Wind-induced symmetric and asymmetric static torsional divergence of flexible suspension bridges

Abstract

Abstract Two types of wind-induced static torsional divergence of suspension bridge structures are addressed in this work in terms of analytic and numerical approaches. The vertical stiffness of a single cable, torsional stiffness of the cable system, bridge deck, and negative aerodynamic stiffness are analyzed via generalized models simplified by ignoring bridge towers’ deformation. It is found that the main cable system’s symmetric torsional stiffness can be one order of magnitude larger than its asymmetric counterpart. The bridge deck’s symmetric torsional stiffness is only a quarter of its asymmetric counterpart. The aerodynamic torsional stiffness, on the other hand, is found to be independent on the symmetrical characteristic of the structure’s deformation. The generalized models show that, while the asymmetric torsional stiffness is unaffected by the structure’s vertical deflection, the symmetric torsional stiffness is subject to a critical vertical deflection, beyond which the part of stiffness contributed by the cable system vanishes and the whole system’s stiffness degrades to a very low level contributed uniquely by the bridge deck. The finite element simulations indicate that, because of the degradation nature of the symmetric torsional stiffness, the wind-induced symmetric static torsional divergence may emerge ahead of the asymmetric one even in the case of the symmetric stiffness being much greater than the asymmetric stiffness. The numerical simulations indicate that asymmetric divergence is characterized by ‘twist-locking’ phenomenon, signified by two half-spans’ abrupt rotating to opposite directions and swift tighten up of the tension in main cables. The numerical results also show that increase in the deck’s torsional stiffness not only enhances the critical wind speed of static torsional divergence, but also shifts the pattern of instability from asymmetric to symmetric divergence.