3D Laser Scanning Technology Research for Erection Line-Shape Monitoring of Long-Span Steel Truss Arch Bridge

The rapid development of bridge construction level leads to the increasing demand for bridge construction volume and construction risk dynamic assessment. Line-shape monitoring is crucial in the erection process of large steel truss arch bridge, which can ensure that bridge line-shapes accurately extend. Guangzhou Mingzhuwan Bridge is a six-span continuous steel truss structure with full cantilever construction method in the erection process. The erection working conditions account for a large proportion when the cantilever length is more than 200 m, and most of the rods are splice with high-strength bolts outside the joints in terms of the freedom of adjustment of the rods is large, so that the erection alignment is difficult to control under the influence of wind, temperature and other natural factors. The 3D laser scanning technology can obtain the coordinate information of massive structure surface through fast scanning, realizing the rapid real-time monitoring of multiple key points in the structure. The research has monitored the erection line-shapes of the main arch and girder of Mingzhuwan Bridge by using 3D laser scanning technology. Three key erection conditions of main arch girder erection, main arch closure and main girder closure were monitored through reasonable arrangement of scanner monitoring positions, and several structural scans were carried out mainly around the cantilever of main girder and around the tower crane. In the monitoring process, the rear intersection method was used to obtain the coordinates of scanner erection position and then calculated the coordinate information of the point cloud of the monitoring target. Moreover, the point cloud processing software Cyclone was used to realize the overall model splicing of the main bridge point cloud through noise elimination and point cloud association, and then multiple linear monitoring indicators were extracted for analysis by combining the point cloud data processing technology. At the same time, the same point coordinates were obtained by the total station for verification. The results show that the average difference of the relative distance between the cantilevers of the main arch and girder is 19 mm for the 3D laser scanning and total station measurement methods, which is within the relative point position measurement error between adjacent points allowed by Technical Specifications for Construction of Highway Bridge and Culverts(JTG/T F50—2011), indicating that the linear monitoring method based on 3D laser scanning can meet the engineering application requirements. The verticality index between the tower crane joints is generally within ±3‰, thus the bottom of the tower crane does not reach the ultimate bending moment without structural failure. However, when the main arch or girder is large cantilever, the upper structure of the tower crane has a large vertical deviation, and it is still necessary to strengthen monitoring during the erection of the main bridge. The monitoring line-shapes of the main arch and girder are basically consistent with the theoretical line-shapes, and the mean values of transverse and elevation deviation indexes are 2.9 mm and 16.6 mm, which are in line with the deviation threshold of the longitudinal axis and height difference of steel arch bridges in JTG/T F50—2011. Moreover, the cantilever rods on both sides of the main arch and girder constantly approach the theoretical value during the erection process, indicating that the erection alignment of the main bridge well control and meeting closure needs.