Wind Spatial Distribution Characteristics In The U-shaped Valley And Its Effect On Buffeting Response Of Long-span Bridges

The designs and constructions of long span bridges for crossing valleys and rivers in the mountainous areas are increasing dramatically since the 21 st century in China.At the same time,the strong turbulent flow in the valley area is a sufficient condition for bridge buffeting.At present,scholars have focused on the on-site wind field characteristics of several mountainous areas,as well as the influence on bridge buffeting responses.However,the distribution law of wind characteristics in the valley area is still unclear,and the wind parameters of the Specification are difficult to be applied to the wind resistance design of mountainous valley bridges.Therefore,the main work carried out in the present study is as follows:(1)The field measurement was carried out in a U-shapped valley.It is found that the wind speed profile deviates seriously from the power exponent and logarithm law in the Specification.The average wind speed along the deck exhibits uniform and parabolic distributions.The measured wind attack angle is out of the range recommended by the current design Specification.The gust factor and turbulence intensity show a linear correlation.The preeminent frequency of fluctuating wind increases with the increase of height.The difference between the measured power spectrum and the power spectrum models in the Specification is significant.A concept,deflection rate of wind direction,is defined to describe the variation of wind direction with the elevation.A three-parameter power spectrum model which is closer to the measured spectrum is proposed,and the probability distributions of the three parameters and their relationship with elevation are also clarified.The applicability of the coherence function defined by Hui is verified.The reasonable elevation of the meteorological mast instrument at the bridge site is suggested.(2)Numerical simulation and wind tunnel test were carried out to study the influence of oncoming flow characteristics and valley characteristics on wind parameters.The results show that the spatial distribution of wind characteristics tends to be stable with the increase of oncoming wind speed.When there is an angle between the oncoming wind direction and the valley strike,at the same position,the wind speed decreases,the wind attack angle and turbulence intensity increase.The oncoming wind direction has the most obvious effect on the wind attack angle and wind deflection angle.The surface roughness distribution pattern in the valley has no obvious effect on the wind speed.When the ratio of width to depth decreases,wind speed acceleration effect,wind attack angle and turbulence intensity all decrease.A method to simulate the roughness in the valley is given,the relationship between the ratio of width to deep and the power spectrum model coefficient is proved,and the evaluation model of the influence degree of the oncoming flow characteristics on the wind parameters is established.(3)Based on Navier-Stokes equations,the characteristics of Couette flow and Poiseuille flow are analyzed,the distribution mechanism of wind speed in the U-shaped valley from the perspectives of pressure gradient,wall friction and U-shaped valley characteristic size can be explained.The spatial wind speed distribution in the U-shaped valley is deduced and verified,the simplified envelope function of spanwise distribution of wind speed is put forward.(4)The buffeting response of a long-span bridge in a U-shaped valley is analyzed based on the speed distribution model of oncoming flow,the oncoming wind speed and the location of the bridge.The results show that the buffeting response will be overestimated when the flow is uniformly distributed.The buffeting displacement response has a nonlinear relationship with the oncoming wind speed.At different positions,the buffeting displacement of the cable-stayed bridge is smaller than the recommended value.The buffeting response of the suspension bridge is reduced with the increase of relative height.Some suggestions are given for selecting reasonable wind characteristics when studying buffeting response of bridges in valleys.