Identification Of Natural Vibration And Aerodynamic Damping Of Long-span Suspension Bridge Based On Ambient Excitation

Long-span suspension bridges featured by lightweight,flexibility,and low damping are particularly sensitive to wind load and can produce various wind-induced vibrations.Inherent dynamic characteristics of structures are the foundation for finite-element model updating,damage detection,and vibration control.While dynamic characteristics including natural frequencies,damping ratios,modal shapes could be influenced by various loads such as traffic,temperature variation,earthquake and wind.Thus,the modal frequency and modal damping ratio of the long-span suspension bridge under different wind speeds are accurately estimated,and the evolution law of modal parameters of the Xihoumen Bridge under steady wind load has been discovered using Natural Excitation Technique(NEx T)and Eigensystem Realization Algorithm(ERA)base on a large amount of measured wind speed data and response acceleration data near the Xihoumen bridge site.The main research contents are as follows:(1)The modal recognition method using NEx T and ERA is systematically summarized and derived.The possibilities and advantages of combining the two methods are illustrated and problems that may occur during the modal identification process using this method are pointed out.(2)A MATLAB program for identifying structural modal parameters using the NEx T-ERA method was compiled and the selection methods of sample duration,window time interval and Hankel matrix order in the modal identification process are discussed.Through parameter analysis,the conclusion was obtained as follow: The increase of sample duration and window time interval will improve the accuracy of modal parameter recognition.If any one of the two parameters is too small,the result of modal recognition will be abnormal;The selection of the order of Hankel matrix needs to be determined manually according to specific engineering background and a certain standard.(3)The round test method and the Hilbert-Huang transform method are used to analyze the non-stationarity of the amplitude and frequency of the measured wind speed sequence and the acceleration of the main beam at the site of the Xihoumen Bridge.The researchs show that the measured wind speed sequence and the main beam acceleration signal at the site of the Xihoumen Bridge are both stationary time series,satisfying the assumption of the NEx T-ERA method;The non-stationarity of the wind speed sequence is obviously greater than that of the main beam acceleration response under the corresponding wind speed.(4)Using NEx T-ERA modal identification technology,a total of more than 3,000 sets of time series of the Xihoumen Bridge under seven different wind speeds were analyzed,and the vertical,lateral and torsional modal frequencies and modal damping ratio of the bridge structure under different wind speeds were obtained.The conclusion was obtained as follow: The suspension bridge has a relatively low damping ratio.The distribution interval of the first three-order vertical damping ratio is 0.2%-2.1%,the distribution interval of the first three-order lateral damping ratio is 1.3%-2.8%,and the distribution interval of the first three-order torsional damping ratio 0.3%-2.3%.The change of wind speed has little effect on the modal frequency of the long-span suspension bridge,which can be ignored,while the effect on the damping ratio is significant.The overall performance is as follows: As the wind speed changes,the first three orders of vertical damping ratio increase significantly Large,the lateral damping ratio remains unchanged,while the torsional damping ratio is significantly reduced.The change of wind speed has a significant effect on the dispersion of the identification results of the damping ratio.The dispersion of the identification results of the vertical damping ratio increases as the wind speed increases.The dispersion of the result decreases as the wind speed increases.