This is non-linear system. CAESAR II manual explains this case as Mr.Rich say.

Another way is to take out the supports which are lift-off in your model. And you check the sustained stress and deflection. When the sustained stresses are whithin allowable limit, it is O.K, that is, leave the lift-off support in your system because those resting supports will be helpful for hydro-test or the other abnormal condition. When overstressed, you add springs in the lift-off point.
This is very simple and common way to check lift-off case's stress.

[This message has been edited by Soon Ryang, WEE (edited February 27, 2001).]