Just by being so thin and by having large spans that are unsupported (span here means relative to the thickness of the fabric) the fabric will not have any ability to support compressive loads without buckling. Buckling in FEA is more than a bit tricky. One would normally use a membrane element that does not have any bending stiffness which is not available in SWX Sim. The other thing is these problems are inhernetly non-linear as the normal to plane deformations are often more that 1/2 the thickness of the structural depth of the fabric (its thickness). If you have the non linear package then there are some techniques that may help you solve some aspect of this problem.