Allowable Fb for Compact Sections 1

[JCWilson]

I am looking for commentary on the use of Fb = 0.75xFy from ASD 9th Ed. Equation F2-1. Can this be liberally used for shapes or portions of shapes that are considered compact as defined by the manual? I have been trained too much in the conservative use of .6xFy for most all of my bending calcs. Now I have a significant need to use the .75 factor, but I am having a hard time convincing myself hit is okay.
For my example, I am designing a continuous angle with a transverse loading that effectively bends the plate of the leg about its weak axis. The shape therefore complies with the definition of "solid rectangular sections bent about their weaker axes," but it is still an angle.
Is .75xFy valid for this type of condition?

 

[jcox]

I'm not quite clear on your loading application. The reason that 0.75xFy is used in weak axis bending because there is a higher moment reserve capacity. Just compare the ratios in a WF section of Zx/Sx to Zy/Sy. In a WF once the Flanges reach the yielding limit, there is not much reserve capacity. If the bending is in the weak axis, there is more reserve capaicity in the weak axis. If you are using an angle as a beam, I would think that you would have to limit yourself to the 0.6xFy range, with any applicable Q factors. Section F2 does not apply to angles. Read pg 5-147 for a better explaination. Read pg 5-309 for specifications for bending on single angle members. It's a lot of work, and makes you not want to use and angle for bending unless just absolutely necessary.

 

[JCWilson]

To clarify the loading condition, it is similar to a shelf angle supporting a continuous load. A load P acting at the edge of an L3x3x1/4" would induce a moment of 3in x P at the base of the leg. The section modulus of a unit length of the leg would be the same as that measured for a rectangle, or 12in x 1/4in^2 / 6 per foot of angle.

I am trying to reason that the behavior of the leg in this loading condition is the same as that of a compact solid rectangular section. It just happens to be part of an angle.

 

[jcox]

If I understand you correctly, you are concerned with the bending on the leg of the angle as if it were a cantilevered plate. If this is the case, 0.75xFy should be just fine.

 

[JAE]

The .75 factor is also due to the fact that a weak axis member will not fail in lateral torsional buckling (see the commentary).

It is not intended for a shelf angle loaded as you describe. The angle will still be capable of lateral torsional buckling and therefore you should not use 0.75.

AISC has a specification called: "Specification for Allowable Stress Design of Single Angle Members". Angles are inherently unstable and difficult to verify strength as you will find in this specification. Simply taking out a "piece" of the angle and using .75 is not appropriate in my opinion.

 

[JAE]

I might add, though, this:

If you are checking the bending in the LEG due to a load such as brick, then the bending of the leg would certainly be based on 0.75. An example of this would be when you have a lintel angle attached to a concrete beam or wall (say at 18" o.c.) over an opening and brick is sitting on the horizontally extended leg. In this case the angle itself is not bending across the span of the window and the horizontal leg is like a continuous cantilever extending out from the beam/wall. The leg is bending downward and is bending across a weak axis. 0.75 is OK.

If you are checking the bending of the whole angle along its span (across a window opening) where the angle is not attached to the backup, but bending across a primary x or y axis, then the 0.75 is not appropriate.