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Home > Forums > Engineering Codes, Standards & Certifications > Engineering Codes, Standards & Certifications > AISC (steel construction) Code Issues Forum

I Values for double angles

thread172-319791
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BVPE (Structural)
10 Apr 12 9:17
I have been using the 9th edition AISC for I values for double angles.  When looking at the 13th edition, I am perplexed why I values are not given for double angles.  I see them for single angles.  Am I missing something?
frv (Structural)
10 Apr 12 10:02
It's just double that of a single angle bent about the same axis.
JoshPlum (Structural)
10 Apr 12 12:41
frv -

That's only true for one direction.  For the other direction, there is a re-location of the centroid....  So, it is not so straightforward to calculate.  

If I had to guess, I would think it is a matter of AISC trying to keep the number pages down.  They want a single manual (as opposed to the two volume silver books) and they wanted a combined ASD / LRFD manual.  Therefore, some things had to get the axe!  
frv (Structural)
10 Apr 12 13:41
JoshPlum-

I'm nut sure I follow.  For all cases of double angles either both angles are bending about the y axis or both angles are bending about the X axis. How would the centroid be affected?
MiketheEngineer (Structural)
10 Apr 12 13:43
Different size legs on the angles??
frv (Structural)
10 Apr 12 13:45
Mike,

Was that comment in response to the one I just posted?
JoshPlum (Structural)
10 Apr 12 14:18
frv -

Take a look at the ninth edition tables:
An 8x8x1 has an Ixx of 177 in^4.  That's exactly equal to twice the Ixx value for a L8x8x1.  Howerver, the ryy varies depending on whether the angles have a back-to-back spacing of 0, 3/8, or 3/4.  Since r = sqrt (Iyy/A) the I value changes based on the spacing, right?  

For a LL8x8x1, the Iyy would be 346.8 in^2... That's approximately 4 times the Iyy value so a single L8x8x1.  
 
msquared48 (Structural)
10 Apr 12 14:31
frv:

In one direction you are correct, but not the other due to the presence of a connecting plate between the angles, the space increasing the effective I and S.  

If there is no plate then you are correct, but ususlly there is.  

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
 
frv (Structural)
10 Apr 12 14:51
I see.

I guess I wasn't picturing bending about the Y-Y axis as a "2L" case. I've actually had to calculate this value before, and I assumed that the reason this wasn't provided in the manual is because it would depend on the gap.

I don't have the 9th Ed., so it didn't occur to me that the Iyy value was specified previously.

Thanks for the clarification.
hokie66 (Structural)
10 Apr 12 17:23
Depends on whether the angles act together (compositely) or separately.  And there are lots of ways angles can be oriented, e.g. toes together like a channel, toes together to make a box, corners together to make a cross.  If anyone needs I for a non-typical configuration, just revert to first principles.
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