one trick i use is instead of applying hard constraints to the model, I support the model on RODs (or springs) so that the model responds better. you should be able to set the stiffness to be what you expect, and the results should be good (without the high stresses/loads associated with rigid constraints.
"Since it is symmetrical pressure will be equal in all direction hence there is no need of constraining." you know this, I can accept it, 'cause we're smart and understand the problem. FEA is dumb and assumes things are flying in space unless it is told otherwise. I'd've thought you'd have some weight loads into the ground.
Maybe you should read up on constraining rigid body motion. If you are constraining each support in 3 dof, even with springs, you are doing it wrong (over constraining the model).
another day in paradise, or is paradise one day closer ?