Min. Spacing Requirements

[bisandcan]

I have a co-worker that insists the min. spacing requirements of s=15(40000/fs)-2.5cc do not apply to foundations, this is a one way p-ile cap, but has no real explanation why the requirement doesn't apply. I am a fairly new engineer, and from reading the commentary (ACI 318-08) I don't see an out to use 12 in. spacing when there is a 3 in clear cover to the bottom reinforcement. Could someone provide an explanation for me please?

 

[cliff234]

There really is no "out" to this. I think 12" spacing is reasonably and is probably a good idea for crack control. You are not adding any tonnage of steel to the project. Just use whatever bar size is required for the 12" spacing.

 

[steellion]

I agree with your co-worker. From ACI 318-05, the provision that you are referencing is in Section 10.6. Paragraph 10.6.1 clearly states that it only applies to beams and one-way slabs. I think 12" maximum should be used anyway, and I would refer to CRSI Manual guidance for design and detailing of rebar for pile caps.

 

[TXStructural]

Your one-way pile cap is a beam, albeit one that is essentially buried. You could consider it a grade beam of sorts. I would probably apply the beam requirements for such a configuration.

CRSI just published a new design guide on pile caps. It maintains most of what our Design Handbook previously required. There is expanded coverage for heavy piles and higher loads. As for applying ACI 318, the requirements for structural slabs transferring loads to piles would apply to traditional (2-way) pile caps. 318-14 Chapter 13.

I do not think you will find it desirable to space bars wider than 12 inches in most pile caps. Large bars take more length to develop and when hooked they take greater space. Our tabulated values do include some configurations where bars are greater than 12 inches apart in large pile caps.

 

[bisandcan]

The problem is that if you run the numbers using a 3 in. clear cover, you get 7.5ish spacing requirements, using fs=40,000. So, then 12 in. spacing is too far apart to prevent cracking in that case. So, if you use 12 in spacing, that doesn't conform to the way the code reads.

 

[TXStructural]

The formula you quote (ACI 318-11 Eq 10-4) is for control of crack width AT THE TENSION SURFACE. Provision 10.6.4 does not apply in your case. You have no need to detail for this condition in a buried member.

If the element is subject to moisture (wet in service) or other deleterious conditions, you may want to limit strain in the steel to keep crack widths narrow. See 10.6.5 in such cases. The strategy is to limit fs which effectively increases the area of reinforcement crossing potential cracks and keeping them tight.