Anchor Bolts - Concrete Breakout in Tension

[ScratchyFilm]

I'm struggling to understand what's going on with the equations from ACI for Concrete Breakout Strength in Tension.

I am doing a check on some light pole footing anchor bolts, and the embedment depths are quite deep on these (~38" so my ca1 values are <1.5hef on all sides of the footing). Looking at the equation andremoving the modification factors, it is Anc/Anco * Nb.... According to ACI, Anco should be 9*hef^2, and Anc needs to be calculated with the modified hef value. This ends up meaning that Anc/Anco is a very small number.

The part that I'm getting confused is that Nb is supposed to use the modified hef value, but shouldn't this be the full hef value since you would be multiplying it by a fraction of what the actual failure plane would be? Otherwise I would be taking a Anc/Anco (small fraction) x Nb (small number based on modified hef).

It doesn't make sense that concrete breakout in tension would be a limiting factor here with the embedment depth. Thanks for any feedback.

 

[ProgrammingPE]

It's hard to know for certain if you have an error or not without seeing your calcs, but is it possible that you are still using your actual hef value of 38" when you calculate Anco? You should instead be using the hef value that has been reduced due to the close edge distances.

Basically the reduced hef provision says that once your embedment is deep enough that your breakout is through three sides, you will get no additional capacity by going any deeper. You can then just calculate the capacity like the embedment is actually at that reduced embedment depth.

 

[AskTooMuch]

Light pole foundation? So it's on a drilled pier or on a pedestal/pier. You will want to consider the effects of reinforcement.

Google "design of anchor bolts in reinforced pedestals"

 

[ScratchyFilm]

ProgrammingPE I had a hunch that this is what should be done, but I didn't see any language in ACI to suggest doing that to Anco, only Anc. That would clear a lot of things up. Thanks!

AskTooMuch I was reading about using reinforcement for concrete breakout in ACI. They do provide a diagram, but their detail shows a hook on the bar. I'm guessing this is just for illustrative purposes and shouldn't be required for concrete breakout. Thoughts?

 

[AskTooMuch]

I'm not sure what you mean requires hook. See link...

https://www.sefindia.org/forum/file...nforcement_a_petrolium_project_papers_199.pdf

 

[ProgrammingPE]

I didn't see any language in ACI to suggest doing that to Anco, only Anc.

The wording in ACI 318-14 says:

"Where anchors are located less than 1.5hef from three or more edges, the value of hef used for the calculation of ANc in accordance with 17.4.2.1, as well as for the equations in 17.4.2.1 through 17.4.2.5, shall be the larger of (ca,max)/1.5 and s/3, where s is the maximum spacing between anchors within the group."​

I see the wording could be interpreted to mean that the reduced hef value is only being used for ANc, but it is actually saying that the reduced hef is used in all of those equations. You know this since ANc only shows up in one of the referenced equations. Also, the part I underlined wasn't included until ACI 318-11.

Structural Engineering Software:

http://www.structuralcentral.com

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http://www.youtube.com/structuralcentral

 

[bridgebuster]

Are you dealing with an unusually tall light pole that requires a design? All the agencies I work with have predesigned light pole foundations. 33"+ J bolts are typically used

 

[ScratchyFilm]

It's a rural area, so no standard. As I run through these calculations, I am not really seeing the necessity for such a large embedment depth, though. The concrete breakout strength gets limited by the diameter of the footing fairly quickly, and the method shown by ACI is good up to 25 inches of embedment.

 

[WinelandV]

ScratchyFilm,

I will wager a guess that they made the anchors so long so as to put the breakout cone at a depth conducive to lapping the pier vertical bars. This is a fairly common way to deal with insufficient breakout area.

Please note that is a "v" (as in Violin) not a "y".

 

[ScratchyFilm]

Winelandv,

I've just started reading about the reinforcing of the anchorage zone yesterday based on recommendations earlier in the thread. As I was reading about it, I always figured it was the lap BELOW the anchor bolt head. Are you suggesting that perhaps it is that deep so that there is also sufficient lap ABOVE the head?

 

[STpipe]

Winelandv,

I've just started reading about the reinforcing of the anchorage zone yesterday based on recommendations earlier in the thread. As I was reading about it, I always figured it was the lap BELOW the anchor bolt head. Are you suggesting that perhaps it is that deep so that there is also sufficient lap ABOVE the head?

Anchor reinforcement has to be developed on both sides of the potential breakout surface.

 

[ScratchyFilm]

I appreciate the insight.. This would likely describe why the embedment lengths are as long as they are.

 

[Bendog99]

Scratchy, see AISC Design Guide 7. There is a chapter on anchor bolts and an example of how to transfer tension in the anchor bolt to supplemental reinforcing. Hooking the bars reduces the development length of the reinforcing.