Concrete Column Cm factor

[SEARUN]

If I have a concrete column (non-sway) that I am checking for axial load only (no moments- M1 = 0, M2 = 0) but is tall enough to warrant the slenderness check, what would the Cm factor be? Would it be 0.6 or 1.0?

 

[JoshPlumSE]

You will have moments.... based on the M2,minimum equations. Based on those code provisions, you can probably extrapolate it out so that M1/M2 = 0 and you'd have a Cm of 0.6.

Personally, I'd probably use a Cm of 1.0. But, I think that M2 minimum equation woudl allow you to justify a Cm of 0.6.

 

[PicoStruc]

You forgot to account for minimum moment caused by excentricity prescribed by code.

M2 need to be at least Cm*M2_Min !

 

[PicoStruc]

Sorry for double answering..... I didn't saw the first anwser of JoshPlum !

 

[PicoStruc]

Personnaly I use Cm=1.0 because the column subjected only to minimum moment will be in single curvature.

The ration M1/M2 = 1.0 where M1=M2=M_Min is discutable because of the slab dead load cause slab bending that can reduce the M1/M2 ratio...

To be safe I use Cm=1 & M1/M2 = 1.0 for minimum moment ..but I know that not all engineer aggree with me - they find that too much conservative !

 

[SEARUN]

Thanks everyone for your response. I knew about the minimum moment and was accounting for that in the design. It sounds like Cm = 1 is the way to go.

 

[shin25]

Cm accounts for reverse curvatures of columns to your benefit to account for end moments with same sign.

Since, analysis doesn't give you end moments except for code required minimum, I believe, you cannot take advantage of Cm being less than 1.