I opened your model and have a suggestion. All parts in the structure have a thin wall compared to the length and width. The best practice to simulate this type of structure is to use SpaceClaim or your CAD system to replace every solid body with a midsurface. Mechanical will mesh with shell elements instead of solid elements. This type of mesh will have a much easier time converging and provide a much more accurate evaluation of stress in the structure.

Good Luck!