app. d

huangyhg

app. d
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?
here is the attachment.
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.
i think it has to do with confinement. the difference is reinforced vs. unreinforced concrete. if you had ties around your studs, i think you can treat it like an ordinary development length (and lap splice) problem, not per appendix d. i'd be interested to see if others share this view.
i believe you can lap headed anchors with reinforcement regardless of confinement steel.  
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.
structuraleit, you're right.  there's many issues that we ignore for standard reinforcing that are critical for anchoring to concrete, like edge distances, spacing, etc.  i can't explain it.
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.
you bring up a very good point...one that i've often pondered as well.  i also have no explanation for it.  either app d is very conservative for headed anchor design or we are all very unconservative when designing standard reinforcement.  but a spreadsheet or anchor design program is a definitely a must for anchorage calcs.
i hate appendix d.
i think perhaps multiple factors.  one is the difference in the bar versus bolt- with all that load going at one point or being distributed along the bolt.  one thing is a lack of analysis.  just because there's not a requirement for breakout check doesn't mean there shouldn't be.
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.
i had considered that maybe the concrete failure with rebar is a bond failure (or crushing directly above the hook then bond failure) versus a cone failure with headed studs, so the cone interactions with edges and with each other do not apply in the typical case with rebar. but that doesn't seem to add up. there must be some type of cone like interaction as you go along the length of each rebar/deformed bar.
   
i'm not concerned at all with a hooked bolt, i would never spec one.  i'm concerned with the difference in behavior and the drastically different capacities that can be generated.
maybe i'm missing something here, but it looks like you've got a block of concrete with two bars sticking out of it, you figure the pullout capacity of those bars, but then in the plane just behind them, you have an unreinforced concrete cross-section that is loaded in tension?  is that correct?