Questions on AISC F4 and F5 1

[Lion06]

I have a few questions on AISC spec section F4 (which also has some overlap with F5).
What exactly is hc? I can't find this defined anywhere and I'm assuming that it's the portion of the web that is in compression in the elastic stress range.

Has anyone else had to use these provisions? The equations seem very long and tedious. I'm making a spreadsheet because I know that the architect will want to play with sizes and I'm not spending days re-calc'ing all these properties for built-up sections.

 

[WillisV]

hc as used in this section is defined in the symbols list on p16.1-xxxix (it should be defined in F4 the first time it is used but wasn't).

 

[Lion06]

Thanks! I was looking in the general nomenclature in the index, but it's not listed there.

 

[271828]

"Has anyone else had to use these provisions?"

I do a lot of metal bldg consulting and use these ad nauseam. Yes, F4 is insane and spreadsheets or programs are mandatory.

What's REALLY funny is what happens to Eq. F4-8 if Iyc/Iy<=0.23!! Check it out!

 

[Lion06]

I do see that. Why would they put a 0 in the denominator?

 

[SteelPE]

To me, it appears that you would only use J=0 for equation F4-5 when you are calculating Fcr and not when calculating Lr.

 

[271828]

That is not correct, SteelPE. Think about what Lr represents. It is the min length for which elastic buckling is the behavior and Mcr (w/Cb=1) applies. If Lb<Lr, then the compression flange tips yield and inelastic buckling is the behavior. Because Lr is basically setting Mcr equal to the moment that causes first yield, including residual stresses, it makes no sense to use J=0 in Mcr and not in Lr.

StrlEIT, it is simply a mistake. If the sqrt(J/Sxc*ho) is "unfactored-out" then the Jin teh denominator goes away. The next version of the Spec. will have this modified form of Eq. F4-8.