Nonlinear characteristics of flutter and vibration bifurcation phenomenon of a centrally-slotted box deck and their mechanisms

In order to investigate the flutter stability of super-long-span cables supported bridges at large angles of attack (AoAs), the nonlinear flutter characteristics, vibration bifurcation phenomenon and mechanism of a centrally-slotted box deck with AoAs ranging between ±10° were investigated through wind tunnel tests of the sectional model. The results show that no flutter occurs for the sectional model system with AoAs between −2°～10°, nonlinear flutter with vibration bifurcation is observed with AoAs of −3° and −4°, and flutter appears automatically without any artificial initial excitation with AoAs between −5° ～ −10°. Both kinds of flutter are coupled vertical bending and torsional motions and finally attain a stable state of the limit circle oscillation (LCO). The onset wind speed of such nonlinear flutter descends with the increasing absolute value of the negative AoA, and the coupling extent increases with the reduced wind speed. The mechanism of the nonlinear flutter can be well explained by the curve of the system damping ratio against the amplitude. With zero points of the curve representing equilibrium states of the system, a positive slope indicates the stable equilibrium, corresponding to the steady-state amplitude, and a negative slope for the unstable equilibrium and the onset amplitude. For the nonlinear flutter with vibration bifurcation, the system has one stable equilibrium point and one unstable equilibrium point, while for the flutter not requiring any artificial excitation, the system has only one stable equilibrium point.