problems with the vertical earthquake (FPS)

[ariannatonello]

# ^Y
# |
# o 3 __
# | hb [mm] Dispositivo FP
# o 2 __
# | |
# | |
# | |
#(elem1) L [mm] Pila
# | |
# | |
# | |
# o 1 ---- -------->X


Good morning! I'm doing a comparison between models that vary only the element between nodes 2 and 3 (see the example the figure model). The analyzes are time history.
The model is composed of: in elasticBeamColumn element column, and by an isolator. The mass is applied only in the third node.
I modeled the insulator with 1) the law through bilinear "Steel01", 2) element FPS (Coulomb), 3) rigid link.
I noticed that if I apply a static vertical load to the node 3 and a vertical earthquake "accelseries", I get different results in terms of vertical reactions, and in the node 3 the reaction is almost nothing, why?