Minimum Ag For Concrete Pedestal Reinforcement

[JoshH726]

If I have a pedestal (54"x28"x72" tall), Ag=1512 in^2, and the cross-section is larger than required for the loads, so 318 10.8.4 will allow you to reduce the Ag by 1/2, or 756in^2. Is 1/2Ag (756 in^2) then used for 15.8.2: 0.005Ag requirements, so 1/2*0.005*Ag=1.89 in^2 (min. reinforcement then controls), or do you still use 0.005Ag = 3.78 in^2? It seems counter-intuitive that 10.8.4 allows you to reduce the Ag, which you seem to be able to use for 10.9, but you can't use it for 15.8.2.

 

[Leftwow]

Assuming you did a moment analysis of the column, and came up with a low Rn, (which happens often) I would have to refer you to
This side note:
The basis of 10.8.2, 10.8.3, and 10.8.4 is that it is satisfactory to design a column of sufficient size to carry the factored load and then simply add concrete around the designed section without increasing the reinforcement to meet the minimum percentages required by 10.9.1.

 

[KootK]

so 318 10.8.4 will allow you to reduce the Ag by 1/2

.

I don't think that this is quite correct. That clause doesn't allow Ag to be reduced by half, it allows the reinforcement to be calculated based on half of Ag. There's a subtle difference there.

I'm also not sure that the commentary to 10.8.4. absolves the designer of the need to provide 0.005 Ag in the column. Rather, I think that's it's just the justification for using less than 0.01 Ag.

The commentary to 10.9.1 indicates that the intent of that clause is ductility which leads me to believe that the 0.005 Ag ought not be reduced below that value.

I agree that the code is a bit vague on this. However, my interpretation is that:

1) The minimum amount of reinforcement in your pedestal should be 0.005 x 100%Ag and;

2) The minimum amount of dowel reinforcing crossing into the footing should be 0.005 x 100% Ag

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JoshH726]

Thanks for your clarification KootK. Taking this one step further, is there flexibility to analyze the pedestal section (where height <3 x least dimension) as unreinforced? These are for pedestals in low seismic areas with Pu < 20k and Vu < 1k. Would this preclude the 0.005Ag requirements, and instead allow to use 0.0018Ag steel for temp/shrinkage?

 

[KootK]

Well, keep in mind that it's just my opinion.

I suspected that we'd eventually wind up discussing unreinforced concrete as that's the natural extension of the discussion. Personally, I think that you could justify an unreinforced concrete solution. That being said, I've always found the applicability / inapplicability of the unreinforced concrete provisions to be thoroughly confusing and sketchy so I'm probably not the best fellow to comment.

If this were my baby, and I were supporting a real structural column, I'd be putting 0.005 in there. You know, at least until a contractor bullied me into doing something less.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

Agree with KootK about the 0.5%Ag. I wouldn't leave a pedestal unreinforced. And I don't see what T/S reinforcement has to do with it, as a pedestal like that is free to shrink in the vertical direction.