In the present study, non-linear global buckling analysis of tri-axial composite structures subjected to bi-axial loading is performed using the updated Langrangian incremental finite element formulation. The analysis first considers simple tri-axial structures and then proceeds to more complicated structures. These structures include basic tri-axial structure, modified tri-axial structure and enlarged tri-axial structure. The non-linear finite element analysis results reveal that these structures approximately have the same buckling behavior, but with different values of buckling load. When more X-crossovers are involved in the tri-axial structures, their buckling loads tend to approach some certain value. The sensitivity of the buckling behavior of the basic tri-axial structure to the change in the boundary conditions and to the imperfection due to initial configuration is also investigated. In order to provide confirmation to the numerical solutions and to provide simpler ways to obtain the results, approximate analytical solutions corresponding to the different tri-axial structures have been derived using equivalent anisotropic plate theory. Both numerical solution and approximate analytical solution are comparable. The techniques used for approximate analytical solution can be used to obtain the buckling load of a real life tri-axial structure.
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