moment of inertia (structural shapes)

[famine]

I'm working on a project to replace hollow structures, (pipe, sq. tube, ect...) with open structures, (channels, angle iron, ect...). It's been a while and I think that all I would have to do is compair the moment of inertia for each of these shapes and make sure that they match or are larger than that of the original shape.

example:
3" sch40 pipe, 3.5" OD, 3.068" ID
I=3.016

equivalent angle iron
4"x4"x1/4"
I=3.04

Sorry if this is a stupid question, it's been years since I've had to do anything like this.

Thanks

[calebking]

You might need to see if there are any other questions as to torsional rigidity etc. Depending on the loading involved, lateral-torsional buckling and/or torsional rigidity might come into play.

[famine]

Thanks calebking

Tortional loading shouldn't be a problem in this situation

[sardonicus]

For stiffness comparing the moment of inertia is correct. Not sure what it's for but I agree with caleb, the torsional properties are quite different, and they are usually important. For example your steel angle will naturally twist when in bending and will eventually provide only it's lowest stiffness, Ixy (the I about a line 45 degrees to the legs, it becomes a V rather than L). S

[famine]

So if I'm looking at this right the I that I would be getting withy a 4x4x1/4 angle would be somewhere in the realm of 1.22in^4 and that is what I need to be matching with the I of a round pipe? So for a 4x4 to match up with a 3" pipe I would need somewhere between 5/8"-3/4" plate to get an I above 3.

[sardonicus]

famine, you are picking up what I am laying down, yes between an L4x4x5/8 and an L4x4x3/4. That's unrestrained and in bending, again, do not know what your example is with the limited info provided. S