AbstractThe galloping problem of a three-degree-of-freedom (3-DOF) system with strongly coupled vertical-horizontal-torsional motions has drawn wide attention due to the high risk of galloping and subsequent serious damages caused, but the galloping mechanism remains unclear because of the complex interaction of structural coupled motion and aerodynamic forces. In the present work, a 3-DOF system with close frequencies in three directions was analyzed to reveal the galloping mechanism under a strongly coupled motion. Based on quasi-steady theory, theoretical models of a 3-DOF system with a single section and multiple subsections were studied by using a perturbation method based on order analysis, and a galloping stability criterion considering aerodynamic stiffness and small-frequency detuning was established. This criterion reveals the promoting mechanism of aerodynamic stiffness on galloping initiation. Galloping tests on a segmental 3-DOF model of an eight-bundled conductor with D-shaped ice accretion were conducted to examine the validity of the theoretical model and proposed galloping stability criterion under various wind conditions. For the strongly coupled system in the tests, aerodynamic stiffness was able to trigger galloping even under positive aerodynamic damping in all directions, which could be explained by the proposed criterion. Numerical examples were also employed to validate the proposed galloping stability criterion under attack angles of 0°–180°. The numerical study showed that aerodynamic stiffness can promote galloping regardless of attack angles, especially under high wind speeds. The results reveal the galloping mechanism of a strongly coupled 3-DOF system initiated by aerodynamic stiffness and provide a new insight into the prediction of galloping.Practical ApplicationsGalloping can cause great damages to slender structures, especially transmission lines. Understanding the galloping mechanism is essential for researchers, engineers, and designers because the galloping mechanism is the foundation for antigalloping design. It has long been suspected that the structures with closely tuned frequencies are prone to galloping because the coupling effect between different directions is very strong theoretically. For example, bundled conductors are more likely to gallop compared with single conductors, and the close frequencies of bundled conductors have been considered as an important reason. However, whether the strong coupling effect can promote or suppress galloping remains unclear, because there is a lack of an analytical solution to clarify this galloping mechanism. This study answers the question of why structures with close frequencies are prone to galloping. With the proposed criterion, the prediction of galloping initiation is greatly simplified, and this can provide important information for antigalloping design. Engineers can know more easily the wind attack angles where galloping is likely to occur, and appropriate galloping-suppression measures can be adopted, such as separating frequencies and antigalloping devices for conductors with close frequencies.