Concrete anchor bolts in tension 3

[IceVol]

I am digging through the ACI code D.5.2.1 (Concrete breakout strength) to see if a group of anchors can handle a massive 40K force. The answer is probably not, but I am having difficulty decipering the Ncbg formula to prove it. It looks somthing like this

Ncbg= (Anc/Anco) (Psi ec), N (psi ed), N (Psi)c, N (psi) cp, N^Nb

I assume N is the number of anchors but I'm not totally sure. No idea what to do with the commas, but multipling all these factors is probably not right. My Nb is about 8000 psi making N^Nb rediculious. The (psi) factors are all about 1 which makes sense. ANc/ANco is about 1.6 which sounds good to someone who has never done this before. How should I be interpreting this?

I'm fresh out of school forgive my ignorance.

 

[Lion06]

N just denotes that it's for a tensile load. The number of anchors never actually enters into the formulas - it's taken care of with Anc/Anco.
The factors are (?ed,N)*(?c,N)*(?cp,N). Nb should actually be kips and is it's own term, it's not an exponent. I think once you see the equation correctly it will make much more sense. Anc/Anco=1.6 is only good if you have 2 anchors. If you have more, they're too close or the embedment is too deep. If you have one anchor, there's a math error. If you can't get it to work out with App. D, you can develop the anchors with rebar to get away from the concrete breakout, but you still need to do the other checks for App. D.

 

[JStephen]

"ed,N" is the subscript of the first psi.
"C,N" is the subscript of the second psi, etc.

Nb is lbs, not kips.

 

[Lion06]

It will be in kips when you divide by 1000. Don't work with pounds, the numbers will get too big.

 

[asixth]

Agree with all of the above, you are getting confused with the position of subscripts as they are written on paper. Equation D-5 reads:

Ncbg=Anc/Anco*(?ec,N)*(?ed,N)*(?c,N)*(?cp,N)*Nb

 

[IceVol]

That makes things much more clear and much easier. Ya'll are a huge help. Once I understood how the symbols were written its a piece of cake. I have figured out a way to solve my problem.

 

[eit09]

IceVol,

Check out D.5.2.3 in ACI318-08! I overdesigned for a while untill I came accross this section in code. This is additive to structuralEIT's first excert.

 

[weab]

ICE VOL

I fully understand why you had that problem. The way that the code put so many variables with capital and small letters along with commas is confusing. When I first saw that formula, I did the same thing. ACI writers deserve a whipping with a wet noodle for this one.

 

[IceVol]

One of my Co-workers checked my calculations and brought up some points that had us scratching our heads.

hef
I took this value to be 4.75 according to Hilti's recomendation. But in the field the anchor is much longer closer to 8 in. What assumption is hilti making to limit the effective embedment to 4.75? Its the KB-TZ

ANC formula
I hastily used the ANc formula in Fig. RD.5.2.1(b) with the four anchorbolts. But it says conditionally "If ca1<1.5hef"
My Ca1 is not < 1.5hef so my shear cone does not go outside the edge which is good. Do I have to do something to modify the formula under these conditions though? The way it is written it makes it seem like that formula is only valid if the shear cone goes outside the edge.

If you read D.5.2.1(b) it says "ANc shall not exceed nANco, where n is the number of tensioned anchors in the group. Anco is the projected concrete failure area of a single anchor with an edge distance equal to or greater than 1.5 hef." Sounds like I'm good.

So in my case (see attached file) can I use that formula? Just from the make up of the formula it still makes sense.

 

[IceVol]

forgot to put in the link sorry

 

[Lion06]

The first thing I would note is that Anc = 623 in2, not 640.
This is a result of your ca1>1.5hef. You have to limit the value to ca1 or 1.5hef, whichever is smaller.

You have the right numbers in your Anc calc, but you come up with the wrong answer. Punch it in your calculator again. It happens.

It doesn't matter how long the rod is as long as they provide the proper embedment. An 8" rod seems reasonable for a 4.75" ebmed. They need length for grout, plate, and nut. 3.25" doesn't seem unreasonable.