Lower-bound estimate of slender section capacity based on compact section contained within it

[bugbus]

Can a slender steel section ever have a section capacity which is less than that of a smaller (but more compact) section contained within its geometry?

Put another way: is it always conservative to 'trim off' the slender parts of a section and check the section capacity based on something that has less area but is more compact? Most of the codes seem to allow some form of this simplification.

Or -- could the very slender elements of the section undergo local buckling, which might buckle the rest of the section with it and result in less than expected capacity?

 

[SWComposites]

Yes, yes.

 

[ProgrammingPE]

There's sometimes anomalies in the codes where the capacities do not transition smoothly due to some hard cut off point. For example, AISC's shear strength calculation allows a Phi factor of 1.00 as long as h/tw <= 2.24 √E/Fy. Once this is exceeded, Phi becomes 0.90. This means that a W12x14 would actually have a higher capacity if it had a depth of 11.7" instead of 11.9" since it would then meet that limit and get to use the higher Phi factor even though it would have less area (70.2 kip instead of 64.3 kip). Obvious, though, the W12x14 doesn't care about this code limit, and its actually shear strength will transition smoothly.

Structural Central

http://www.structuralcentral.com

http://www.youtube.com/channel/UCOj2mXXf3ZbZtgxTc6BtkGg

 

[bugbus]

ProgrammingPE, I think that's a good point. I'm aware of other anomalies like that throughout the various Australian codes too.

I think you picked up on the intent of my question, which was more to do with how the structure will actually behave, regardless of the code.