Concrete anchor bolts in tension
Concrete anchor bolts in tension
(OP)
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.
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.






RE: Concrete anchor bolts in tension
The factors are (ψed,N)*(ψc,N)*(ψcp,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.
RE: Concrete anchor bolts in tension
"C,N" is the subscript of the second psi, etc.
Nb is lbs, not kips.
RE: Concrete anchor bolts in tension
RE: Concrete anchor bolts in tension
Ncbg=Anc/Anco*(ψec,N)*(ψed,N)*(ψc,N)*(ψcp,N)*Nb
RE: Concrete anchor bolts in tension
RE: Concrete anchor bolts in tension
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.
RE: Concrete anchor bolts in tension
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.
RE: Concrete anchor bolts in tension
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.
RE: Concrete anchor bolts in tension
RE: Concrete anchor bolts in tension
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.