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

Suspension bridges are highly susceptible to aerodynamic instabilities that may cause structural damages and malfunction. These instabilities include flutter, which exhibits negative damping accompanied by an oscillation increase due to disturbance, and static torsional divergence (STD), which is characterized by a decrease in the torsional stiffness accompanied by torsional deformation. Specifically, the classical STD concept has attracted significant research attention lately. This concept has been described in terms of critical flow speed and unlimited structural rotation growth for a long time. However, this definition is limited and fails to account for the related aerodynamic effects fully. Based on the previous findings, different methods for evaluating the critical wind speeds of STD and their impact on bridge structures have been proposed.

The negative aerodynamic stiffness is of great significance in the classical theory of STD, even though the STD of airfoils and suspension bridges exhibit significant differences. However, the evolution of structural stiffness during the wind-induced response process remains poorly understood despite its practical importance. The stiffness degradation has been found to cause symmetric and upward motion that further reduced the cable sag by unloading its tension. To date, there are two main deformation modes associated with the STD of bridges, namely, symmetric aerostatic torsional divergence (SSTD) and asymmetric aerostatic torsional divergence (ASTD). However, the related mechanism is yet to be understood.

Herein, Professor Zhitian Zhang from Hainan University together with Professor Ledong Zhu from Tongji University, investigated the wind-induced SSTD and ASTD of flexible suspension bridge structures. Specifically, the authors investigated their macroscopic characteristics and related mechanisms. A generalized model that was simplified by neglecting the deformation of the bridge tower, as well as FEM-based numerical simulations, were utilized to analyze the vertical stiffness property of an individual cable, bridge deck, the torsional stiffness of the whole cable system, and the negative aerodynamic stiffness. Their related influence on the entire bridge was also investigated. Their research work is currently published in the Journal of Fluids and Structures.

The research team showed that the symmetric torsional stiffness associated with the main cable system was larger than that of the asymmetric system. Consequently, the symmetric torsional stiffness of the bridge deck was significantly lower than that of its asymmetric counterpart. This indicated that the contribution of the bridge deck to asymmetric torsional stiffness was negligible. Wind-induced symmetric static torsional divergence appeared ahead of its asymmetric counterpart in all scenarios possible. ASTD exhibited twist-locking after the critical speed, which resulted in an abrupt increase in the cable tension. In contrast, SSTD achieved a free cable state after unloading the tension. Furthermore, an increase in the bridge deck torsional stiffness exhibited two main effects: first, it increased the critical wind speed, and second, it completely altered its pattern and nature.

In summary, Professor Zhitian Zhang and Professor Ledong Zhu studied the underlying mechanisms of wind-induced ASTD and SSTD of flexible suspension bridges. Based on the findings, ASTD was found to differ from SSTD in various ways significantly. The aerodynamic torsional stiffness was not affected by symmetrical characteristics associated with the deformation of the structure. The authors said their study will advance development of robust design of durable and high-performance flexible suspension bridges.