The superstructure construction stage is the key period for the structural alignment control of suspension bridges, which requires a lot of construction survey and monitoring survey. Usually, in order to ensure the accuracy of construction monitoring survey and achieve the purpose of rechecking, based on the construction survey control network, it is necessary to optimize the local control network according to the requirements of the realizability of structural geometric parameters monitoring and the convenience of field operation. In order to facilitate the comparison and application of measurement results, the monitoring survey is carried out on the principle of the same network and reference point as the construction survey.

linear measurement includes tower top deviation measurement, actual position measurement of main structures, installation linear measurement of main cables, installation position measurement of cable clamps, and linear measurement of main cables and main girders during sling tensioning. The structure of self-anchored suspension bridge is shown in Figure -1.

1 tower top displacement measurement

Because the main tower bears a considerable part of the load through the suspenders and main cables during construction and completion, the main tower will be deformed in different degrees under unbalanced load, atmospheric temperature difference and sunlight. In order not to affect the cable force adjustment, it is necessary to master the variation law of the main tower under natural conditions.

before the cable tower re-survey, the survey data of the previous project should be carefully consulted, which is the direct basis for monitoring calculation and monitoring survey. At the same time, in the subsequent construction stage, the deformation of the tower column during the installation and closure of the main beam should be tracked to provide reference data for the installation control of the main beam.

the displacement measurement of the main tower includes the displacement measurement along the bridge and across the bridge, and the displacement monitoring of the tower top is carried out by high-precision total station. In order to provide reference for the displacement of the tower top, observation points are arranged on the tower top and the tower body, with the circular prism as the sighting target, and marking measuring points are set at the center line around the tower wall, and the measuring stations are arranged at appropriate positions on the bridge axis.

in order to monitor the influence of sunshine temperature difference on the displacement of the main tower, the tower column displacement test was carried out for 24 ~ 48 hours on a day with large temperature change before the catwalk was set up. Record the time-temperature-displacement curve, understand the vertical and horizontal bridge deflection law of tower column with temperature change, and provide reference for subsequent construction process measurement.

The specific re-survey work at this stage is as follows:

(1) Measure the longitudinal position control points of the cable tower and mark them. Combined with temperature measurement, determine the position and elevation of the control points at the top of the tower and the plane position of the main tower column, and compare them with the calculated values to give the construction errors of these values. Because these values directly affect the subsequent construction, they need to be measured carefully.

(2) Because the control point at the top of the tower is difficult to observe, in order to facilitate the monitoring of the main tower under subsequent working conditions, it is necessary to design the marking point at the position closest to the top of the tower, which is convenient for measurement and not easy to be damaged, and take this point as the representative point of the position and elevation of the top of the tower during subsequent working conditions measurement. The position (x, y, z) of this marker point relative to the control point needs to be measured in the current working condition, so that the elevation and longitudinal position of the bridge tower can be obtained by measuring the marker point in the future.

2 monitoring and measurement in the process of catwalk erection

Catwalk is a necessary temporary structure in suspension bridge construction, which provides a construction operation platform for erection of main cables, installation of cable clamps and slings, and is also a transportation channel for materials and tools. The catwalk operation is all aerial work, and it is difficult and dangerous to set up the catwalk due to the influence of natural environment. In order to make the catwalk construction line shape as close as possible to the design line shape, in addition to better measuring and controlling the catwalk cable sag, the horizontal displacement of the bridge tower under this working condition should be re-measured, because the catwalk sag is closely related to the distance between the cable tower and the loose cable suite. Because the catwalk alignment of self-anchored suspension bridge is spatial alignment, the different internal forces of catwalk cables between two spans will lead to the deviation of the bridge tower, which will affect the construction quality of catwalk cables, so it is necessary to monitor and control the horizontal displacement of the bridge tower during catwalk construction.

during the catwalk construction, the horizontal displacement of the bridge tower can be monitored by coordinates, and the marked points can be measured at the top of the tower. The initial value of this monitoring point is taken according to the measurement results of the actual construction environment, and the difference between the real-time measurement coordinates and the initial value is the horizontal displacement of the tower top under this working condition.

3 measurement of actual position of bridge tower, main saddle and main cable anchorage structure before erection of main cable

After erection of catwalk, the static deformation of tower top elevation and plane position affected by temperature and its deformation law have a vital influence on tower deflection monitoring, span change monitoring, sag control of reference strands and main cable and cable clamp stocking in suspension bridge superstructure construction. At this time, the catwalk and the pylon form an integral structure, and the anchorage structure of the pylon, main saddle and main cable should be measured, which is the direct basis for the calculation of cable strand erection. The specific monitoring work at this stage is as follows:

(1) Measure the horizontal displacement and elevation change of the pylon caused by the catwalk, and measure the temperature at which the displacement occurs; Take the average of the two measurement results for 1 nights and compare them with the bare tower results to get the offset value.

(2) Calculate the pre-deflection of saddle installation according to the theoretical pre-deflection and the vertical displacement of tower column caused by catwalk erection. According to this pre-deflection, the construction unit can accurately pre-deflect the loose cable saddle and the main saddle, and the actual pre-deflection of saddle installation is measured and confirmed by different personnel for many times, and then horizontal support is added to limit its longitudinal sliding.

after measuring the anchorage structure of the bridge tower, the main saddle and the main cable, the pre-deflection of the cable saddle and the erection control line shape of the reference strand shall be recalculated in combination with the latest structural dead load data, and the calculated values at the design temperature shall be submitted to the unit designated by the owner for review and confirmation. Check whether the newly calculated pre-deflection is consistent with the set pre-deflection. If there is any change, adjust the pre-deflection of the cable saddle.

4 monitoring and measurement during the erection of benchmark strands

Before the erection of benchmark strands, it is necessary to re-measure the pre-deflection of the cable saddle to confirm that the position is accurate and unchanged. After the cable saddle is pre-deviated, the benchmark strand of the main cable can be erected. The parameters that affect the line shape of reference cable strand are temperature change and deviation of bridge tower. The specific monitoring work at this stage is as follows:

(1) Determine the monitoring and measuring method of the absolute sag of the benchmark cable strand

The sag measurement of the benchmark cable strand adopts one-way trigonometric leveling, and the correction of the local atmospheric refraction coefficient and the correction of the earth curvature are considered in data processing, while the acquisition of the local water atmospheric refraction coefficient can be obtained through experiments under different local climatic conditions according to the method of opposite trigonometric leveling test. The obtained atmospheric refraction coefficient is representative, and the observed vertical angle can be corrected. Finally, the elevation of the point to be measured is calculated by using the corrected vertical angle. If this method is used properly, the absolute sag measurement can reach the accuracy of 1.1 ~ 1.5 cm in the range of 511 ~ 1111 m, which can meet the accuracy requirements of the benchmark cable sag measurement.

(2) Monitoring the relative sag between the reference cable strand and the main cable at one side of the bridge tower

The relative height difference of the reference cable strand is also planned to be measured by this measurement method. Firstly, the method of hydrostatic leveling, that is, connecting pipe leveling, is used to directly measure the relative sag between the upstream and downstream reference cable strands when the wind is light, the temperature changes little at night and the cable strands are stable, and then the relative sag between the upstream and downstream reference cable strands is checked by the triangular elevation intermediate method; The relative height difference between upstream and downstream main cables is also monitored by this method. This monitoring project is in good condition in the implementation of Humen Bridge, Jiangyin Bridge and Haicang Bridge, and the difference between the two measurement methods is generally below 5 mm, which can meet the accuracy requirements of construction monitoring of super-large suspension bridges.

(3) Monitoring at the stage of stable observation after the erection of benchmark strands

After the benchmark strands are adjusted, the benchmark strands should be stably observed for at least 3 days. Measure the span variation of each span strand, the longitudinal position and top elevation of the marked point in the span of each span strand three times every night. The monitoring personnel will determine the erection accuracy of the benchmark strands according to the results of each review. If the requirements are not met, the benchmark strands will need to be readjusted.

5 Monitoring and measurement of tower deformation and beam end displacement during the erection of general cable strands

The horizontal force acting on the tower will change due to the traction of general cable strands, temperature, alignment errors and other factors, which will lead to the deformation of the tower and the displacement of the beam end. In order to control the alignment of cable strands and main cables and ensure the safety of the tower from the monitoring point of view, it is necessary to monitor the tower deformation and the displacement of the beam end.

in the general cable strand erection process, the tower deformation and beam end displacement are generally one-dimensional variables, and the main deformation direction is along the bridge. The selected deformation monitoring method should be simple, fast, reliable, efficient and easy to realize, so as to effectively cooperate with the construction. In the superstructure construction of Humen Bridge, Haicang Bridge and Runyang Yangtze River Bridge, the distance difference monitoring method of rangefinder or total station is used to monitor the cable tower deformation and beam end displacement under various working conditions of superstructure construction, and good monitoring results have been achieved.

6 Re-survey of the results after the erection of the main cable

After the main cable is tightened, the empty cable shape should be accurately measured. The measurement method is the same as the monitoring method during the erection of benchmark strands. Before measuring the empty cable shape, the longitudinal constraint of the temporary support at the loose cable sleeve should be removed to get the real empty cable shape.

the linear measurement elements of the empty cable state are as follows: the vertical and horizontal coordinates of the tower top, the elevation, the average temperature of each span, the mid-span position and elevation of the main cable, the rotation of the loose cable saddle and the temperature of the anchor span. The empty cable shape should be measured three times when the temperature is stable at night.

the measurement results will be used to calculate the erection accuracy of the main cable in the construction monitoring simulation analysis system, and to calculate the stress-free length of the main cable according to the geometric shape of the bridge tower and the measured empty cable line shape, and to calculate the influence on the completed main cable line shape.

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