Field Measurement And Wind Tunnel Test On Wind-sand Effect Of Low-rise Buildings

Sandstorm and typhoon are the main wind disasters that affect our country.They cause huge damage in transportation infrastructure,residential buildings and environment every year.The economic loss induced by them is enomous and the civil protection issue arised thereby.All these problems urged the research on the effect of wind disasters on buildings,however,at present,the research on wind resistance of buildings in China mainly focuses on the impact of typhoon,which mainly happen in the southeast coastal areas.Meanwhile there is little research on wind resistance of buildings in the northwest areas,where sandstorm occurs frequently.In addition,there is lack of knowledge on the characteristics of wind sand flow as well as that on the action of wind sand on buildings during sandstoms.To bridge such gap,we study the impact of wind-sand flow on low rise buildings in typical desert area,based on field measurement and wind-tunel tests.The investigated topics inculde the near surface wind field of desert area,the wind sand flow field,the wind pressure on the external surface of low rise buildings,and the wind sand forces acting on the low rise buildings.The main results are provided as follows:The results on wind field from field measurement are as follows:(1)A long-term field monitoring platform is set up at Shapotou observation site.The near surface wind field characteristics of typical desert landform are obtained through field measurement.The results show that the wind profile is consiste nt with the exponential law;the measured turbulence intensity has certain deviation from suggested value in current Construction Specifications and Design Standards.The turbulence integral value is between that from the Japanese Stansard and that from US Standard,the downwind gust factor has a negative correlation with the average wind speed and a positive correlation with the turbulence degree;the power spectrum of fluctuating wind speed is consistent with the von Karman spectrum.The results on wind pressure from field measurement as well as wind tunel tests are as follows:(2)Through field measurement and wind tunnel test,the features of wind pressure acting on low rise buildings in typical desert area are obtained.The results show that: in the case where the incoming wind direction is approximately vertical to the windward wall of the house,the average wind pressure coefficient is not related with the wind speed;the fluctuating wind pressure coefficient decreases with wind speed.Under the same wind direction,the wind tunnel test results show that: the average wind pressure coefficient of the middle area at the same height of the windward wall is greater than that of the two sides,and the absolute value of the average wind pressure coefficient of the middle area of the leeward wall is less than that of the two sides;the fluctuating wind pressure coefficient of the windward wall is greater than that of the leeward wall on the whole.The absolute value of the average wind pressure coefficient of the eaves and ridges of the windward roof is greater than that of the middle area,and the leeward roof shows the opposite law;the fluctuating wind pressure coefficient of the windward roof is greater than that of the leeward roofin general.(3)The shape coefficient of scale model surface in wind tunnel is analyzed and compared with the measured shape coefficient.It is found that some of the measured results are unconventional.Combined with the wind tunnel test results,the correction method of problem data is proposed.Through the correction,the shape coefficient of low buildings in typical desert area is determined,which provides a reference for the wind resistance and sand resistance design of buildings in this area.The research results on wind sand flow field fromwind sand tunnel are as follows :(4)In the wind sand tunnel tests where the sandstrom environment is simulated though blowing falling sediment,the vertical distribution of sand concentration,wind velocity and turbulence intensity arestudied.The characteristics of the wind sand field under differnet test condition are compared as well.The results show that the distribution of sediment concentration with height is related to the amount of sediment fall and wind speed.When the wind speed is the same,the sediment concentration at the same height is positively correlated with the amount of sediment fall;when the amount of sediment fall is constant and the wind speed is small,the sediment concentration decreases with the increase of hei ght,but when the wind speed is large,the sediment concentration first decreases and then increases with the increase of height.The sand movement reduces the wind speed and the reduction increases when the amount of falling sand grows.On the other hand,sand movement enhances the turbulence intensity profile,and the greater the amount of sand fall,the stronger the effect is.(5)In the wind sand tunnel tests where typical wind carrying sand environment is simulated though blowing still sand,the verti cal distribution of sand concentration,wind velocity and turbulence intensity are obtained,and the characteristics of wind sand flow field at different test conditions are compared.Different from the results from previous ones,the near surface sediment concentration decreases with the height under each wind speed.When the sand bed is the same,the sand concentration at the same height increases with the wind speed;when other conditions are the same,the sand concentration on the mixed sand bed is the largest,followed by the fine sand and the coarse sand.Sand movement weakens the wind speed near the ground,and the weakening degree increases as sand concentration increase;it has certain strengthening effect on the turbulence intensity near the ground,and the greater the sand concentration is,the stronger the effect is.The results on overall wind sand force from wind sand tunnel tests are as follows :(6)When the blowing falling sand tests are carried out in wind sand tunnel the overall stress features of low rise building are analyzed.The results from pure wind tests and that from tests of different sand condition are compared.They indicate that the resultant force coefficient of the average base shear force and the average roof lift coefficient of low rise buildings increase under the combined action of wind and sand,and the greater the amount of sand fall,the more obvious the effect is.After comparing the results of sediment fall in 0.5 hole and 1 hole,it is found that the average stress coefficient increases from 2%-8% to 6%-13%,compared with the pure wind case.(7)When blowing sitll sand tests are carried out in wind tunnel,the characteristics of overal stress against the low rise building are analyzed.The results from pure wind tests and that from tests of different sand condition are compared as well.They show that the combined action of wind and sand increases the average value of the resultant force coefficient of the base shear force and the lift coefficient of the roof.Compared with the net wind,the average stress coefficient of coarse sand,fine sand and mixed sand increases by 2%-7%,4%-10% and 6%-11% respectively.It shows that the effect of wind blown sand is positively related to the surface sand concentration.