ACI moment magnification factor for nonsway frames
ACI moment magnification factor for nonsway frames
(OP)
The moment magnification factor for nonsway frames equals
Cm/[1-Pu/(0.75Pc)]
which is equation 10-9 in ACI 318-02.
As the column gets longer, the critical load Pc gets smaller, and the factor approaches infinity as 0.75Pc approaches Pu.
My question is, what happens if Pu > 0.75Pc?
This results in a negative value for the moment magnification factor. Is 0.75Pc the maximum limit for Pu?
The code does not address this, so I am wondering if anyone has any experience with this situation. Thanks.
Cm/[1-Pu/(0.75Pc)]
which is equation 10-9 in ACI 318-02.
As the column gets longer, the critical load Pc gets smaller, and the factor approaches infinity as 0.75Pc approaches Pu.
My question is, what happens if Pu > 0.75Pc?
This results in a negative value for the moment magnification factor. Is 0.75Pc the maximum limit for Pu?
The code does not address this, so I am wondering if anyone has any experience with this situation. Thanks.






RE: ACI moment magnification factor for nonsway frames
RE: ACI moment magnification factor for nonsway frames
I agree, just looking for some reassurance. A colleague told me that since the moment magnification factor couldn't be less than one, a negative factor should be taken as one. Obviously that's not right, but I just wanted some clarification.
So I guess in this case 0.75Pc is the real critical load. Thanks.
RE: ACI moment magnification factor for nonsway frames
Honestly, this would be true for a sway frame as well. It's just that ACI requires you to run a 2nd order analysis (P-Delta) which to some extent will end up predicting that failure.
The 2008 ACI code has a bit more guidance on using "analysis" to predict 2nd order effect rather than relying on the Cm / (1-Pu/0.75Pc) equation. Unfortunately, I don't have this with me right now. So, I cannot comment in detail....other than to tell you to take a look.
Josh