Automatic Constraints

One further constraint that users should be aware of is the Automatic Constraint. These are constraints set up by the Gss solver to enable the solution of shell models where the elements do not have stiffness in all directions. Shell elements do not have a local zz degree of freedom. Thus when the elements are transformed into a non-global plane they appear to have stiffness is all directions but one of the principal stiffnesses at the nodes is zero. To avoid problems of zero stiffness at the solution stage these degrees of freedom have to be removed from the solution. The method of doing this is termed applying Automatic Constraints.

There are a number of ways to apply Automatic Constraints but the default method is to look at how flat the surface is at a node. If all the elements at a node seem to be co-planar then the number of degrees of freedom at that node are reduced to five rather than the normal six. Gss builds a list of automatic constraints, which can then be examined by the user after the analysis stage.