I have a design of 2 panels connected to each other with stiffeners so that load may transfer from one panel to another.All are shell models with thickness in a range of .1in to .3 in. In a case of Mild steel i am not using any fasteners but in the case of composite i am using fasteners because composite layers(8) of panels may get separated from each other in the case of buckling. So which type of connector element do i need to choose beam,hinge,planar, etc so that fasteners take some buckling load to themselves instead of over constraining my model. And also what element behavior do i need to include. As an instance i am getting eigen value of 9000 in case of mild steel but in the case of composite i am getting 1.1e+5 which is way to much in case of composite i believe. All the section and material properties are consistent. Any kind of help will be appreciated. Thank You

First Picture Stiffeners. 2nd 2 panels with stiffeners inside.