An integrated equivalent stiffness model is developed to describe a grid structure with or without laminate skins. The in-plane bending and shear of ribs are included without introducing any extra degree of freedom so that Mindlin's plate theory can still be adopted. As a result, the model can be directly incorporated with the existing FEM technique and take advantage of commercial codes. The proposed model is demonstrated to be efficient and reasonably accurate. If high precision is required, the equivalent stiffness model can be incorporated with the exact FEM modeling to obtain a refined stress analysis. Hygrothermal behavior and local buckling are also taken into account. Using the model as a tool, the performance of composite grid structures can be compared to traditional laminates, sandwich plates and metal grid structures. A method of optimum design is established to design a grid structure with or without laminated skins. Multiple loads and multiple failure mechanisms are considered. Discrete design is also included. Furthermore, the method can be easily incorporated with FEM to perform structural optimization. The optimization code is demonstrated to be a very powerful design tool. Finally, a user-friendly computer code is developed to perform the above tasks.
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