In ASME B31.8, Page 102 & Clause A842.27 it is given that Support or any attachment should be welded to encircling member & not to pipe directly.

Also on Page 28, Clause 834.5, if pipe is designed to operate at a hoop stress of more than 50 % of the specified minimum yield strength, and then support will be attached to encircling member & not to pipe directly.

Can anyone guide me about these criteria? Why this criterion is been set for? I mean why criteria of 50 % hoop stress is been set?