| Downburst is a local strong convective phenomenon which always accompanied by thunderstorm. The special wind field structure and wind speed profile make its action quite be different from the normal wind of atmospheric boundary layer. Impressive destruction on transmission towers, large span space structures and cantilevered roofs is brought by the downburst, as an extreme wind load, and this brings huge cost to human and massive damage to property. Currently, the downburst-induced wind load has been one of the hottest topics in structural wind engineering. However, duo to a variety of objective factors in downburst, the strong randomness of damage location and dominant regional difference, abruptness, removable, short duration and so on, the development of this research fields is restricted. Statistics and analysis on the spatiotemporal distribution of downburst based on the practically observed data should be solved urgently as an important method to understand characteristics of downburst and a key problem to accelerate process of this study. In addition, the development of computer technology has drew lots of researcher into the subject on downburst wind action by numerical simulation. But the unitary modeling results are far from satisfactory because of the complexity of downburst wind field, although the computational fluid dynamics has made a great progress and come to maturity. In contrast, the experimental simulation is a more effective way to reveal the downburst wind characteristics. Moreover, the current theory of wind-resistant design for building structures which focus on normal wind is apparently inapplicable of downburst. Thus there needs more works and researches to be done on the downburst-induced wind load.In allusion to the problems mentioned above, firstly, a series of statistics and analysis on the spatiotemporal distribution of downburst have been conducted. Secondly, the downburst wind field considering of surface roughness has been simulated in the lab. Thirdly, wind loads acting on prism-shaped buildings and an open-type roof have been measured separately. And then, the effects of the distance and azimuth angle of the downburst on the distribution of wind pressure have been studied. At last, the calculation of downburst wind load for building structures based on characteristic coefficient has been proposed. The main contents of this thesis include as follows:(1) Take the data of four meteorological stations located in Dalian area for instance, the temporal distribution of downburst was analyzed. On the basis of information of 182 international communication stations, the spatial distribution of the annual downburst-day’s number and annual duration were calculated and figured. Based on the annual downburst-day’s number, a partitioning method of the downburst-resistant design on buildings was provided. According to the observation data of 707 meteorological stations from 1971 to 2000, the mean annual downburst-day’s number, mean annual duration, the occurrence frequency and period of downburst were obtained for each partition.(2) Based on impinging jet model, the downburst wind field considering of surface roughness was simulated in the lab. The wind speed profile of radial sections in downburst under terrain B was observed. And then, the radial and vertical section of the horizontal and vertical wind velocity were figured respectively. Furthermore, the horizontal wind speed near the ground was descripted detailedly and the vortex locations in downburst wind field were obtained. On basis of wind speed profile of downburst and the streamlines of wind field, the downburst wind field could divide into three regions: core region, developing region and outside region.(3) Three prism-shaped building models, common shape in practical engineering, were chosen to simulate downburst action on the experimental platform, and three different heights of models were selected according to the effect range of downburst field. Through the changes of the position and angle of the building models, the effects of distance and azimuth angle of the downburst on the pressure distribution were observed. And the maximum wind action and the mean wind pressure coefficient were obtained. Furthermore, the downburst-induced loads were gotten in some transient positions under moving downburst.(4) The open-type roof which was the most vulnerable building model subjected to downburst was selected to simulate downburst-induced load in the lab. The wind pressure distributions of the model surfaces were obtained, and the total wind load acting on the model was calculated. With consideration of different positions and angles of the building model, the changes of wind loads on both surfaces and wind action of total open-type model at different distance and azimuth angle were observed.(5) Wind profile of horizontal downburst wind velocity along the height considering terrain condition was approximately similar with The Gaussian function and the experiential function was proposed by data fitting and analysis. Based on mention above, the wind pressure height coefficient of downburst was deduced and calculated. According to the calculation thought of current load code for the design of building structures, the downburst characteristic coefficient was introduced to consider the special wind field of downburst and a computational method was proposed for calculating the horizontal and vertical downburst loads of building structures. On the basis of the experimental results, the downburst characteristic coefficients were calculated and corresponding suggestive values were provided. The feasibility and validity of this method is validated through instance analysis.It would be encouraging that this thesis could make some contributions to step further research on downburst-resistant design for building structures. |
