Wind Load Characteristics Of Long-span Flat Roof Structure Under Downburst

In recent years,gusts,thunderstorms and other transient winds are considered to be the cause of extreme wind speed in many parts of the world,and have attracted more and more attention.Among them,downburst is a very destructive strong wind,which often occurs in thunderstorm weather,and has great damage to the building structure.Largespan flat roof structures are widely used in large infrastructure,cultural and sports venues.They are light in weight and flexible,and are sensitive to wind loads.Many large-span structures are in the range of extreme wind profiles of downburst currents.Therefore,based on the impact of the downburst,the characteristics of long-span flat roof structures are analyzed in this paper.The main contents include:(1)Physical experiment of steady-state downburst impact wind: Three-dimensional scale physical experiment of downburst impact wind was carried out on flat terrain.The downburst wind profiles at different radial positions were analyzed and compared with the empirical models and measured data.The wind tunnel test of pressure measurement for large-span buildings in downburst wind field is completed,and the characteristics of average wind pressure,fluctuating wind pressure,roof shape coefficient and three-direction wind force coefficient of buildings at each radial position are analyzed.According to the analysis results,the location of the distance between the downburst center and the windward side of the building is determined to be 1 times the diameter of the jet for subsequent numerical simulation analysis.(2)Steady-state numerical simulation of downburst impingement wind: The building is placed at a distance of 1 times the jet diameter from downburst center,and the average wind profile and wind pressure on the building surface obtained by numerical simulation are compared with the experimental results to verify the validity of numerical simulation.By changing the wind direction angle of the inflow of a rotating building,the wind load characteristics of the building at the wind direction angles of 0,15,30 and 45 degrees are calculated,including the average wind pressure characteristics,fluctuating wind pressure characteristics,three-direction wind force coefficients,etc.(3)According to the numerical simulation results of steady downburst,the wind pressure time history of roof measuring points under different wind directions was analyzed.The distribution characteristics of Gauss and non-Gauss of roof measuring points were analyzed by probability density method,high-order statistics discriminant method and Kolmogorov test method.The non-Gaussian distribution is obviously asymmetric.The non-Gaussian distribution is obviously asymmetric.The larger the average wind pressure coefficient of roof,the more regular the fluctuation is near the mean,though the wind pressure time history fluctuates greatly.Meanwhile,the Translated-Peak-Process Method(TPP method)is used to calculate the peak factor of non-Gaussian distribution of measuring points,and the area-time history method is used to calculate the maximum average wind pressure coefficient of measuring points under each wind direction angle,and the most disadvantageous wind pressure coefficient isoline of the whole wind downward roof is obtained.(4)Flow field characteristics of unsteady downburst impinging wind and analysis of wind load characteristics of buildings under its action: The development process of unsteady downburst was simulated by means of self-defined entrance function.The instantaneous wind pressure characteristics of long-span structures before and when downburst impacted buildings,and the time history values of overall three-direction wind force coefficients were studied.The development process of "annular vortices" of downburst,the alternating shedding phenomenon of "vortices" around buildings,the change and development process of "columnar vortices" and "conical vortices" produced in the roof during fluid operation are presented intuitively.Combining with the wind load characteristics mentioned above,the distribution and unbalanced development of wind pressure on roof and the fluctuation of lift and drag coefficients of building as a whole are explained.