Fixed base

The fixed base can be set up by selecting the “none” option from the Dynamic model conditions in combination with a line prescribed displacement. Via the line prescribed displacement, the user can specify the input motion as a displacement, velocity or acceleration history. The fixed base is a fully reflective boundary. Throughout all Plaxis versions the program expects the input of the so-called within motion (i.e. the sum of upward and downward travelling waves) for a fixed base. This input motion may, for example, be derived through a deconvolution analysis using other (equivalent linear) programs like SHAKE or Deepsoil.

Compliant base

The compliant base is in general used with a line prescribed displacement. Via the line prescribed displacement, the user can specify the input motion as a displacement, velocity or acceleration history. Internally the prescribed displacement will be translated to a load history such that this boundary condition allows for applying an input motion while also absorbing incoming waves.

From PLAXIS 2DAE.02 on, the program expects users to input only the upward travelling part of the input motion (i.e. the incoming or incident motion) for the compliant base. This input motion may, for example, be derived through a deconvolution analysis using other (equivalent linear) programs like SHAKE or Deepsoil.

Also see this related article for a trick on how to explicitly check the input signal into the compliant base boundary.

Note that in previous Plaxis versions different input motions were expected for the compliant base:

• 2DAE.00 and 2DAE.01: an outcrop signal was expected were the upward travelling part of the wave is equal to the downward travelling part of the wave and as such the input motion transferred through the compliant base was automatically divided by two by the program;
• 2D2012/Classic: only the upward travelling part of the motion just like 2DAE.02