Vertical ground motion in soil domain

[KyriakosTryphonos]

Hello,
I have read the example in Opensees site for the 2D Dynamic effective stress analysis of a slope. At this example earthquake is simulated as a horizontal Force History at the base of the soil domain, with Lysmer dashpots to represent the underlying rock.

My question: Is it possible to apply Vertical ground motion with that method at the base of soil as a Force History?

If it’s not possible to apply Vertical ground motion with that method, is there any other way to do this?

Thank you for your time!

[fmk]

from a usage point of view you can do vertically what you do horizontally, not being a geotech I am not sure if the method is limited to the waves coming in from the side.

[KyriakosTryphonos]

Yes i thought of that but there is an issue. When the base motion is horizontal, the base nodes are restrained in vertical translation DOF. I removed that vertical restrain so i can apply vertical force directly to them, put the Lysmer dashpots in vertical direction and use Vp instead of Vs for their constant.
Now i can apply a vertical force but the base nodes are unsupported since i removed their vertical restrain.
I tried to do this but the base nodes slowly go down during the analysis since the only force resisting to their vertical move downwards is the viscous force from the dampers.

I don't know if that is correct from physical point of view but is there a way to restrain that nodes, for example by using some vertical springs at the base?

[fmk]

you can use vertical springs at the base (or fix them and use the Penalty method to enforce the constraints .. results in same thing)