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# Steel axial compression equation

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## Steel axial compression equation

(OP)
In ASD steel manual, ninth edition, page 5-54, equation H1-1:
fa/Fa+Cmx fbx/((1-fa/Fex')Fbx) +Cmy fby/((1-fa/Fey')Fby)<=1.0
Should "Cm/(1-fa/Fex')" or "Cm/(1-fa/Fey')" be always greater than 1.0?
Jack McCormac's "structural steel design ASD method" book metioned that it should be greater than 1.0.
I prefer to take 1.0 as its minimum value, but some engineers think it could also be less than 1.0 because
ASD steel manual doesn't give any limit on it and engineers should design structures based on manual instead of text book.

What do you think?

### RE: Steel axial compression equation

jackzhong:

The chances of actually getting the term "Cm/(1-fa/Fey)" to be less than one seems to me to be very unlikely.  You would have to get an extraordinarily small fa in order to make the term <1.  However, when fa/Fa is less than or equal to 0.15, AISC permits equation H1-3 in lieu of equation H1-1.  Equation H1-3 is equivalent to equation H1-1 with the factor equal to 1.

In my opinion, equation H1-3 would be the upper limit on combined stress ratios and therefore, the term "Cm/(1-fa/Fey)" should never be taken less than 1.

~dison

### RE: Steel axial compression equation

jackzhong,

Yes, Cm/(1-fa/Fex) could be less than one since Cm could be less than one when the ends of the member are restrained against rotation.     But remember that the member has to comply also with equation H1-2.

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