Experimental Study On Wind Load Of The Local Openings In Vulnerable Areas Of Double-Sloping Low-Rise Building Roofs

In the wind-induced damage of low-rise buildings,the proportion of open-hole damage in the vulnerable area of the roof under high wind is high,while the steady-state opening and transient opening damage are the main ways of destruction.Based on the the Terrain Category A,B and C,the double-slope low-rise buildings with a scale ratio of 1:40 and a slope angle of 18.4° are taken as the research object,and the steady-state and transient open-hole wind tunnel tests of the roof vulnerable area are carried out.The analysis focuses on the steady-state openhole internal pressure distribution and the fluctuating wind pressure characteristics of the roof vulnerable area,and the transient overshoot effect and internal and external pressure characteristics during transient opening.The transient and steady-state characteristics of the open-hole in the vulnerable area under different terrain category are compared.The comparison between the internal pressure of the test and the standard value after opening and the rule of the net wind pressure extreme value of the roof during the most unfavorable effect are discussed.The results show that the pressure distribution in the vulnerable area of the roof is relatively uniform,but the degree of influence on the opening position and the incoming wind direction is different.The wind-induced internal pressure is related to the opening ratio and the length of the inflowing orifice,and it is weakened when the leeward opening is opened.The internal pressure of the roof changes with the wind direction angle,and the variation of the leeward opening with the wind direction angle is significantly different from that of the windward.The pressure spectrum of the roof external pressure is distinct in different opening positions,and the internal pressure spectrum is significantly affected by Helmholtz resonance and vortex shedding,and the pressure spectrum is obviously affected by the wind direction when the leeward is opened.The correlation between the internal and external pressure of the roof opening is positive.The internal and external pressures in the orifice are mutually offset,and the correlation between the internal and external pressure on the unopened side is significantly smaller than that on the open side,which is unfavorable for low-rise buildings.When the roof vulnerable area is opened in an instant,the overshoot ratio varies greatly with the position of the opening,and the leeward opening will produce a more significant overshoot effect.Compared with the unopened,transient and steady-state phases roof external pressure,the open-hole damage has obvious transient effects on the external pressure,and the steady-state external pressure also has a large difference compared with the unopened external pressure.Therefore,it is necessary to consider the transient effects and the effects of openings in the wind resistance design of low-rise buildings.In the internal pressure of three different states of the roof vulnerable area,the steady-state internal extreme value and the transient fluctuate internal pressure distribution are relatively uniform,and the net wind pressure extreme value under the action of internal and external pressures varies greatly with the position of the measuring point.The most unfavorable position of the net wind pressure extreme value is mainly distributed in the orifice near the eaves side,the windward roof corner and tail area,the leeward roof corner,eaves and the ridge area.Under the Terrain Category A,B and C,with the increase of turbulence,the internal pressure of the roof increases gradually,and the range of fluctuations in the wind-induced internal pressure is also more severe.Under the three terrain category,the calculated value of the specification value is always smaller than the experimental,and the accuracy at different opening positions is related to the distance of the incoming flow acting on the orifice.Therefore,it is recommended that when the low-rise building has only one dominant opening in the roof and the opening ratio is greater than 0.02 and less than or equal to 0.1,the internal pressure can be taken as 0.5-1.0 times the external pressure coefficient of the orifice in category A;it can be taken as 0.7-1.0 times in category B;it can be taken as 0.3-1.0 times in category C.As the turbulence increases,the transient overshoot increases linearly.The net wind pressure extreme values of category A and B are relatively close,while that of category C is significantly larger than that of A and B,and the combined effect of internal and external pressure on the orifice near the eaves,the corners and tails of the roof,and the ridge of the leeward roof is most pronounced.