When the curvature of the membrane surface increases, loads such as wind and snow will cause large deformation. In the case of the support membrane, curvature reversal caused by wind load at large curvature also occurs from time to time, and the design of reversal is necessary when it is difficult not to allow some degree of reversal at maximum wind load.

In the analytical design of membrane structure curtain wall, should the nonlinear and anisotropic properties of the material be considered?

Usually the prestress does not exceed 5% of the tensile strength, the working stress does not exceed 20% of the tensile strength range, for the simple design calculation, generally can assume that the film material is orthotropic linear elastic material, it is considered that the film material is in the elastic stage, that is, the nonlinearity of the material is not considered.

When the tensile prestress and design work stress are large, the nonlinear and anisotropic properties of the material should be considered: the fabric film is a nonlinear composite material: the constraint between the fibers changes with the different warp and weft stress ratio; The performance of the fiber itself under load is nonlinear; The properties of the coating are nonlinear and affected by time; The woven warp and weft fibers are relaxed in the initial state, and the coating has a restraining effect on the straightening of the fibers.

The film is composed of substrate and surface coating, and the substrate is woven from warp and weft yarns, so it presents strong orthotropic properties, and the difference in warp and weft deformation ability is about 5 times (prestressed film has been improved). When the orthotropic material is subjected to the stress in the inelastic spindle direction, it presents the performance of the anisotropic material, that is, the tensile stress produces shear deformation in addition to the tensile direction and the other direction perpendicular to the tensile direction.

At the same time, in addition to shear deformation, shear stress also leads to tensile deformation. Therefore, the anisotropy problem of the material, or the angle between the warp and latitude direction of the membrane material and the direction of the principal stress, must be considered when accurately analyzing the shape and load analysis of the membrane structure. Similarly, this factor needs to be taken into account when determining the clipping strain compensation rate. Unfortunately, most software today does not have this capability.

Safety factor of membrane structure curtain wall.

The membrane material is a flexible fabric, and only a certain stress field exists inside the fabric to obtain a certain stiffness. Therefore, for membrane structures, stress-free states are not allowed in the membrane under any circumstances. Under high stress, the higher the tensile strength of the membrane material, the less likely it is to creep and age; Secondly, in the large-span membrane structure, the stress in the membrane is often large, and the safety requirements for the membrane are high, generally under the long-term load, the safety factor is 6~8; Under short-term loads, the safety factor is taken. The safety factor takes into account the following unfavorable factors: the strength and elasticity constant of the membrane material itself have large variability; the assumptions of membrane structure calculation and the inaccuracy of the calculation results; The difference between the bidirectional stress test of the membrane material and the actual situation; Aging of film materials under ultraviolet irradiation; fatigue of membranes; Scratches and folding during film production and construction, etc.

The membrane material of the curtain wall membrane structure must have high tensile strength and tear resistance. However, the compressive and flexural strength in the membrane plane is almost zero, the shear strength is also very low, and it has certain anisotropic and geometric nonlinear properties of the material, and it is easy to occur stress relaxation and creep when used, so how to correctly determine the mechanical parameters of the membrane and the relationship with the structure in the membrane structure analysis and calculation will be an extremely cumbersome and essential thing.