Study On Wind Load Distribution Characteristics Of Long-span Roof Structure Under Downburst

Formed by the strong downdraft in thunderstorm weather,downburst is a kind of sudden strong wind,which hits the ground violently.It has a wide geographical distribution and frequent occurence,and has a destructive effect on the building structure.However,at present,there is no relevant regulation on downburst wind field and wind load value in China's wind resistance design code.It is of great theoretical significance and engineering practical value to study downburst and its load characteristics for building structures from the perspective of structural design.In this paper,the simulation method of downburst in the atmospheric boundary layer wind tunnel and the influence of wind pressure distribution on the roof of large-span flat roof building structure are studied.The influence of wind direction angle,relative distance between building and thunderstorm center on the average wind pressure and extreme wind pressure distribution on the roof are investigated.The results are compared with the corresponding test results in exposure category B.The conclusions are as follows:(1)Based on an improved downburst generator,a 1:300 downburst wind field suitable for wind pressure distribution test of long-span roof structure is simulated ideally in the atmospheric boundary layer wind tunnel.The average wind speed profile is in good agreement with the classical theoretical model,and the turbulence profile is similar to the simulation results of the appropriative thunderstorm simulator.A dimensionless horizontal wind speed spectrum mathematical model of downburst wind field at the best wind profile location is established.The model is in good agreement with the experimental results,and is very close to the experimental results reported in recent international journals.(2)The average wind pressure coefficient of the large-span flat roof under downburst is negative,which is less affected by the radial distance.The highest average wind pressure coefficient is-4.20,which is caused by the conical vortex;(3)The extreme negative pressure coefficient is-9.85,which is 78.7% higher than that of-5.54 in exposure category B.The conical vortex is still the main cause of the high negative pressure of the roof.It is found that the power spectrum of the fluctuating wind pressure of the roof under downburst is significantly higher than that of in exposure category B.(4)The maximum wind pressure coefficient of flat roof in exposure category B is close to or less than 0.However,in different wind directions under downburst,the ratios of the roof area with the maximum wind pressure coefficients over 0.15 are between 51% and 75%,and the local maximum coefficient reaches 0.35.The high positive pressure in a wide range of areas will further affect the bearing capacity of the structure,which should be noted.