ACI Appedix D Concrete Pryout Strength D.6.3 1

[jetboat]

I am designing anchors embedded into a 2' square pier. The pier has eight vertical bars evenly spaced 3" from the perimeter. I consider the vertical bars as supplemental reinforcement and ignore the concrete breakout check for tension since it doesn't govern. I was wondering about the Ncbg value for concrete pryout in section D.6.3. Does anyone have guidance on weather or not using a larger value for Ncbg than D.5.2 guides? Due to supplemental bars I would assume that alarger value of Ncbg could be used.

 

[civilperson]

The ties around the eight vertical bars and anchor rods will increase the Ncbg. Three each number #3 in the top five inches will help.

 

[sdz]

The anchor rods have to be embedded far enough for the reinforcement to develop the required strength on both sides of the potential failure surface. Once you satisfy that concrete pryout should not be a problem. Failure surface should be either a horizontal plane or inclined at 35 deg to horizontal (consider both).

 

[NS4U]

No, I disagree.

Refer to ACI 318-08. D5.2.9 to be exact.

It discusses the addition of supplemental reinforcement in detail. You can not account for both concrete breakout strength AND supplemental steel strength. The reason being that the only way to engage the supplemental steel is to crack the concrete in tension, but if you crack the concrete in tension you have no tension capacity.

Also, sdz is right, if you are going to account for supplemental reinforcement, the bars need to be developed on each side of the failure plane to actually be able to take into account their strength.

For more discussion on this see "Anchorage in Concrete Construction" by Eligenhausen, Mallee, and Silva.

sdz, why do you suggest considering a horizontal plane?

 

[Lion06]

NS-

I don't have ACI 318-08, but the 05 version says that you can get a larger phi factor if you have supplemental reinforcement. I assume that to mean that the reinforcement does not need to be developed on both sides are you would get much, much more of a bump than 0.05 in your phi factor.

That being said, the rebar would have to be awfully close to the actual studs to be in the breakout zone. Additionally, the breakout zone for pryout is smaller than for tension breakout (i.e. there is little to no breakout to the left of the stud - if the direction of the shear load is to the left, or to the right of the stud if the direction of the shear load is to the right). They just use that as a simplifying model.

 

[jetboat]

Structural/EIT,

I agree with what you have stated but in general what is your take on awfully close( Within 6" or you thinking closer).


No Soup,

Whould you not consider the horizontal ties as intersecting the pryout shear plane?

 

[Lion06]

My initial reaction is to say closer than 6", and here is why - typically pryout only has a chance to control for shallow embedments. That being the case, the failure plane will not extend very far from the stud - consequently, the bar has to be pretty darn close to get some length on both sides of the crack.

For an example, if you have a stud with 4.5" embed, the failure plane can't be expected more than 6.75" from the stud. If you leave 1.5" clear at the top of the footing/pier and the vertical bar is 3" from stud then that only gives 1" of bar above the crack. If you get it to 2" from the stud, then you have 1.67" of bar above the crack. It's obviously up to engineering judgment as to how much you need to constitute "supplemental reinforcement" per ACI 318-05 App. D, but that's cutting it awfully close.
If you have much deeper embedments where it doesn't have to be quite so close then pryout isn't likely to govern anyway.

 

[NS4U]

EIT-

Please direct me to your 318-05 reference, I do not recall seeing anything stating this. I find it odd that something so subject would make it in 318. But either way, I do not agree that you get "much much more" strength by adding developed steel. You can not count your concrete and your steel capacity together.

No more than you can count on concrete's tension capacity plus rebar capacity when analyzing a beam for flexure. It's either one or the other.

Also, I believe you misunderstand the pryout failure mode. From your explanation you appear to think that as you load the top pf an anchor in shear, the bottom head of the anchor kicks back. This is not the case. Test results have shown that as the top of the anchor bends (due to shear), it pushes down and into the top of the concrete, essentially putting itself in tension. This is why the code has you check pryout using tension capacity.

I will try and sketch when I get chance.

Jet-
The horizontal ties should hook around the anchor bolts. Not the vertical bars. Again, "Anchorage to concrete" goes into a good discussion about this. Essentially by hooking the bars around the anchor you prevent the anchor from deforming giving you better performance from the steel. Tests support this conclusion. If your ties aren't in contact with the anchor you can lose up to half of the capacity of the supplemental reinforcement.

Seriously, I can't echo this enough... Appendix D is based on empirical data, "Anchorage to concrete" explains the test data, the equations that are based off the data, the failure modes that are considered and why, the reasoning and detailing behind supplemental reinforcement. All of the question you are looking for are answered in one chapter of this book. You are probably much better of reading it in there. Then when you have questions we can discuss. While I definitely don't consider myself an an expert in anchorages, after reading some of that book I feel like I understand Appendix D a million times better. I think you will too.

 

[sandman21]

D.5.2.9 is not talking about supplemental reinforcement, this section talks about anchor reinforcement designed to take the load in the bolts. If you design per this section concrete breakout does not need to be checked, only reinf. spaced less than .5hef can be used, something I think is wrong.

 

[Lion06]

I was saying that if the bars were completely developed on both sides than you wouldn't even have to count on the concrete in most cases, not that ACI gives different values for different lengths of embedment of supplemental steel.

 

[sdz]

NS4U (Structural)
6 Feb 09 12:49

sdz, why do you suggest considering a horizontal plane?

NS4U,

In a pedestal we should consider potential, reasonably probable, failure surfaces. One is to consider a failure surface inclined upwards from the bolt head at 35 degrees to horizontal; but this is the (approximate) failure surface you get where the bolts are remote from the edge. This might not form in a pedestal where the horizontal dimensions are relatively small. Instead we might get a horizontal failure surface across the pedestal.

How do we know which is more likely to form? I don't know and I have not seen any definitive research. Therefore I would consider both as upper and lower limits, and ensure development length is provided for both cases.

 

[jetboat]

NS4U-

I do not have access to the ref. you have stated at this time but I will work on that! Have you seen the ASCE publication "Wind Loads and Anchor Bolt Design for Petrochemical Facilities"? If so what is your take? Over the years I have used this publication as a guide to take the pedestal reinforcement also as supplemental steel, for increased shear and tensile resistance when concrete failure is present. If you are not familiar with this publication thanks for the previous input!