Study On Downburst Characteristics And Wind Load Effects On Large-span Bridge With High Piers

With the intensification of global warming,the frequency of weather disasters is increasing every year worldwide,and engineering structures are inevitably subjected to more extreme wind loads.The downburst is one of the most frequent typical local strong winds,and the strong airflow generated after impacting the ground is significantly different from the synoptic wind in the atmospheric boundary layer(ABL).The existing codes on structural wind engineering still considered less about the characteristics of such specific wind loads and its effects.Furthermore,the disaster mechanism for bridges and structures under downburst was far from being clarified.This paper conducted field measurements,computational fluid dynamics,wind tunnel tests and finite element numerical calculations to investigate the characteristics of downburst wind field and its effects on large span bridges with high piers.The main research works are shown as follows:(1)Based on the on-site wind observation system at the location of Linyi Yellow River Bridge,the wind field data of a four-year observation period were processed and identified,and the typical downburst wind events were identified.The wind field parameters of downburst wind,such as time-varying mean wind speed,gust factor,time-varying turbulence intensity and power spectral density function of non-stationary turbulence during the event period and their correlations were investigated.Moreover,the parameter characteristics of non-stationary non-Gaussian processes at different time intervals were analyzed.Meanwhile,the parameters mentioned offer the support to numerical model of the downburst wind field in the following sections.(2)A three-dimensional CFD numerical simulation of the impact jet was applied to simulate the mean wind field of the downburst,and an inlet intensity evolution function was introduced to obtain time-varying mean wind speed data of the downburst flow that was closer to the field measured data.Furthermore,the non-stationary turbulence velocity data of the downburst flow was obtained by using the harmonic synthesis-based non-stationary turbulence velocity decomposition method based on the mean wind model.Combining the mean wind model and the turbulence wind model,a complete life-cycle wind field model of the downburst was constructed.Furthermore,the structural responses of the Commonwealth Advisory Aeronautical Research Council(CAARC)model under the downburst wind were calculated,and the numerical results were compared with the wind tunnel test results of the CAARC under downburst wind.(3)Taking the Linyi Yellow River Bridge(LYRB)in Shanxi as the engineering background,using the full aeroelastic model wind tunnel test method,the wind-induced vibration responses of the LYRB with maximum incremental launching cantilever were investigated in the uniform wind field and the ABL turbulent wind field,respectively.Moreover,the displacement responses of the launching nose end under different wind yaw angles in the ABL turbulent wind field were analyzed and compared.Using the equivalent wind spectrum method based on Sears function to consider the influence of aerodynamic admittance function,the buffeting responses of the LYRB with maximum incremental launching cantilever under the ABL turbulent wind field were analyzed in time domain.Furthermore,the numerical results were compared with the experimental results.The downburst numerical model was applied to calculate the buffeting response of the LYRB with maximum incremental launching cantilever.Moreover,the numerical results of the LYRB under downburst wind were compared with the experimental results of the bridge structure under the ABL turbulent wind field.(4)Relying on the LYRB in Shanxi,the investigations on the windresistance measures during the construction period of the large-span bridge with high piers were carried out.The tied-down wire ropes were proposed to suppress the wind-induced vibration of the launching nose of the LYRB.Moreover,the effects of the vibration suppression scheme were evaluated and compared in the ABL turbulent wind field and downburst wind field by means of the time-domain buffeting analysis method,respectively.