Vision-Based Methods for Relative Sag Measurement of Suspension Bridge Cables

Abstract

Main cables, comprising a number of wire strands, constitute a vital element in long-span suspension bridges. The determination of their alignment during construction is of great importance, and relative sag is commonly measured for the efficient sag adjustment of general strands. The conventional approach uses the caterpillar method, which is inconvenient, difficult-to-implement, and potentially dangerous. In order to realize the high-precision measurement of cable alignment in a strong wind environment, a vision-based method for relative sag measurement of the general cable strands is proposed in this paper. In the proposed measurement system, images of pre-installed optical targets are collected and analyzed to realize the remote, automatic, and real-time measurement of the relative sag. The influences of wind-induced cable shaking and camera shaking on the accuracy of the height difference measurement are also theoretically analyzed. The results show that cable strand torsion and camera roll have a great impact on the measurement accuracy, while the impacts of the cable strand swing and vibration, camera swing and vibration, and camera pitch and yaw are insignificant. The vision-based measurement system tested in the field experiment also shows a measurement error within 3 mm, which meets the requirements for cable adjustment construction. At the same time, the vision-based measurement method proposed and validated in this paper can improve the measurement accuracy and efficiency of strand alignment in a strong wind environment. Potential risks involved in the manual measurement, e.g., working at heights and in strong wind environments, can be eliminated, facilitating the automation of the cable erection process.

Construction-Monitoring Analysis of a Symmetrical Rigid Frame Tied Steel Box Arch Bridge in Southwest China Based on Segmental Assembly Technique

“…The control of a main beam's alignment is a gradual process that may significantly affect subsequent stages. During bridge construction, experts have started carefully considering whether a girder's alignment matches that designed for adjustments [16][17][18]. Zhang et al [10] accurately identified deflections during construction and derived the relationship regarding strain for evaluation.…”

Application of Emerging Technologies to Improve Construction Performance

“…Huang et al [22] proposed a vision-based method to accurately measure the cable alignment of long-span suspension bridges in a strong wind environment. Aided with pre-installed optical targets, the relative sag of cable strands was monitored remotely, automatically and in a real-time manner.…”