Design criteria of column and wall

[auatipu]

Hi all'
i m confused about the design criteria of column and wall. ACI 318-11 code describe, for special moment resisting structural member which axial force is greater than (Agf'c)/10 must be design as column i,e least dimension to long dimension ratio should be within 2.5 and minimum dimension should be 12 in. But in another section they distinguish column and wall design as width to length ratio i,e if ratio is less than 2.5 than column on the other hand if ratio is greater than 6 than member should be design as wall. now my question is if we have any member for architectural requirement width to length ratio greater than 6, minimum dimension 10" but axial force is greater than Agf'c/10. what should we do in this situation.

Thanks

 

[TXStructural]

ACI 318-11 21.6 applies to moment frames. A very stiff element, like a wall, in a moment resisting frame would need to be designed as a shear wall, which results in a different structural system.

 

[RobertHale]

I think you need to make the distinction between "column" and "wall" in terms of behavior. If code reading and my memory from grad school serve me correct, the idea of the limitation on axial load from 21.5 is meant to keep elements that are either transferring gravity load to ground (e.g. column) or transferring shears from the diaphragms in the form of axial force (collectors and struts) much stronger than any pure flexural member as sort of redundancy (for lack of a better word). It kicks you into the detailing requirements for a column rather than the detailing requirements for the beams. In your case I would think that the dimensioning requirements lead you into 21.9 for special structural walls. That width length ratio is there to make sure the behavior fits with the corresponding R and Cd values for seismic behavior. If the column is wide enough then the behavior is no longer dominated by flexure but rather shear deformations (think of the difference between a beam and a "deep beam"). If I understand where you are getting your information, Section 21.9.8 is trying to make sure at wall discontinuities (openings) where that length to width ratio drops, the same change in fundamental behavior is dealt with adequately. You can think of these elements like moment frame columns fastened to a very stiff beam above and below.
I would think that your length to width ratio kicks you into behavior like special structural walls. Unfortunately this means that you now have a dual seismic system and you need to re-evaluate your seismic force calculations, or you may be able to eliminate that wall from the force system and skirt around the seismic detailing requirements. I hope this helps and I am sorry if I missed something that makes this just a high sounding tangent.

 

[auatipu]

Thanks TXStructural and RobertHale. Now its clear to me.