Alright, I live in New York City & I have inspected many residential buildings that have gravity tanks and DO NOT HAVE FIREPUMPS.
15fls x 8 ft = 120
Lobby =12 ft
basement =12 ft
tank above roof = 15ft
(17) 16 inch slabs =272 inch / 12in = 22.6 ft
=181 ft
181 ft x 0.433psi = 78.373psi
water...