Wind-induced Vibration And Aerodynamic Measures Of Long-span Suspension Bridges With Steel Truss Girder In Complex Mountainous Canyon

In complicated mountainous canyon area, over which more and more long-span bridges have been built with the development of the highway traffic in Midwest China. Wind-resistance of long-span bridges is a hotspot and difficulty in the wind engineering realm, and will become more outstanding due to the particularity of complex inflow in mountainous canyon area. Therefore, in this study, the following studies were carried out to investigate wind-induced vibration and aerodynamic measures of long-span highway suspension bridge with steel truss girder in complex inflow of mountainous canyon.Firstly, a method of energy analysis for galloping instability was developed. Application of the CFD dynamic mesh technique, the occurrence possibility of galloping can be judged by calculating the input energy of aerodynamic forces during the forced harmonic vibration of cross-section. For the galloping of square cylinder, the unstable region was calculated by the Den Hartog criterion and the energy method respectively, and the results of the two methods were compared. The effects on the input energy by aerodynamic forces were further discussed under the condition of various vibration frequencies, amplitudes, wind velocities. The maximum amplitude of galloping and the minimum damping ratio for vibration suppression were determined. For the wake galloping of two circular cylinders, the unstable region of downstream suspender was calculated and the mechanism from work done by aerodynamic forces perspective was analyzed. The effects on wake galloping were further discussed under the condition of various radiuses of circling motion, vibration frequencies and velocities of wind flow.Secondly, according to the large wind direction in mountainous canyon area, the numerical simulation was adopted to analyze the unsteady flow around two square columns with round corner in tandem arrangement according to an actual bridge tower. The effect of wind velocity, wind direction and spacing ratio on aerodynamic coefficients of the tower cross section were analyzed. The magnitude and the change trend of lateral force of upstream and downstream column were compared to study their effect on the increase of tower torsional moment. The effects of the wake of bridge tower on the aerodynamic characteristics of long suspenders nearby the pylon were further analyzed under the condition of various wind velocities and directions. The possibility of vortex-induced vibration of long suspenders nearby the tower in the wake of bridge tower was researched by analyzing the flow field characteristics and the change of aerodynamic forces of suspenders. By energy analysis, the occurrence possibility of wake galloping of long suspenders nearby bridge tower was also discussed. The phenomenon of suspender vibration in the wake of bridge tower was found by wind tunnel test and the characteristics of its track were discussed.According to the large angle of attack in mountainous canyon area, a simply supported bridge with ideal plate was chosen as an object of study to analyze its flutter modality at different angles of attack by numerical simulation, and the mechanism was discussed from work done by aerodynamic forces perspective. The effects and mechanisms of central slot and central stabilizer on flutter stability were further analyzed at large angles of attack.According to the non-uniform inflow in mountainous canyon area, a 3D analysis method using ANSYS was achieved to analyze the flutter performances of bridge in non-uniform inflow. The effects of the position and range of non-uniform inflow, including angle of attack and wind speed, along the bridge span on flutter performance of bridge were studied after determining the non-uniform inflow can make the flutter stability decrease.Finally, the flutter performance of a long-span highway suspension bridge with steel truss girder and its optimization by the two aerodynamic measures were analyzed by wind tunnel test after determining the influence of the vibration direction difference between the test device and actual structure on critical flutter wind speed. A comprehensive optimization method was presented and its mechanism, as well as the effect on wind environment of lane, was analyzed by a simplified plate-truss CFD model.The results demonstrated as follows:the energy method agrees well with classical research results and the energy method can be used to estimate the occurrence possibility of galloping, wake galloping and flutter of cross-section. Vortex-shedding from bridge towers has a strong influence on the aerodynamic forces of suspenders at a wide range of wind directions. The suspenders vibrate in both longitudinal and transverse directions. The large-amplitude vibration of suspenders is observed when the vortex-shedding frequency from the towers coincides with the frequency of the suspenders. At large angle of attack, central slot makes the plate further present the characteristic of bluff body and is not favorable to the flutter stability of bridge. Central stabilizer can improve the critical wind speed of bridge by hindering the movement of vortex formed in front of the plate if the stabilizer and the vortex are on the same side. Compared to the average angle of attack or wind speed along the bridge span, the non-uniform inflow makes the flutter stability of bridge decrease and has the greatest influence on mid-span. When the angle of attack of mid-span is less than that of two ends, the flutter critical state of long-span suspension bridge with steel truss girder is even controlled by antisymmetric torsional mode with higher freauencv. For long-span suspension bridge with steel truss girder, closing the central slot and setting central stabilizer is an effective measure to improve its flutter stability. The wind environments of all lanes on the bridge can be improved when the angle of attack is less than or equal to zero.