A fundamental understanding of the flutter stabilization of streamlined box girders is of critical importance for ensuring the structural safety of superlong span bridges. This study systematically investigated the flutter characteristics of thin plate sections by performing theoretical analysis in conjunction with wind tunnel tests with the aim of evaluating the flutter mechanism of superlong-span bridges with streamlined box girders. From the results of a two-dimensional, two-degree-of-freedom (2D-2DOF) flutter analysis, it can be seen that with increases in f_torsion/f_heaving, the aerodynamic damping part D of the torsion motion becomes more important and the level of participation level of heaving motion gradually decreases. Thus, the relationship between the structural torsional–heaving frequency ratios (f_torsion/f_heaving) and the critical velocities of flutter can change with changes in the structural frequency ratio in different ways. Furthermore, the method of flutter energy analysis was used to investigate the components of energy input and energy dissipation, and the results show that the components associated with flutter derivatives A₁* and A₂* produce the maximum positive and negative energy inputs, respectively. The maximum energy dissipation has a strong correlation with the mechanical damping force. In addition, the measurements of flow field using particle image velocimetry (PIV) demonstrate that the scale of vortex streets becomes larger with increases in wind velocity, while the bobbing movements of the vortex streets could lead to the flutter instability of thin plates.

Slab track structures become deformed under the effects of differential subgrade settlement. According to the properties of the China Railway Track System (CRTS) II slab track on a subgrade, a three-dimensional (3D) coupled model based on both the discrete element method (DEM) and finite difference method (FDM) was developed. The slab track and subgrade were simulated using the FDM and DEM, respectively. The coupled model was verified. The deformation of the slab track and contact forces of gravel grains in the surface layer of the subgrade were studied under differential subgrade settlement. The effects of settlement wavelength, settlement amplitude, and other types of settlements were also discussed. The results demonstrate that the settlement amplitude and settlement wavelength of the subgrade have significant effects on track deformation. The deformation amplitude of the slab track increases nonlinearly with an increasing settlement amplitude of the subgrade. Increases in the settlement wavelength and amplitude of the subgrade significantly increase the maximum value of the contact force of the gravel grains in the subgrade. The maximum contact force of gravel grains near the boundaries of the settlement section can reach two to three times that of the unsettled condition, which makes it easy to accelerate the plastic settlement of the subgrade.