Steel Tank Anchorage Uplift

[LicensedToPEe]

I am looking at a reference for calculating maximum uplift loads for anchorage (at the outter diameter) for steel tanks. The max uplift for both Wind and Seismic loads are calculated as (4*M/D)-W ,where M is moment due to seismic or wind load, D is dia. of tank and W is dead load of steel shell minus corrosion allowance. Why is there a factor of 4 for the moment?
Any help is appreciated.

 

[rd78]

Equate a bending stress to an axial stress, then convert to a force. Essentially divide M by section modulus (pi*D^2*t) and divide by area (pi*D*t). For anchorage, also divide by the number of anchorages to get the uplift on a given anchor.

 

[JStephen]

There's a 4 missing in rd78's equation-
Moment of inertia is pi*R^3*t, section modulus is Pi*R^2*t, or Pi*D^2/4 * t and that's where the 4 comes in.
It's just P/A +/- MC/I.