Single Angle Factors and When they Apply

[CBrech]

I am having trouble distinguishing when factors to my yielding moment apply during my calculations of a single angle beam. I am analyzing the capacity of an equal leg, compact angle iron using the 14th edition of AISC. Since it is compact, local leg buckling does not apply. Yielding is easy to calculate using F10-1. My force can be applied from any direction so I would have to check both principle and geometric capacities. Please let me know if I'm mistaken. To check the principle axis I used F10-4. To check tension in the geometrical axis I used F10-6b with a .8 factor for my yielding moment (My). To check compression I used the user note in section (b)(iii) and did not factor my (My) at all.

My question is if I find that Me is larger than My; for tension and compression do I always factor My by .8? Also, if the user note can by used for Mn to equal My, do I factor My by 1.5?

I hope this didn't confuse you, and I'm sorry for the long drawn out explanation. I great appreciate your help. Thanks.

 

[CBrech]

bump

 

[Hokie93]

Is your angle laterally unbraced over its span? If so, my advice is to resolve the applied moments into components about the minor and major principal axes and go from there. For an equal leg angle bending about the major principal axis, use Eq. (F10-4) to determine Me. Compare Me to My and use either Eq. (F10-2) or Eq. (F10-3), as appropriate, to determine Mn. For an equal leg angle bending about the minor principal axis, Mn = 1.5My. Then use AISC Eq. (H2-1) for interaction purposes. I skipped over the fun of determining the section modulii and moments of inertia for the principal axes. Luckily for you, those are provided in the on-line version of the AISC Shapes Database. Example F11.C on the AISC website mostly covers this but the example has a few errors.

If you don't need every kip-inch of capacity you can muster, tabulated capacities for laterally unbraced single angles are provided in a paper from the AISC Engineering Journal, first quarter 1984, titled "Safe Load for Laterally Unsupported Angles".

 

[WillisV]

Hokie93 - what are the errors?

 

[Hokie93]

Points "A" and "C" on the cross-section in Figure F.11C-1 should be reversed. Point "A" should be on the lower right corner and point "C" should be on the upper left. This shows up on page F-57, in the leg local buckling check, where section modulus S[sub]wA[/sub] should be used for Sc rather than S[sub]wC[/sub] because point "A" of the cross-section is in compression when subject to moment Mw, not point "C". It does not change the result since S[sub]wA[/sub] = S[sub]wC[/sub] for an equal leg angle. It would make a difference for an unequal leg angle. This corrected location of points "A" and "C" is the "key" used in the on-line AISC Shapes Database to define dimensions z[sub]A[/sub], z[sub]B[/sub], z[sub]C[/sub], w[sub]A[/sub], w[sub]B[/sub], and w[sub]C[/sub].

Another AISC Engineering Journal paper that gives good treatment to the subject is "Design Aspects of Single-Angle Members" from the fourth quarter, 2009.

 

[CBrech]

Thank you for your replies. I will check out the example on the AISC website, and hopefully it will be more clear.