Study On The In-field Measured Data, Wind-rain Action And Wind-induced Fatigue Of A Long-span Suspension Bridge

With the rapid increase of bridge spans over years, the suspension bridge becomes more flexible and especially more sensitive to the wind action; the structural safety of long span suspension bridge under wind action has become an important issue of great concern. The wind resistant study of long-span bridge belongs to the late started research areas and there are still many unresolved issues. Relying on the Natural Science Fund Project " prototype observation and verification of dynamic disaster of long-span bridge under wind action "(item number:90815022), the long-term field observation of wind environment and structural responses of a long-span suspension bridge, wind-rain combination, fatigue life of wind-induced vibration and safety assessment of bridge component under extreme wind load are studied by using the field measurement and theoretical analysis methods, the main researches and conclusions are as follows:(1) In order to provide reliable basis for wind resistant evaluation of a long-span suspension bridge, the structural health monitoring system was installed and the long-term wind data at the bridge deck and the top of bridge tower were recorded. The average wind speeds and directions, variation of wind speeds with height, turbulent characteristics and spatial correlation are analyzed by using statistical methods and spectral analysis. It is found that the dimensionless exponent decreases with the average wind speed increases; the measured turbulence intensities are greater than the recommended values, the turbulence power spectrum can well fit the standard spectrum, however, the measured spectral values are considerably smaller in low frequency ranges; the mean wind speed of downstream reduces and the turbulence intensity increases significantly, the spectral characteristics of downstream also change obviously.(2) Based on the vibration monitoring system of a long span suspension bridge, the measured wind-induced responses of stiffening girder and hangers are discussed; also the theoretical responses are analyzed and compared with the measurement. The results indicate that the RMS of acceleration response of stiffening girder has a linear relationship with the average wind speeds vertical to the bridge deck, and its dynamic characteristics are stable with the wind speed increases; the dynamic response of hanger under wind is a high frequency resonance vibration and the resonance frequency increases with the wind speed increases; the buffeting responses of theoretical analysis are significantly greater than the measured values, the buffeting analysis still need more refinement.(3) Based on the basic characteristics of rainfall, consider the impact effect of raindrops on the bridge deck, the impact load and element damping model of rainfall are proposed, and the wind-rain induced vibration of a long-span suspension bridge are calculated. The results indicate that, the heavy rainstorm will increase the structure's mean displacement response of along wind direction, vertical and torsion mean displacement responses are less affected, while due to the damping effect of rainfall, the RMS values of the displacement and stress responses will be reduced.(4) Based on the damping model of rainfall, the wind-rain flutter analysis methods are proposed, and the flutter of a long span suspension bridge is analyzed. The results indicate that due to the damping effect of rainfall, the structure's critical flutter wind velocity is further increased, however, due to the small quality and the low vertical landing speed of raindrops, the effect of rainfall to the bridge flutter is relatively small.(5) Based on the continuous observations of mean wind speeds at a long span suspension bridge site, the distribution of fatigue wind' speed and direction are discussed and the wind-induced fatigue life is analyzed. The results indicate that the fatigue wind speed distribution obeys the Weibull distribution, the bridge stiffening girder's wind-induced fatigue life is419.24years; the cross-sea suspension bridge locates in the marine corrosion environment, the combined effect of corrosion and fatigue will greatly reduce the hanger's wind-induced fatigue life.(6) Hangers are primary members for suspension bridge. The hanger's strength model is established considering broken wire and corrosion, and its reliability analysis method under extreme wind load is proposed by Monte Carlo method. The hanger of a long span suspension bridge is analyzed in detail and the results indicate that the hanger's reliable indicator has a almost linear attenuation relationship with the rate of broken wires increases, the corrosion of steel wire can not be ignored.