Thanks for the reply Mr. Wenlong and Mr. Sdeogeka 

But if I am having a model (say a rectangular bar) which is given periodic symmetry (because say the model is cut out from a very long rectangular Bar).

If I give one end face 'low position' and the other end face 'high position', then how can I give a displacement boundary condition to one end face. (because I want to give an axial tensile load to a very long rectangular bar)