Fixities are very flexible: by default we have standard fixities, which fix the bottom of the model in all directions and the vertical sides in the horizontal directions.

In addition to this, a user can specify the boundary conditions for each side independently according to the following options:

• Free
• Normally fixed
• Horizontally fixed
• Vertically fixed
• Fully fixed

The option to apply different fixities by manually creating prescribed displacement surfaces (or lines/points) and specifying different conditions for these is still possible.

A prescribed displacement can be - per direction - fixed, free or have some specific displacement value (for lines and surfaces it can also be a function of the position). Activated user-defined prescribed displacements (set to fixed or with specific displacement value) will overrule the default deformation boundary conditions.

In the case of setting a prescribed displacement to free, the following rules exist (in decreasing order of priority):

1. point prescribed displacement wins for all directions
2. line prescribed displacement wins for all directions, with the exception of:
   • a point prescribed displacement
   • boundaries of the line (i.e. its two end points)
3. surface prescribed displacement wins for all directions, with the exception of:
   • a point prescribed displacement
   • line prescribed displacement (even its end points)
   • boundaries of the surface (edge lines and edge points)
4. default fixities (if none of the above is applied)

It is also possible to disable the automatic default fixities completely for one or more phases and rely completely on manually defined fixities. The latter is done by deactivating the Deformations group (checkbox) in the Model Explorer or via the command line.

PLAXIS 3D AE

The default boundary conditions in the Model conditions section of the Model Explorer under the Deformations tree-menu should look like this:

Examples of commands to change them can be found below.

To deactivate the entire Deformations item (so no fixity applied):

 deactivate Deformations Phase_1

To change one of the model sides:

 set Deformations.BoundaryXMin Phase_1 "Free" set Deformations.BoundaryXMax Phase_1 "Normally fixed" set Deformations.BoundaryYMin Phase_1 "Horizontally fixed" set Deformations.BoundaryYMax Phase_1 "Vertically fixed" set Deformations.BoundaryZMin Phase_1 "Fully fixed"

PLAXIS 3D 2013

Since PLAXIS 3D 2013, the boundary conditions are now stored in the new Model Conditions section of the Model Explorer under the Deformations-node:

The command to change it, is now:

 set Deformations.UseDefaultFixities Phase_1 False

PLAXIS 3D 2012

The format for PLAXIS 3D 2012 is:

 set Phase_1.Deform.UseDefaultBCs False

In PLAXIS 3D 2012, this setting is also available in the improved phases window.

PLAXIS 3D 2011

The format for PLAXIS 3D 2011 is:

 set Phase_1.DefaultDeformationBCs False