Moment Amplification

Certain “slender” vessels are susceptible to increased stresses as a result of the eccentricity of the vessel weight in the deflected position.

Slender vessels are defined as vessels with one of the following conditions:

1. Uniform diameter, and uniform mass distribution, with a height-to-diameter (H/D) ratio exceeding 30.
2. Non-uniform vessels with large loads concentrated in the upper portions with H/D ratio exceeding 15.

Allowance should be made for the increased stresses under lateral loads due to the potential weight eccentricity in the deflected position. The designer may compensate for this concern by adding to the base wind moment (Mw) or earthquake moment (MEQ) a value Mo as defined in Equation 400-29.

(Replace W2 with W1 for computing anchor bolt tension with wind loads.)

The variation of this moment with height is

W1 = Dead weight, kips
W2 = Total operating weight, kips
X = Height above grade, ft
H = Vessel height, ft
D = Vessel diameter, ft
MDesign = (MW or MEQ) + Mo

It is recommended that free standing uniform vessels be limited to a H/D ratio of 40. Vessels with larger H/D ratios should be guyed.

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