Study On Vortex-induced Vibration Characteristics Of High-order Vertical Bending Modes Of Long-span Bridges

The main beam of the long-span flexible bridge has a lower vertical bending frequency,a dense modal frequency,and a small damping.It has been observed that many such bridges' multiple modes have occurring severe vertical bending modal vortex-induced vibration under common wind speed at home and abroad.Besides,in Wind-resistant Design Specification for Highway bridges,the allowable amplitude of the vertical vortex vibration is inversely proportional to the vortex frequency,indicating that the specification has stricter limits on the high-order modal vertical vortex vibration.Therefore,revealing the response characteristics of high-order modal vertical vortex vibration of long-span bridges and establishing the amplitude prediction method of high-order modal vertical vortex vibration response is an important problem to be solved in the study of wind resistance of long-span bridges.In this paper,based on multi-point elastic support aeroelastic model wind tunnel test,the high-order modal vortex vibration characteristics and amplitude prediction method of long-span suspension bridge are studied.The main contents of the experimental research are as follows:(1)A multi-point elastic support aeroelastic model with coil springs as the vertical support is designed and fabricated,then the stochastic subspace method(SSI)and the stochastic decrement method based on analytical mode decomposition(AMD-RDT)are used to identify the first 5th order modes of the multi-point elastic support aeroelastic model under turbulent field conditions and static wind free vibration conditions.The results show that the designed multi-point elastic support aeroelastic model can better simulate the dense frequency characteristics of long-span suspension bridges,and can obtain smaller high-order vertical bending mode damping ratio.The frequency of the model obtained by the SSI and AMD-RDT is consistent,but it is smaller than the calculated value.The damping recognition results of the two methods are not ideal,and may be related to factors such as shorter sampling duration,more non-Gaussian white noise components in the measurement signal,and amplitude.The structural mode shape identified by the SSI method under different working conditions is basically consistent with the theoretical vibration mode calculated by ansys,and is basically consistent with the standard sinusoidal function.(2)The VIV wind tunnel test is carried out for the multi-point elastic support aeroelastic model with different frequency distribution and different modal damping ratios,the vortex-locking interval and response amplitude of the first 5th-order vertical bending mode of the aeroelastic model are analyzed.The results show that the highorder vertical bending mode of the aeroelastic model will have two vortex-locked wind speed intervals,and the amplitude of the high-speed vortex lock interval of the same mode can be smaller than the low wind speed vortex lock interval;in the same vortexlocked wind speed range,the maximum vortex amplitude decreases with the increase of the modal order.The reason for this phenomenon may be due to modal competition,and there is mutual inhibition between different modal VIV,which leads to the failure of fully developed modal VIV response.(3)By carrying out multi-point elastic support aeroelastic model higer-mode pressure-measuring wind tunnel test,the pressure data of different sections of the aeroelastic model are measured,and the lift coefficient of each section is calculated.The spanwise correlation of the vortex force with the wind speed in the static and vibration state of the model,vortex amplitude and vortex lock interval is analyzed.The results show that for different vertical bending modes,the maximum value of the lifting force spanwise correlation coefficient in the second vortex locking interval is located in the rising section of the vortex locking interval,the maximum value of the lifting force spanwise correlation coefficient in the first vortex-locking interval is located at the maximum amplitude of the vortex-locked interval.In static state,the lift spanwise correlation coefficient of aeroelastic model increases with the increase of wind speed(Reynolds number),and tends to zero with the increase of spacing.(4)Based on the distribution law of the vortex force spanwise correlation of the aeroelastic model,the calculation formula of the vortex vibration response of the multipoint elastic support aeroelastic model based on the Ruscheweyh model is derived and compared with the experimental results.The results show that the amplitude response of each mode vortex estimated by the Ruscheweyh model is larger than that of the experimental results.The value error of the rectangular section lift coefficient,the resonance assumption and the mutual overlap of the modal vortex lock intervals and the identification error of the damping ratio may be the main reasons leading to the prediction deviation of the Ruscheweyh model.