Standard hooked rebar with 0" cover

[Prestressed Guy]

I have a client that is asking me to use a detail from another firm. The connection is on a non-composite insulated precast wall with integral continuous corbel to support hollow core plank. They have placed both the a_s corbel reinforcement and the out-of-plane wall anchorage reinforcement with the tail of the hook in direct contact with the foam insulation. I assume that the thinking is that no cover (environmental) is required because the concrete is in a dry condition within the composite wall.

Ingnoring the merit of that argument, will the hooked bar be developed (assuming that full l_dh is provided) if there is no concrete on the outer edge of the hook tail?

It seems to me that the tail still needs to develop tension. When the tail starts to load in tension as the load tried to pull it around the bend, it will deform up into the foam and have no anchorage in the tail. If the bend was sufficient without the tail being anchored, we would not need any length to the tail beyond the bend.

If you cast a sample with a bar tight to the form (0 cover) and test it for tension I don’t think it would take much load before spalling out the face. In Section 12.2.3 EQ 12-1, l_d would result in a division by 0 error due to c_b and K_tr both being 0. As c_b approaches the limit, l_d is going to infinity so no development length would satisfy. In the commentary on Section 12.2.2 it states “With less cover and in the absence of minimum ties or stirrups, the minimum clear spacing limits of 7.6.1 and the minimum concrete cover requirements of 7.7 result in minimum values of c_b equal to db. Thus, for “other cases,” the values are based on using (c_b + K_tr)/d_b = 1.0 in Eq. (12-1).”
This tells me that 12.2.2 other cases is not applicable for c_b Less than d_b.

 

[JAE]

I wonder if section 12.5.4 applies here...similar condition where the tail of the hook is along a surface plane of concrete (i.e. the end of beam = your inside face of panel surface at the insulation)

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[JAE]

And technically - doesn't 7.7.1 require 3/4" cover for your situation? How can you provide 0" and still meet this section of the code? Just because it is on an inside surface of a wall - it is still a terminating wall / concrete surface that requires minimum cover.



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[Prestressed Guy]

It does not appear to me that section 12.5.4 applies. It states “For bars being developed by a standard hook at discontinuous ends of members with both [highlight #FCE94F]side cover and top (or bottom) cover[/highlight] over hook less than 2½ in.” Neither of these conditions apply in this case. Section 12.5 does not address the cover required on the tail of the hook, so it would default to the standard cover requirements. The minimum cover required for walls in 7.7.2 & 3 is ¾” regardless of environmental concerns. In commentary section R7.7 it states "The development length given in Chapter 12 is now a function of the bar cover. As a result, it may be desirable to use larger than minimum cover in some cases.

 

[Prestressed Guy]

Jal, Let me repeat, this is not my detail and I do not think it will work. I have a client that is asking me to use this detail from another firm (which is much larger than mine so they must be correct.
I think that this other firm is thinking that they have 5" of cover (2" of foam plus 3" of facing concrete). This might make sense if the cover is just for environmental rust protection and that the tail did not need to be developed. To me I could only justify l_d of 6" and would ignore the hook.

 

[JAE]

I agree with your assessment.

I guess I thought at the very least, section 12.5.4 shows that the end cover on a hook IS an issue with hook development in general.
Section 7.7.1 is what would apply to you.

Our precast engineers usually eliminate the insulation in small areas where tie-in or hooks or embedded plates with anchors is needed.

As far as engineers in larger or smaller firms - I would pose the JAE axiom: Despite public ignorance and delusion, firm size has nothing to do with engineer talent.



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[TehMightyEngineer]

I agree with you Haydenwse, this doesn't appear to be acceptable as the hooks needs to be developed. I'd see if they can swap it out for headed bars or change the hook geometry. Or they could cut out a section of insulation to allow the rebar to be fully enclosed in concrete.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

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[TLHS]

Even if hooks didn't need to be developed, you don't have confinement of the hook and it will act significantly differently than one with confinement. It can't hold load on the inside radius of the hook as well as a confined hook would, because the tail of the hook will bend outward into the foam when you load it instead of getting at least nominal restraint from the concrete behind the hook.

 

[Prestressed Guy]

Thanks for the replies. I have control of my details but was looking for some chapter and verse reasons why I cannot use this other detail. I will keep using my own standard which is to call for #3 or #4 with ¾" cover and then adjust spacing based on a_{s req}/a_{s prov}. It doesn't meet the 6" minimum requirement but I think it can be justified because it would require pulling out the full thickness of the wall which would include all reinforcement and associated shear friction capacity of the shear cone pullout.
In other words, I can see l_dh of 5¼" with 3/4 cover on the tail but not 6" with no cover/development of the tail.

 

[bones206]

Since you technically won’t be meeting the 6” minimum embedment for a hooked bar, could you do an Appendix D style anchorage calc to see what kind of capacity you get? It seems like a fairly important connection and I’m not sure that the 5-1/4” embedment is fully defensible, even though I agree it’s better than the 0” tail cover option. If you treat it as a hooked anchor bolt and you get the capacity you need that way, then I think you’d be able to better justify going below the minimum knowing that you covered the bases. You mentioned cone pullout, so it got me thinking you probably ought to check that.

 

[TLHS]

Or do an embedded plate style detail if you want to go all the way to the back of the concrete. Weld a piece of plate to the back of bar to anchor it, and leave that exposed to the foam. Then treat it as a plate bearing on the concrete.

 

[Mjkkb2]

I really do not like this detail. I think you are correct that if any tension is applied to either of these bars, the back/hook will just kick back into the foam. So essentially you just have a straight bar with no hook. I think appendix D approach is a good idea. If you can justify the capacity with an anchor in tension approach, then I would be ok with keeping the bars in contact with insulation.
But this is generally a bad idea to get oneself talked into a detail that cannot be easily defended by citing code language; which in large part comes from testing. And I’m pretty sure there was no testing done on the hooked bar development with no cover.