"Referred to the compression flange" in AISC-05 1

[JStephen]

In AISC-5, Section H1, Iyc is the "the moment of inertia about the y-axis referred to the compression flange". What does this mean? I know what the moment of inertia is, can calculate it about one axis or another, but don't have a clue what they mean here. It sounds like it would be the moment of inertia about the compression flange, only then the ratio in question, Iyc/Ic, would inherently be larger than 1, rather than between 0.1 and 0.9.

Ideas? Thanks!

 

[nutte]

I've wondered that, too, as the 2005 specification doesn't elaborate on it. The draft version of the 2010 specification says the following, which makes it abundantly clear what Iyc is:

... where Iyc is the moment of inertia about the y-axis of the compression flange.

Why they thought "referred to" was better than "of" is beyond me.

 

[JStephen]

That makes sense now.

I'll bet I know why they changed it. That could be interpreted as the moment of inertia of the compression flange area about the y axis, or as the moment of inertia (of the section) about the compression flange's y-axis. Makes me wonder if someone didn't scribble in "referred to?" as a question about what it meant, and it got put into the text.

Thanks!

 

[271828]

nutte, it's the MOI of the compression flange about the y-axis of the section, right?

 

[271828]

I'm sure what I typed was right. Look at the definition of Iyc in the up-front definitions and in Section F4. The Ch. H definition is incorrect and physically meaningless, LOL.

 

[nutte]

271828, yes, that's how I would interpret it.

Iyc = tf*bf^3/12

Which should be close to (but less than) half of Iy for most rolled shapes.

 

[271828]

Right, but the point is to catch the shapes that are much less symmetrical than rolled shapes.