Allowable Fb for Compact Sections
Allowable Fb for Compact Sections
(OP)
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?
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?





RE: Allowable Fb for Compact Sections
RE: Allowable Fb for Compact Sections
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.
RE: Allowable Fb for Compact Sections
RE: Allowable Fb for Compact Sections
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.
RE: Allowable Fb for Compact Sections
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.