Research On Skin Effect Of The Typical Greenhouse

The greenhouse is one of the most important agricultural facilities,its internal environment and structural safety have a great impact on agricultural development and social benefits.In recent years,extreme weather events occur frequently.The damage to the greenhouse caused by strong winds or snowstorms has had a great impact on farmers' income and social order.The safety of greenhouse structure is attracting more and more attention.The covering materials of greenhouse are usually divided into flexible plastic covering materials,rigid plastic covering materials and glass.When the greenhouse with rigid plastic covering materials or glass covering,the covering materials could work with the greenhouse frame to bear the load and improve the overall stiffness and mechanical properties of the structure.But now,due to the immaturity of technical specifications,the design of the greenhouse structure only considers the force of the main structure,ignoring the actual skin effect.Not only does it conform to the actual force of the structure,structure in some cases there have been unexpected damage,resulting in security incidents.For those reasons,this paper studies the skin effect in greenhouse structure.This paper summarizes the mechanism of the skin effect,the basic conditions of the skin effect,the common method of skin effect analysis,the main components of the greenhouse structure and the type of connector through reading the research results of the skin effect both at home and abroad.Then two types of greenhouse including venlo greenhouse and Huabei-type greenhouse were chosen to be prototypes,and the numerical simulations on the force state of whole frame structure,whole frame structure with PC board covering,whole frame structure with plate glass covering under wind load and snow load were carried out by using finite element analysis software ANSYS with consideration of geometric imperfections,geometric large deformation and material nonlinearity.And the displacement and internal force of the structure are compared and analyzed.The result shows that:1.The displacement of the greenhouse after considering the skin effect is significantly lower than that of the skin effect,which indicates that the covering material can effectively reduce the lateral displacement and improve the resistance lateral stiffness.Moreover,the effect of the skin effect on the lateral displacement of the gable is more obvious than that of the middle frame.The decrease of the structure displacement of the glass as a covering material is larger than that of the PC board.2.The maximum bending moment of most of the components in the greenhouse after considering the skin effect has decreased.The most obvious effect of the skin effect on the structural bending moment is to significantly reduce the bending moment of the purlin and the roof beam,which greatly enhances the bending resistance of the upper part of the greenhouse;The most obvious effect on the shear force of the structure is that the decrease of the shear force of the gable is more obvious than that of the middle frame,and the covering material greatly enhances the shear capacity of the gable;Axial force changes are more complex,the gable column,horizontal rod,purlin's axial force rise,the other components of the axial force decreased in varying degrees;In general,the skin effect of covering material reduces the internal force of the structure and make the internal force distribution of the structure uniform.3.The frequency of the first 1-2 steps is higher than that of the frame structure when the greenhouse is covered with the skin material,but the frequency of the later steps is smaller than the frame structure.The frequency becomes dense and the dynamic characteristics of the greenhouse become more complex.The frame structure shows horizontal vibration,vertical vibration,torsion and other vibration forms,while the structure with the covering material shows the horizontal vibration of the covering material and its connecting parts,indicating that the skin effect reduces the deformation of the frame,but the integrity of the cover material itself is vulnerable to damage has become a threat to the safety of the greenhouse structure.4.The skin effect makes the greenhouse structure stress state and dynamic characteristics change greatly.In the design to consider the skin effect on the one hand can make the structure more tending to the real state of the force to ensure its safety performance,on the one hand can reduce the part of the cross-sectional size to obtain good economic benefits.However,it should be noted that the covering material is not a high-strength materials,plastic deformation capacity is also very limited,its own vulnerable to the characteristics of the design needs to pay special attention.