Study On Mechanical Response Of Membrane Structures Under Hail Load

Membrane structure buildings have been widely employed in many different kinds of great span structures because of its advantages of low weight,broad span,and excellent light transmittance.Since membrane constructions have a wider range of applications,their inherent stiffness is rather low,and when subjected to varied external stresses,issues such damage and severe deflection deformation are increasingly evident.Hail load is one of the external stresses that membrane structures regularly experience in daily life.Hail load concentrates on a specific region of membrane surface damage,unlike evenly dispersed loads like wind and rain,which are more likely to lead to stress concentration and local failure on the membrane surface.Therefore,this thesis conducts in-depth research on the dynamic response of membrane structures under hail loads to clarify the impact mechanical behavior and damage characteristics of the membrane under hail loads.The specific research content and results are as follows:(1)The numerical simulation of ETFE rectangular tensile membrane under individual hail loads was conducted.The effects of the diameter of hail loads and the prestress of the membrane structure on the dynamic response of the membrane were compared and analyzed from the perspectives of maximum stress,maximum displacement,cross-sectional deformation,and plastic strain.The results show that the maximum displacement of membrane surface increases exponentially with the increase of hail diameter,and decreases linearly with the increase of membrane prestress;The maximum stress at the impact point is linearly related to the hail diameter,and there is no clear rule between the maximum stress and the membrane prestress.In terms of permanent damage to the membrane,hail of each diameter can be divided into three steps: the first step is that when the diameter of the hail is 2 cm or less,the plastic strain caused by the impact load of the hail on the membrane is 0;The second step is a hail load with a diameter between 3cm and 6cm,resulting in a maximum plastic strain of 0.29%,2.6%,and 8.5%,respectively,lower than the strain rate corresponding to the second yield limit of the material.The third step is when the diameter of the hail is 8 cm or more,and the maximum strain rate of the film is much higher than the second yield point strain.The film in the impact area has completely lost its original mechanical properties.(2)Based on the dynamic response of ETFE rectangular tensile membrane under single hail,the interaction of multiple hail events on the membrane surface was studied,and the deformation state of the entire membrane surface under different working conditions was obtained.For planar randomly distributed hailstones with a single particle size model,the maximum displacement of the membrane surface occurs near the center point.The maximum difference between the maximum displacement of the center point of the membrane surface and the maximum displacement of the entire membrane surface is 9.5%,and the maximum difference between their respective average values is not more than 12%.The stress and strain generated when hail loads with the same diameter act on different locations on the membrane surface are related to the distance from the impact point to the boundary.The closer the impact point is to the boundary,the greater the stress and strain on the membrane surface caused,which is more likely to lead to the failure of the membrane.In the combined particle size mode,the center point displacement curve of the membrane surface is between its displacement time history curves in the corresponding diameter single particle size mode,and the maximum displacement of the membrane surface is closer to the maximum displacement caused by the single particle size hail with the largest diameter.Under spatially randomly distributed hail loads,the amplitude of the displacement time history curve at the center of the membrane surface gradually increases with time.(3)Based on the study of dynamic response and damage characteristics of planar tensioned membrane under various hail loads,in order to compare the influence of membrane structure on its anti hail impact performance,various dynamic corresponding parameters of saddle membrane structures with different span to height ratios under spatial random hail loads were compared.In most cases,the impact resistance of saddle membrane structures with a span to rise ratio of 1/8 to hail loads is better than that of saddle membrane structures with a span to rise ratio of 1/12.This is because the curvature of the membrane increases and the resistance to deformation increases as the rise to span ratio increases.Under the same vertical load,the maximum displacement of the membrane impact point decreases.Comparing the maximum displacement and maximum stress of the saddle membrane and the plane membrane under the same hail load,it can be found that the anti hail impact performance of the plane tensioned membrane is the worst,and the anti hail impact performance of the saddle membrane with 1/8 span to rise ratio is the strongest,which indicates that the structural safety of the membrane structure can be improved by properly changing its own shape in hail weather.