referred to the compression flange in aisc-05
 

"referred to the compression flange" in aisc-05
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!
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.
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!
nutte, it's the moi of the compression flange about the y-axis of the section, right?
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.
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.
right, but the point is to catch the shapes that are much less symmetrical than rolled shapes.