if you have a piece of your model supported only by springs you have to be very careful. Do the springs react loads in all three directions ? if not, you have to constrain the missing direction. This shouldn't be a problem to the solution (it should be a zero energy freedom); check the constraint load to ensure it is small/negligible.

another day in paradise, or is paradise one day closer ?

Hello rb1957,
Let me explain you the entire thing, its the pressure vessel(bullet). Huge size which going to be placed under geound hence the soil will act as a spring and vessel is free from all the sides. Not fixed anywhere. Hence i can not completely fix it anywhere. Since it is symmetrical presaure will be equal in all direction hence there is no need of constraining. Hope u understand. I tried differet stiffnes values but result is not coming exactly correct.and high stresses are at the spring location whih is not correct.
Pls comment.

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 ?

Yes i need to read more on it. I use help file but many practical aspects of the commands are not explained in the help file. Do u know about any reference for this......?