AISC 360-22 - Single Angles

[alex2000junio]

Hello everyone,

I'm kind of confused with AISC 360-22 and the design examples.

I'd like to know, regarding single angles, when to use the principal axis and when to use the geometric axis (mostly in the Chapter F10). It seems like to me that we should always use the principal axis for all of the calculations, but when I get to see the Example F.11C in the design examples, they use some of the geometric data. It's confusing to me because the single equal angle has got the same value for the moments of inertia and for some of the AISC 360-22 topics they use the "major/minor inertial axis".
If we should calculate taking the geometric axis as the correct one, some of the important inputs we can't get, right? Like the inertial moments, I have no idea how to get this values!

Thanks in advance!

 

[BulbTheBuilder]

I don't if this is too late for your homework or anything.

This is from AISC 360-16
"Single angles with continuous lateral-torsional restraint along the length are permitted to be designed on the basis of geometric axis (x, y) bending. Single angles without continuous lateral-torsional restraint along the length shall be designed using the provisions for principal axis bending except where the provision for bending about a geometric axis is permitted. "

So, I won't say "always."

The reason can be seen in the commentary F10.

"When bending is applied about one leg of a laterally unrestrained single angle, the angle will deflect laterally as well as in the bending direction. Its behavior can be evaluated by resolving the load and/or moments into principal axis components and determining the sum of these principal axis flexural effects."

"For such unrestrained bending of an equal-leg angle, the resulting maximum normal stress at the angle tip (in the direction of bending) will be approximately 25% greater than the calculated stress using the geometric axis section modulus."

To keep it simple, I'd use
geometric axis for lateral-torsional restraint angles (equal and unequal)
principal axis for angles (equal and unequal) without lateral-torsional restraint