Bresse-Bryan formula refers to INFINITE length cylindrical shells, and that's why you will not find the Length/Diameter (L/D) ratio within this formula. It is a theoretical formula used only for preliminary and conservative estimations.

ASME Sect. VIII (UG-28)calculation procedure regards all possible practical situations (I mean short length, medium length and long length cylindrical shells), and, what it cannot be ignored, involves also the safety coefficients that take into account the inherent initial distortions of the shell wall.

I suggest you to use ASME Sect. VIII method (applied for the LARGEST L/D ratio that is available). Moreover, as probably you noted, ASME B 31 Codes (B 31.3 for instance, at par. 304.1.3 - Straight Pipe Under External Pressure) IMPOSE this approach ("To determine wall thickness and stiffening requirements ..., the procedure outlined in the BPV Code, Section VIII, Division VIII, UG-28 through UG-30 SHALL BE FOLLOWED ...").

The exceptions are also described there and refers to Diameter/Thickness (D/T) ratio values lower than 10.

Regards,

Dorin Popescu