Navigation:  Calculation Modules > Analysis > ENERCALC 3D > Ribbons > Create > Constraints > Diaphragms

Create > Constraints > Diaphragms prompts you with the following dialog.  

 

 

 

It allows you to define regular or generic rigid diaphragms (in-plane) in a 3D model.  For example, to model horizontal concrete floors, you may select one node on each floor and apply regular diaphragms to the selected nodes in XZ plane (with normal in the global Y direction).  Instead of using plate elements, rigid diaphragms allow you to model stiff in-plane actions quickly.  The program further provides the option to ignore the rigid diaphragm actions as an analysis option (Analysis > Analysis Options).