External stiffening rings shall be attached to the
shell by welding or brazing [see UG-30]. Internal stiffening
rings need not be attached to the shell when the rings are
designed to carry the loads and adequate means of support
is provided to hold the ring in place when subjected to
external pressure loads. Segments of rings need not be
attached when the requirements of UG-29(c) are met.
Except as exempted in (f) below, the available moment
of inertia of a circumferential stiffening ring shall be not
less than that determined by one of the following two
formulas