Problem:

What does "Remove Lift-off Supports" mean on AutoPIPE's Analysis sets dialog? 

Solution:

To be able to calculate the sustained stress when in cold condition, or when in hot condition is a code requirement.

 From AutoPIPE help:

Enable this feature for non-linear analysis sets to remove supports that lift off during hot sustained.

Only enabled for ASME B31.3 2014 and later editions

AutoPIPE's non-code combination GrP1..GrPn would be both hot and cold sustained combination:

If supports, that would otherwise be physically present, are theoretically removed before performing an analysis , it would be considered HOT sustained...
if supports are all left in place regardless of lift-off reported by analysis, it would be COLD sustained...

See Also

Static Analysis

Bentley AutoPIPE