Hi Corne
Thanks for sharing experience. Can you please make it more clear

*****BTW Another example: I've once seen a fixed support being calculated with a for steel structures allowable displacement in piping axial direction of around 20 mm.****

If I read you correctly you use pipe relative anchor to structure & that structure have horizontal displacement of 20 mm ? If displacement is vertical seems reasonable. Any specific reason of your consideration?
Are you taking something offshore design?

Regards

Habib