When using the free field and/or compliant base boundary condition we need to create so-called node pairs along the boundary of the model (for this we manually need to create an interface along the boundary). Between the two nodes of a node pair a viscous damper is created which allows for the transfer of the input and/or free-field motion but also allows for absorbing the incoming waves.

Currently, however we can by default only see one side of this node pair in the Output program:  this is the side of the viscous damper that also moves due to absorption of incoming waves. So, in general, the motion of this node is not equal to the input motion.

To be able to see and to explicitly check our input motion we should make the other node of the node pair visible in Output. To do so we can make use of a trick with a "dummy plate". See the steps below on how to apply such a “dummy plate”:

• in Structures mode select the bottom line of the model;
• create a plate on this line;
• assign a material set to this plate using low stiffness parameters (such that we expect no influence on our results)
• after meshing the project and prior to the calculation select all desired points of interest and make sure you select also a node on the dummy plate at the bottom of your model
• after the calculation create a plot of your signal for the node on the dummy plate, this should be exactly equal to your input signal.

Note: the dummy plate should not be activated during the calculation.

Also, see attached graphs

Figure 1. Node selection for input signal on dummy plate

Figure 2. Accellerogram for soil node (B; red) and dummy plate node (A; blue)