Fixed Constraint
The Fixed Constraint node adds a condition that makes the geometric entity fixed (fully constrained); that is, the displacements are zero in all directions on the selected geometrical entities. If there are rotational degrees of freedom, they will also be zero.
Shell Properties
Interface Selection

Constraint Settings
To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box.
In the COMSOL Multiphysics Reference Manual:
Location in User Interface
Context Menus
Ribbon
Physics tab with Solid Mechanics selected:
Physics tab with Layered Shell selected:
Physics tab with Membrane selected:
Physics tab with Beam selected:
Physics tab with Multibody Dynamics selected: