| Large span vertical and horizontal stiffened girder pedestrian suspension bridge has become the preferred bridge type for crossing valleys in scenic areas in recent years due to its beautiful shape,short construction period and light weight.However,due to its small width span and large flexibility,it is more sensitive to wind load,and it is typical blunt body section,which is prone to vortex-induced vibration under low wind speed.Although the vortex-induced vibration is a kind of self-excited amplitude limitation vibration,which will not cause devastating damage to the bridge structure,the excessive amplitude will cause fatigue damage to the structure and affect the safety and sightseeing experience of visitors.Therefore,in order to grasp the vortex vibration performance of the pedestrian suspension bridge with a small aspect ratio and effectively determine their pneumatic control measures,this thesis takes a proposed pedestrian suspension bridge with the aspect ratio of 7:250 in China as the engineering background and conducts a systematic study on the vortex vibration performance,vortex vibration mechanism,pneumatic control measures and vortex force model of the main girder with the help of CFD numerical calculation method,The details are as follows:(1)The accuracy of the vortex vibration solution method in this thesis was verified by taking a rectangular section with an aspect ratio of 4 as the research object,according to which the vortex vibration response of a vertical and horizontal stiffened girder section of a certain line suspension bridge was numerically calculated,and the vertical bending and torsional vortex vibration responses of the stiffened main girder at ± 5°,± 3°and 0°wind attack angles were obtained,and the effects of wind speed and wind attack angle on the vortex vibration response of the girder were explored,and the vortex off of the vortex vibration occurrence was summarized.The evolution law,the distribution characteristics of wind pressure and vortex vibration contribution coefficient on the surface of the girder are summarized.The results show that the most disadvantageous vertical bending vortex vibration and torsional vortex vibration appear at-5°and 0°wind attack angle respectively,and the wake end of the beam is the main contribution area of vertical bending vortex vibration.(2)Based on the mechanism of vortex vibration in the vertical and horizontal stiffened girder,three different pneumatic control measures of wind nozzle,inverted L-shaped skirt plate and lower horizontal deflector were selected.By changing the angle of the wind nozzle,the length of the inverted L-shaped skirt plate and the lower horizontal deflector,the vibration suppression performance of each pneumatic measure is analyzed with the variation law of the measure size.The vortex shedding evolution law and the surface wind pressure distribution of the vertical and horizontal stiffened girder before and after the installation of aerodynamic control measures are compared.The results show that the vibration suppression effect of the 70 °sharp angle nozzle is the best,the fillet nozzle is the second,and the 90 °angle nozzle is general;with the increase of the size of the inverted L-shaped skirt,the vibration suppression effect is more obvious;the lower horizontal guide plate does not have the vibration suppression effect with the increase of length.(3)Based on the Scanlan empirical nonlinear model for parameter identification,the variation law of the pneumatic parameters with the wind angle of attack is discussed,and the vortex excitation force is reconstructed and its components are analyzed accordingly.At the same time,the reconstructed vortex force values are compared with the simulated vortex force values extracted from the CFD numerical calculation results,and the displacement is back-calculated from the vortex force reconstruction values.The results show that the model describes the vortex-induced force of the stiffened girder to some extent,but it has some limitations. |
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