I am attempting to stiffen an area of interest in my model of plate elements by adding a plate to the top of it (they are fastened together with bolts). I don't want to add this plate in the femap model, but I'd rather just have the original geometry, and increase the bending stiffness of the elements. Essentially, I want to keep the simple geometry without the extra plate, and still have the bending stiffness of the bottom plate represent both plates fastened together. Would this bending stiffness value (12*I/T^3)=2 if both plates have the same dimensions?
Attached are CAD files of the model, with and without the stiffener.
Depends what question you are trying to answer with this change/addition. The stiffness of this two layer plate depends on how "bonded" you wish to assume the plates are with each other... If you assume they are fully bonded (which they are not) then the bending section prop is based on the full thickness. If you assume they aren't bonded (also not right because the bolt shank + pre-load + friction supports shear at the interface), then the bending stiffness is based on two lots of the individual bending stiffness, plus the half-thickness offset of each plate's neutral axis. In reality (and assuming the added plate is not welded around its edges), the bending stiffness is the same as if it were fully bonded right up until the point when the pre-load and friction is not enough to support the shear at the contact interface.