Dear Sir,

I have a question about ASME Formula for Stress calculation of Spherical shell.
as you know
S=PR/(2t)+0.1P
P:internal pressure
R:internal Radious
t:shell thickness

I dont know base of "0.1 P" section
and I can't underestand the reason of this part.
is there any one who can help me?
thanks in advance
Amin