How do you calculate first moment of area of stiffener 2

[LeonEarle]

Can anyone help? I need to calculate the first moment of area of a stiffener being attached to a 75' diameter tank. Stiffener will be 3" x 3" x 3/8" angle.

 

[rb1957]

1st moment of area = Area*(distance to an axis) ...

you can have a local 1st moment of area (that of the stiffener essentially by itself) ... Sum(Area*y) ...
divide the angle into the upright flange and the horizontal flange (i'd make the horizontal flange 3" by 0.375", and the upright 2.625" by 0.375"); take a datum at the center of the horizontal flange (so it has no offset); then 1st moment of area = (2.625*0.75)*(0.375/2+2.625/2)

or maybe you want the 1st moment of area with respect to the tank (5.625*0.375)*37.5' (without fussing too much)

 

[JStephen]

See this link for definition:

http://en.wikipedia.org/wiki/First_moment_of_area

The moment of area needs to be about some axis. If it is the centroid of the angle itself, then you know what that axis is and can calculate accordingly. More often, though, when designing stiffeners on a tank, you'd be using a composite section including part of the tank shell, and would have to figure the centroid and properties of that composite section.

Typically, you'd use the first moment of parts of the angle for calculating shearing stress. The moment of inertia enters into buckling equations, as for external pressure. The section modulus enters into bending formulations, as for wind girders. Make sure you're not confusing properties.

 

[LeonEarle]

Thanks guys. This helps a lot. I am calculating the weld shear flow due to radial shear load on the stiffener. As I currently understand the formula, I am to use the first moment of area of the stiffener without regard to the contribution of the shell where it attaches. Thanks again for your help.

 

[rb1957]

I'm not quite sure where you're going ...

assuming you have multiple stiffeners dividing the tank shell. the stiffeners allow the shell to have discrete changes in shear stress (shear flow) ... as opposed to a shell which would have a continuously varying shear stress. and these different shear flows require an endload in the stiffeners to react them.

it'd make sense to me that you could start with the unstiffened shell to get a nominal shear stress distribution, and then discretise this over your several bays. i think you'll find a difference proportional to the distance from the neutral axis (so that the stiffener endload resembles a bendins stress field).

you're probably working on shear stress = VQ/It, where Q is the 1st moment of area of the cross section above (away from the section neautral axis) the section plane (where you're calculating the shear stress).

hopefully this isn't as fibberish as it sounds (as i read it !)