The dimensions of the standpipe are:
d = 30 ft or r = 15 ft
t = 0.5 in
Water pressure (hydrostatic pressure variation)
at the bottom of the standpipe
p = γw z = (62.4 pcf)(120 ft)
= 7,488 psf = 52.0 psi
From the free body diagram, for equilibrium Fs = pA
For unit thickness (pressure variation negligible in the limit): σh (2r π) (t) = p (πr2)
The hoop stress is calculated as
σh = pr / 2t = (52 psi)(15 ft)(12 in/ft) / [2 (0.5 in)]
= 9,360 psi = 9.36 ksi
Note 1: the tensile strength of steel is on the order of 40 - 80 ksi, so 0.5 in. is sufficient thickness.
Note 2: final plate thickness used in design must consider wind and/or earthquake loads.