Field investigation and analysis of wind-induced vibration of Niujiaoyan cable-suspended bridge

Flutter has been of great interest as one of the control factors in the design of long-span bridges. There are very few flutter recordings of real bridges, hence the audio and video recorded on site and post-vibration studies about flutter are invaluable. At around 2 pm on May 1^st, 2021, two cable-suspended bridges in the Niujiaoyan area of Xiangba Town, Sinan County, Guizhou Province, were subjected to violent flutters. This study analyzes the response characteristics of these flutters and summarizes the field investigation of the bridge condition after the flutter. It is found that the large bending-torsional coupled flutter causes severe damage to the bridge elements, including the significant elongation of the bearing and stable cables on the leeward side of bridge 1#, and the fracture of the bearing and stable cables on bridge 2#. The relative height difference and the torsion angle increase continuously from the two sides to the mid-span, reaching the maximum values of 0.480 m and 4.58°, respectively, at the mid-span. By comparing the dynamic characteristics of the bridge under the field measurement and the finite element modelling, it is found that the bending and torsion frequencies change little after the flutter, while the transverse frequency decreases significantly. Analyses of the flutter videos recorded in the field show that the flutter is a classic bending-torsion coupled flutter with a vertical amplitude of about 5.2 m and a torsional amplitude of about 52°, and the phase difference between the vertical and torsional vibrations is about 90°. When the flutter occurs, there is a constant conversion among the kinetic energy of the vertical vibration, the angular kinetic energy of the torsional vibration and the potential energy; when the kinetic energy is at its maximum, the angular kinetic energy and potential energy are at the minimum.