I Values for double angles

[BVPE]

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]

It's just double that of a single angle bent about the same axis.

 

[JoshPlumSE]

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]

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]

Different size legs on the angles??

 

[frv]

Mike,

Was that comment in response to the one I just posted?

 

[JoshPlumSE]

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]

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]

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]

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.