For a dynamics analysis, the user can choose several boundary conditions: Viscous, Free field, tied degrees of freedom and no specific boundary (None).
With the option Tied degrees of freedom, the calculation will connect the nodes at the left and right model boundaries such that the corresponding nodes will undergo exactly the same displacements. This option may be chosen to model e.g. a one-dimensional shear column in order to perform a site response analysis.

This type of boundary condition is only available for the vertical model boundaries (Xmin, Xmax). If this option is selected for one of the vertical model boundaries, it must also be selected for the other vertical model boundary. These Tied degrees of freedom boundary conditions can only be applied if the distribution of nodes along the two vertical model boundaries is identical. In other words, the corresponding nodes left and right should have the same y-coordinate. If this does not happen automatically, you might use geometry lines to force the mesh generation process.

In PLAXIS 2D, these boundary conditions will only work on boundaries that are free to move, i.e. no fixities should be applied to these vertical boundaries.

It's important to note that when using the Tied Degrees of Freedom boundary condition, adding interfaces at the same location is unnecessary. Including interfaces, could lead to incorrect results. Hence, interfaces should not be added.

In PLAXIS 2D 2015 and later, the default fixities for the vertical model boundaries default fixities (Xmin, Xmax) should be set to free in Model Explorer > Model conditions > Deformations),  see Figure 1.

Figure 1. PLAXIS 2D 2015 and later: setup of model conditions for tied degrees of freedom

In PLAXIS 2D AE this will mean that the default fixities should be switched off (see Model Explorer > Model conditions > Deformations), see Figure 2.

Figure 2. PLAXIS 2D AE: setup of model conditions for tied degrees of freedom

In both versions, any desired boundary condition at the bottom can still be applied (e.g., a fixed base, prescribed displacements with a dynamic signal, or a compliant base boundary).