Experimental And Numerical Study On The Internal Pressure Induced By The Local Failure Of Low-rise Building Roofs

During the strong wind passing through,the roofs of low building with double slopes are prone to be locally damaged,resulting in different openings created on the roof.Based on the wind fields of terrain category A,B and C and the double slopes low-rise building model with reduced scale of 1:40 and slope angle of 18.4°,extensive wind tunnel tests were conducted to investigate the internal pressure induced by the local wind damage on roof.The study mainly focused on the influencing factors of peak transient pressure caused by local roof area failure,fluctuating internal pressure power spectrum,the coherence function of internal pressure,the internal-external coherence function,and the internal pressure admittance function.The paper also analyzed the effect of terrain categories to the transient internal pressure,steady-state internal pressure,the characteristic of net wind pressure extremum,and finally compared the theoretical value and experimental value of internal pressure caused by the roof opening.The research results show that the peak value of transient internal pressure and steady-state internal pressure increase with the increasing of opening area ratio.The transient overshoot of internal pressure caused by corner opening is greater than that of leeward roof opening.The increasing of opening length weakens the transient internal pressure overshoot effect at the side of no opening roof.The peak transient internal pressure caused by opening at windward roof is much larger than that caused by opening at leeward roof ridge area.The influence of wind speed on the peak steady-state internal pressure is small,but the influence on the peak transient internal pressure is obvious.The peak transient internal pressure changes linearly with the wind speed.The peak internal pressure caused by windward roof opening increases with the increasing of wind speed,while the peak transient internal pressure caused by leeward roof ridge area opening decreases with the increasing of wind speed.The vortex shedding frequency and internal pressure Helmholtz frequency caused by square and rectangular openings at the roof corner increase with the increasing of the opening area ratio.With the same opening area ratio,the opening shape has little effect on the vortex shedding frequency,but the internal pressure Helmholtz frequency caused by rectangular opening is greater than that caused by square opening.When the building volume is expanded by 2 times,the vortex shedding of the airflow at the opening edge and the Helmholtz resonance of the internal pressure will produce coupling effect and increase the internal pressure fluctuation.The wind direction has little effect on the Helmholtz resonance frequency of the internal pressure,and the vortex shedding frequency is increased under the oblique wind direction.In the low frequency range,the internal pressure is well coherent and the coherence of internal pressure caused by the opening at windward roof is stronger than that caused by the opening at leeward ridge area.Both vortex shedding at opening edge and internal pressure Helmholtz resonance reduce the internal pressure coherence function.The attenuation slope of internal and external pressure coherence function caused by windward roof corner opening is twice that of leeward roof ridge opening.The internal pressure admittance function is greatly affected by the opening shape.The peak frequency of the admittance function related to the internal pressure Helmholtz resonance is not affected by the opening configurations,and the peak frequency related to the flow vortex shedding at the opening edge decreases with the increase of the opening area ratio of the roof.The transient overshoot ratio of internal pressure caused by opening at leeward roof ridge is greater than opening at windward corner under various terrain category conditions.The relationship between transient overshoot ratio and turbulence intensity is linearly increasing.In all kinds of terrain category wind fields,the difference of transient overshoot ratio in different areas caused by windward opening is obvious,while the overshoot ratio caused by leeward roof ridge opening is evenly distributed.For opening at different roof locations,the internal pressure of the roof was evenly distributed in different terrain categories,and the internal pressure of the roof increase with the increase of the upstream turbulence.Under the most adverse wind direction,the internal pressure caused by the opening at leeward roof ridge is obviously greater than that caused by the opening at the windward corner.At the same position and area,the opening shape has a great influence on the internal pressure.Using the method in the current building load code to estimate the internal pressure caused by the roof openings will result in extremely small estimated value.Under the the most adverse wind direction,the roof with opening has the vulnerable area of secondary damage,which is mainly distributed in the gable area of the roof without opening.In terrain category C,the extreme value of net wind pressure caused by roof local damage is obviously greater than that of other two terrain categories.Roof secondary disasters are more likely to be caused by roof ridge damage than roof corner damage.The estimation results of average internal pressure caused by opening at the roof corner are more accurate.For the small opening area ratio condition,the experimental value is more consistent with the theoretical value.The theoretical internal pressure of small opening ratio underestimates the peak internal pressure,while the theoretical internal pressure of large opening ratio overestimates the peak internal pressure.The internal pressure equation is more accurate to estimate the internal pressure caused by simple shape opening,but more difficult to estimate the internal pressure caused by more complex opening.The estimation accuracy of fluctuating internal pressure and peak internal pressure in different building volumes is higher than that of average internal pressure.The degree of agreement between the theoretical internal pressure calculated by the governing equation is little influenced by the incoming flow turbulence.When the theoretical internal pressure is estimated,the calculation accuracy is higher under the condition of large wind speed,therefore,the larger wind speed should be selected to estimate the internal pressure.