by collecting and analyzing the image data, and the field experiment verified that the measurement error of this methodology is lower than 3 mm. However, the method solved the problem where the elevation of the cable strand cannot be measured, or where the accuracy is too poor in the strong wind environment [79], it is still affected by the complex temperature field. (3) Allowable deviation of the sag adjustment  
The adjustment quality of the cable strand sag directly affects the free cable shape. In China, the construction specifications [66] require that the strand shape should be controlled by the allowable deviation of mid-span elevation. The specific requirements have been presented in Table 2.  
The adjusting tolerance of the relative sag difference between the reference strand and the general strands during the construction of the Minami Bisan-Seto Bridge in Japan was \(\pm\)30 mm [57]. However, [PERSON] suggested that this tolerance was too liberal, and perhaps 6 mm of deviation is acceptable [80]. Although the specification has clear requirements for the erection accuracy of general strands, due to the complex factors affecting the erection  
Figure 11: Vision-based system for relative sag measurement. Figure 10: Space position of strands. accuracy of strands, the accuracy control of many projects has not yet adopted a unified standard in China, and the specific statistics have been presented in Table 3.  
As for the erection of general strands, it is generally required to achieve the ideal state of \"half approaching, half separation\" between adjacent strands.