In modeling the soil structural interaction, some engineers allow tension and compression of vertical elastic springs. This is not right. You should allow only compression in vertical direction.
Finite element analysis software give this option in one or two or three direction. usually you restrict the compression in vertical direction and allow tension and compression in the other two directions or you prevent movement in the other two directions. it depends on what you want to study.
Sometimes you have to strict movement at least one node to obtain equilibrium of the system and stop failure of the structural model. Choose wisely or model more structural elements in the structural model until you reach balanced and stable state without error flags.
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