App. D
App. D
(OP)
Here is a very generic question on ACI App. D. I have a very good grasp of this material since I wrote a very detailed spreadsheet for it, but I still question the validity of it. Is the only reason that this is required is because of the high stress concentrations associated with headed anchors? If the anchors were deformed and embedments met development length requirements would App. D even exist?
I'm just thinking about a #6 rebar with a development length of 15" (with no reductions).
Let's say for the attached sketch you have a #6 bar hooked with 15" embedment - you have nothing else to worry about, no breakout checks, no side face blowout checks, no pull-out checks, etc. You also get a capacity of 47.5k of tension (for the two anchors in tension).
Now replace those #6 rebar with 3/4" diameter headed anchors with 15" embedment and design per App. D. Now for the same 2 anchors in tension, you get a capacity of 15.3k and it's controlled by concrete breakout (assuming cracked concrete, but using supplementary steel). That's a HUGE difference!
Is the only reason because of the stress concentrations at the head of the anchor?
I'm just thinking about a #6 rebar with a development length of 15" (with no reductions).
Let's say for the attached sketch you have a #6 bar hooked with 15" embedment - you have nothing else to worry about, no breakout checks, no side face blowout checks, no pull-out checks, etc. You also get a capacity of 47.5k of tension (for the two anchors in tension).
Now replace those #6 rebar with 3/4" diameter headed anchors with 15" embedment and design per App. D. Now for the same 2 anchors in tension, you get a capacity of 15.3k and it's controlled by concrete breakout (assuming cracked concrete, but using supplementary steel). That's a HUGE difference!
Is the only reason because of the stress concentrations at the head of the anchor?






RE: App. D
If the pier shrinks to 18" and the edge distance of the anchors drops to 2.5" then the headed anchor capacity drops down to 6K, from the 47.5k of the rebar. That's crazy.
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I don't believe that confinement is the issue since the basic hook development length doesn't require ties for confinement - you get a reduction if they are present, but they're not required.
RE: App. D
It might have something to do with the fact that reinforcing is developed with much longer lengths than practical with anchor bolts, but logically that can't be the whole reason.
Any time I try to get my arms around Appendix D, I get a massive headache. I follow it, the best I can, but I've never understood the necessity/complexity for it.
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You might also notice that in your original example, if you use a hooked bolt with the same geometery as the hooked rebar, then capacity of the bolt will be less than the headed bolt.
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Mike McCann
MMC Engineering
RE: App. D
1. If you place a #6 hooked bar as a 3/4" headed stud, the investigation method is identical, no differential treatment - the breakout, if it controls stud design, it controls the #6 bar as well. The difference of hooked bar in pedestal and in beam-column joint is due to the confining effect of the rigid joint (small rotation), and nature of the load delivery mechanism - progressively passing from point to point (not concentrated at a single load point as anchorahe does, which causes high local stress concentration as many have pointed out).
Also note, bent bars, or anchor bolt, is poor anchorge device, which prone to pull-out failure.
2. Again, without refer to code. For headed stud, I would utilize the reinforcing dowels to resist uplift. However, the dowels shall have adequate development length above the concrete failure cone. The cone radials 45 degrees from the top of the head of the stud, thus, usually resulted in longer stud, which is conservative. The upper ties shall be kept in a closer spacing (1/3 to 1/2 of the normal spacing) to prevent the dowels from poping out. Also, make sure there is adequate space in between the anchorage and the dowel to allow for bond to develop for both (comply with ACI minimum spacing requirement - would result in bigger pedestal size).
3. I believe the original design concept (anchorages in concrete subject to shear & tension) and analytical method was developed by NRC (Nuclear Regulatory Commitee) for nuclear power facilities, then adopted industrial wide with very conservative safety factor (4 ?), that meant to use in conjunction with ASD. The ACI had been tardy for many years, not until ???, finally, it started to play catch-up games. Well, you know how good it is. Good luck, fellows.
RE: App. D
Also companies do make rebar that is thread at the end, which kicks you out of App. D. Bet they like App. D :)
RE: App. D
Consider the attached figure. You have an upper block of concrete with anchor bolts in the top of it, attached to the lower slab with rebar. If those anchor bolts extended all the way through the upper block into the slab, then App. D gives you equations to check breakout of the bolts from the slab. However, if the bolts terminate where shown, and rebar is extended into the slab as shown instead, there is no requirement in the standard to check breakout. Does that mean that the rebar installation is stronger? Or that you've just missed a failure mode?
Please understand that I haven't made a detailed comparison of App. D vs the rebar anchorage requirements; I'm just trying to explain some possibilities of why things are as different as they are in the results.
RE: App. D
The development length of rebar is determined by the bond strength which is affected by adhesion, friction and bearing of the deformations against the concrete. For the rebar to pullout, the concrete in front of each deformation must crush or the keys of concrete between the bar deformations must shear off.
With a headed anchor, the proximity of the failure cone to the edges and the depth of embedment affect the capacity. There's no consideration for adhesion and friction along the length of embedment of an anchor. Plus the only bearing is provided by the head on the anchor whereas on rebar, numerous deformations provide the bearing along the length of embedment.
sandman21 brings up an interesting subject about threaded rebar. Why go through the hassle of App D, struggle with obtaining capacity, and utilizing rebar lapped to the anchor rod to develop the required resistance when you can use a specified length of rebar threaded on one end to serve as the anchor and not bother with App D?
RE: App. D
App. D does not address the actual member itself, only the anchorage. The issue is not a lack of analysis. With rebar development, there is no "concrete breakout, pullout, etc", it is a bond failure or splitting failure (depending on cover, spacing, etc.). The point I was trying to make is that if you have two identical piers (as I detailed in a previous post) and one has #6 bars, the other has 3/4" diameter headed anchors. They both have identical spacings, edge distances, embedments, etc. the #6 give dramatically higher capacity. I am simply asking if this is a result of the deformations on the bar developing the rebar along its length as opposed to the headed anchor being concentrated at a single point. I don't care about, and have no interest in, what goes on beyond developing the load into the member. In your sketch, the only way that you don't have to worry about concrete breakout is if the rebar have enough cross-sectional area to develop the load AND the rebar extends the required development length above the potential failur plane.
kslee-
The investigation method is not the same. The headed stud requires App. D, while the rebar requires only the necessary development length. There is no "breakout" capacity of a hooked #6 bar, there is only the development length (which is limited by bond or splitting).
RE: App. D
Did code explicitly calls for different method for stud and rebar? If it is the case, I don't feel it's correct. And that's the reason, though I didn't point out, why it stays in "APP D", because it is subjected to potential revisions.
I think something in the appendix is not rigid provision, but a preference/guide. Once the refine works done, it will be moved into the main code body.
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