# Non-Circular Vessels, Apx 13

Hey Folks, my question is related to language in Sec VIII, Div 1, Apx 13, 13-4(k). This para speaks of moments of inertia calculated on a unit-width basis: I = b*t^3 / 12, with b = 1.0. The direction of dimension b is not formally defined in sketches or nomenclature so far as I know.

Given that the MOI is used to calculate bending stress in the side plates, I have always reasoned that b is in the vessel length direction, or looking at, say, Fig 13-2(a)(1) for example, normal to the plane of the figure.

I have recently had cause to explain my interpretation but I have no real confirmation. I’m asking myself “How do you know that?”

What say you?

TIA :)

SnTMan:
I don’t have that Spec. Sec. in front of me, but try this on for size. I = (b)(t^3)/12, with b = 1, and paying due attention to the units in the equation, is the moment of inertia for a unit width strip, for a unit width calculation of stress, deflection, strain, etc. And, ‘b’ is the unit width, in whatever direction you are calculating the stresses, etc. Think of your first/basic beam cross section (or plate strip, for a beam) when we learned to do simple beam stress calcs. You are locked in on the vessel length because we are usually interested in the ring tension (circumferential stress) in the vessel, thus ‘b’ relates to length. In non-circular tanks, we might look at the side plates as spanning from the bottom to the top of the tank, then ‘b’ would relate to tank side length. Sometimes tank plates span btwn. vert. stiffeners, in which case ‘b’ would relate to tank height/depth.

dhengr, thanks, yes that is very much as I interpreted it, but am just wondereing if anything more formal exists.