Hi,How to apply seismic accelaration on the BC in abaqus.thank you!:}

I am confused about apply BC of a model in abaqus. because I find that if I apply the accelaration of earthquake on BC, I must remove the fixed constraints applied on BC in previous step in abaqus. so the model become a free body without any constraints.

 any expert can give me some detailed suggestions?thank you very much!^_^