As AutoPipe limited node to 10000 only, for bigger model we model part of the system and we need to apply boundary conditions at the discontinuous node, if it is defined as anchor and apply node displacement (for thermal growth) this fictitious anchor will receive loads in seismic and gravity analysis where we need it to be active for thermal analysis only. In STAAD pro I can define supports and run analysis for a load case then i redefine supports to run other load cases.