Research On Wind-induced Instability Of Long-span Girder Suspension Footbridge

Long-span footbridge often uses suspension bridge because of its spanning ability,structure reasonable, does not need to build a pier and other reasons, especially suitable for mountain valley and across the deep lake, etc.. As for safety and comfort, the stiffening girder suspension footbridge is better than flexible girder suspension footbridge. However, the stiffening girder of long span pedestrian suspension bridge is still relatively slender and soft.Suspension footbridge girder width is only meet the requirements for pedestrian, generally is2 m to 5m, width span ratio and height span ratio are dramatically reduced with the increase of span, these footbridge are very sensitive to pedestrian load and wind load, structural dynamic stability is poor, and often cross the deep canyon, bad wind environment of the bridge site,easily lead to a larger amplitude of wind-induced vibration, so the wind resistant design is an important aspect of this type of footbridge. However domestic special specification or code provisions on the footbridge design are relatively less, wind resistance research of long-span stiffening girder suspension footbridge is not enough.In order to better serve the long-span stiffening girder suspension footbridge wind resistant design, this paper takes a suspension footbridge with a main span 420 m stiffening girder as the research background, study the aerodynamic performance and wind-induced vibration characteristics of long-span stiffening girder suspension footbridge, establish the whole process analysis on wind resistance performance of long-span stiffening girder suspension footbridge based on detailed research of aerodynamic performance, with expansion and detailed investigation on wind resistant stability of long-span suspension footbridge with stiffening girder.Design and manufacture the wind tunnel test model, and test aerodynamic parameters and the wind induced vibration performance of stiffening girder section with different wind fairing type, deck slot, railing protecting network structure. Based on wind resistance performance the optimized stiffening girder section is presented to meet the wind resistant stability and simple construction. Comprehensive research on wind resistance performance of long-span suspension bridge with stiffening girder, i.e., three-dimensional nonlinear staticwind stability, vortex vibration, buffeting and flutter stability. In this thesis the main research work are:(1) Dynamic characteristics contrast research of long-span stiffening girder suspension footbridge with different structure design scheme for span ratio, wind resistance cable, central buckle. Analysis of the dynamic characteristics of the possible high density pedestrian flow and the extreme state of ice cross section.(2) The expression of the skew wind load is deduced with consideration of the spatial deformation of the bridge. Static wind instability of long-span stiffening girder suspension footbridge is analyzed considering skew wind effect. The static wind response and failure pattern long-span stiffening girder suspension footbridge with and without wind resistant cable are studied. Analysis of the development path and mechanism of instability of long-span stiffening girder suspension footbridge, and the aerostatic stability parameter study.Optimization of aerodynamic section of stiffening girder based on static wind stability.(3) The vortex vibration performance of long-span stiffening girder suspension footbridge are conducted in the wind tunnel tests. Vortex vibration test on the aerodynamic design section of a variety of stiffening girder and some extreme states. The surface pressure distribution of the deck plate the stiffening girder is obtained by the measurement of the surface pressure. The possible states of vortex induced vibration of such bridges are discussed.(4) The improved harmonic synthesis method based on MATLAB is used to compile the spatial wind field simulation program, to simulate the fluctuating wind field of the bridge space structure and to test the effectiveness of the wind field. Considering the nonlinear factors of the static wind additional wind angle of attack, the transient wind attack angle and and the geometric nonlinearity, based on the ANSYS, the 3D nonlinear time domain analysis program is developed for the long-span bridge. Buffeting response analysis of the natural state and extreme state of the long-span suspension footbridge. Optimization of aerodynamic section of the stiffening girder based on the buffeting performance is performed, and the influence parameters are studied.(5) Study on aerodynamic section type selection based on flutter stability of long-span stiffening girder suspension footbridge. A weighted least square method is used to identify the aerodynamic derivatives of the free vibration of the bridge section. The random decrement technique is adopted to deal with the vibration signal of high wind speed, and to improve the recognition accuracy of the aerodynamic derivatives in the high wind speed area. The full mode 3D flutter analysis program is developed based on ANSYS for the bridge with static wind additional wind attack angle, and study on the influence parameters of footbridge flutter.This study is a useful supplement and development of the wind resistance of suspension footbridge, has a certain practical significance for engineering applications. This study especially has important significance for the design and construction of long-span suspension footbridge with large wind load in mountainous area.