Field Measurement of Wind-induced Vibration Response of Long-span Cable-stayed Bridge under Downburst

Taking Sutong Yangtze River Highway Bridge (STB) as an engineering background, a downburst wind and the vibration acceleration responses of the bridge structure were monitored based on the wind-induced vibration response monitoring system of the bridge. Furthermore, the wind speed and direction, vibration responses of the main girder under the thunderstorm were investigated. Firstly, the measured wind speed and wind direction data of the downburst wind at the bridge site were analyzed, the time-varying mean wind and fluctuating wind characteristics of the downburst at the mid-span of the main girder and the pylon tops were obtained. Then, the wind-induced vibration acceleration responses of the main girder under the downburst were analyzed. The research results show that the wind speeds at the main girder level and the pylon tops of the STB changes abruptly under downburst, which lasts approximately 10~24 minutes. The maximum instantaneous wind speeds at the leeward and windward side of the main girder in the middle of the main span of STB are 32.4 m/s and 27.3 m/s, respectively. Meanwhile, the maximum instantaneous wind speeds at the south and north pylon tops are 60.5 m/s and 62.9 m/s, respectively. The turbulence intensity for a time interval of 30 s at the main girder level is about 0.048~0.32, and the turbulence intensity for a time interval of 10 minutes is about 0.43~0.51. The reduced fluctuating wind speed at the downstream of the main girder and the north pylon conforms to Gaussian characteristics, and its power spectrum is in good agreement with the field measurement results by Burlando and other scholars. Obvious short-term vertical and horizontal vibration occurred near the middle of the main girder (that is, the anchorage of NJ26D and NJ32D cables), and the corresponding acceleration response amplitudes were 0.25 m/s2 and 0.10 m/s2, corresponding to the displacement amplitudes of 0.12 m and 0.03 m, respectively. The vertical vibration response of the main girder is obviously larger than the transverse vibration response. The predominant frequency of vertical vibration of the main girder is 0.183 Hz, which is close to the 1st symmetric vertical bending mode frequency 0.174 Hz of the main girder. And the predominant frequency of horizontal vibration is 0.117Hz, which is close to the 1st symmetry lateral bending mode frequency 0.097 5 Hz of the main girder. © 2021, Editorial Department of Journal of Hunan University. All right reserved.