ACI 10.8.4 for Slender Column 1

[iv63]

I am evaluating existing slender concrete pier 24"x36" with reinf 8-#7 (0.56%). Column has very small loads and I have already checked column in accordance with ACI 10.13.4.2 Magnified Moments-Sway Frames. Question: Does ACI 10.8.4 (using reduced effective concrete area) apply for a slender column?
Thank you very much.

 

[Lion06]

I don't see why it wouldn't. What is the lateral system of the building and how tall is this "pier"? Even using the cut-off of kl/r of 22 (the absolute minimum - based on equal end moments producing single curvature), I'm getting that the "pier" could be up to 13.2' tall without having to consider slenderness effects.

 

[iv63]

StructuralEIT:
Column was designed 1992 and customer asked us to justify its compliance with the code. Column (exchanger support) is cantilevered and 26ft tall (kl/r=87).

 

[Lion06]

How much axial load does it have on it? How is the load applied if it is cantilevered? Is it part of the lateral system? What is the lateral system?
I'm having a hard time seeing the moment magnifier being appropriate for this case. My understanding is that the moment magnifier is to capture P-little delta effects. I have to think you are far more concerned with P-big delta effects.

 

[iv63]

StructuralEIT please see attached sketch. Vert load is only about 10 kips and lateral wind load about 100 plf.

 

[Lion06]

Is there any moment induced at the top of the column from the pipe/tank support? Do you have an analysis program such as RISA or RAM Advanse? If so, I would run a second order analysis on it (using reduced EI) and forget about the moment magnifier in ACI. Be sure to break it into smaller segments to account for P-little delta effects (assuming the program doesn't account for them - most don't). Then just do the strength check per the reduced section.

 

[iv63]

Moment at the top is very small 0.25k x 2.5 ft = 0.625 k-ft from wind load only. Unfortunately I do not have program with a second order capability. Do you mind doing a quick run for me? Thanks a lot.

 

[Lion06]

Pu=40.05K
Mu=66.3K

I used 1.2DL+1.6WL. I also added in the wind shear of 0.25k at the top of the column and included the self-weight of the column.

 

[iv63]

Thanks StructuralEIT - big star for you. So I can use reduced section 16”x28” (4” offset all around from 24”x36”) with 8-#7 to check against your second-order forces Pu=40.05k and Mu=66.30k. Is this correct?

 

[Lion06]

Yes. That moment is for bending about the weak axis. Second order effects were minimal, so you would be conservative (but not overly so) to use that moment for both axes (individually).