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.
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:
Ground response analysis
[Publications]
Compliant base and free field boundaries: check on input signal
[Tips and Tricks]
Drift correction for dynamic input signal from file
How to setup tied degrees of freedom
On the use of dynamic boundary conditions
Time step used in dynamic calculation
Using an accelerogram for Dynamics