rafters and columns required
 

rafters and columns required.
in the designing of water storage tank, is there a rule or method of determining if its necessary, to use columns and rafters?. i have a tank with 16 feet x 23 feet and a conical "self supported" roof of 3/16" thick, and a tan 20º of slope.
thanks.
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api-650 gives a formula for thickness of a self-supporting cone roof as t = d/(400 sin(theta)), where theta is the slope angle. with your tank dimensions, a self-supporting cone roof of 3/16" would work okay. this assumes dead+snow load of 45 psf or less.
this equation is in api-650 but is commonly applied to awwa tanks as well. it can generally be shown that roofs meeting this requirement will comply with the compressive stress formulas of awwa as well.