To investigate the dynamic characteristics and flutter performance of suspension bridges with double introverted main cables, a hypothetical bridge, with its main span 1708 m, was taken into account by decreasing cables' transverse distance. Dynamic finite element analysis shows that, with the decreasing of cables' distance, system's fundamental torsional frequency can raise significantly, while the fundamental vertical frequency keeps steady. Meanwhile, a lateral and torsional coupled mode can be found, and its frequency is between fundamental vertical and torsional frequency. Both lateral and torsional equivalent mass of that mode remain in a rather low level. To research the influence of the coupled mode on system's flutter performance, the state-space approach was introduced into the analysis both two-dimensionally and three-dimensionally. It indicates through 2D analysis that, with a closer cables' distance, the coupled mode is more harmful. While by 3D analysis, the participation of that mode is higher when the cables' distance decreases. Conversely, the fundamental torsional mode is more determinative. Moreover, the second vertical mode plays a non-negligible role during the divergence. To sum up, for this kind of introverted-cable suspension bridge, the flutter divergence is an integrated result of multi-modes. Thus, the traditional 2D analysis can be hazardous and a 3D analysis is essential. © 2016, Editorial Board of Journal of Huazhong University of Science and Technology. All right reserved.
