Differences between ACI318-08 and ACI318-11 Section 10.10.6, Cm factor lower bound limit

[wsg SE PE]

I am going over the Moment Magnification Procedure for non-sway columns as described in ACI318-11 Section 10.01.6 and I noticed a difference in the code relative to ACI318-08 that I cannot find discussion or documentation of anywhere on the internet.

Equation 10-16 in ACI318-08 limits the value of Cm to a minimum of 0.4. However, this limitation is gone in ACI318-11. This can lead to a *significant* difference in the final magnification factor which, in turn, can lead to a *significant* difference in the final (magnified) design moment of the column. Further, this difference is in the unconservative direction, the design moment is allowed to be smaller relative to the 2008 version of the code. For example, in the problem I am currently working though (in preparation for the SE exam in the fall), this single change in the code reduced my design moment by a factor of 2 (Cm = 0.2 instead of 0.4).

Can anyone speak to this? Is this an intentional change? Am I misreading the code / overlooking the discussion about a lower-bound limit on Cm?

Thank you for your time.

Will Godfrey

 

[h-badawy]

i don't see any limitation of Cm in this item except "For members with transverse loads between supports, Cm shall be taken as 1.0" and also i don't see any Differences between ACI318-08 and ACI318-11 in this item

 

[wsg SE PE]

Thanks for the reply.

You are correct! The lower bound limit on C[sub]m[/sub] doesn't exist in 318-08, it is last seen in 318-05. That is my mistake. This may explain why I couldn't find documentation of it being removed for the 2011 version of the code.

Regardless, any insight someone might have would be appreciated. I guess I can rule out it being a typo though since it's been this way through 2 revisions of the code. Maybe I'm being too pedantic.

wsg