I'm currently working on aboveground carbon steel piping connected by expansion joint to underground rtrp piping, pipe diameter between 30" to 72", and commodity is cooling water. I observed that CS piping always requires an adjustable base support right before it goes down to underground. But I understand this CS pipe will liftoff from support during operating case. I'm wondering why not use fixed base support instead. Or they are just expecting support settlement. Is there anyone can help me figure this out? Thank you!

Not sure if I understand your explanation but could it be also to assist erection? Compensator is not to align the two pipes. On a short compensator even a small lateral misalignment is no go. Sketch or picture would help.