Does pullout failure need to be checked for a hooked rebar meeting Ldh?

[MJC6125]

I have a 6" CMU parapet wall sitting on top of the edge of a 8" elevated concrete slab. See image below for reference. The wall is cantilevered (vertically) from the slab, so I want to make sure I'm not missing anything related to designing the joint between the CMU wall and the slab. When I have suction wind pressures pulling away from the building this will become an "opening" joint, which I know from previous posts on this site is not a simple thing to detail.

My loads are relatively small, so I was thinking I could justify this connection using ACI chapter 17 anchorage design rules. I'm getting a tension force on the rebar of approximately 7 kips. Everything according to ACI chapter 17 for a 1/2" diameter j-bolt seems to be working (steel strength & concrete breakout), except for pullout strength (ACI 17.4.3). Does a standard 90 degree ACI hook meeting the Ldh requirement automatically preclude a tensile pullout failure from ACI 17.4.3?

Would you add any other reinforcement to the slab edge to control breakout or anything even if the ACI chapter 17 equations aren't saying I need it?

 

[KootK]

1) I use a very similar detail to yours, just with the addition of some hooked top steel at the slab edge. I like the top steel for this because it would tend to act a bit like supplemental anchorage reinforcement for what one would expect the likely shear breakout frustum to be. And it surely helps a bit with the tension break out as well, helping to hold things together and such. I'm guessing that you probably have slab edge top steel in your plans, even if it's not shown in this detail.

2) I can get behind your J-hook check if that's how you roll. Detailed as described in #1, I don't even bother to check the joint explicitly for normally sized parapets. You gotta pick your battles in structural engineering... and this one isn't mine. I've never heard of one of these things failing and, if it did, my money says the parapet would just kind of sag out over the building rather than actually tearing off.

 

[WARose]

Does a standard 90 degree ACI hook meeting the Ldh requirement automatically preclude a tensile pullout failure from ACI 17.4.3?

As far as I know: yes. Mainly because the way the force transfers around that hook (with a ribbed bar) very differently than anchor bolts.

Would you add any other reinforcement to the slab edge to control breakout or anything even if the ACI chapter 17 equations aren't saying I need it?

No. (Although I would detail that slab a bit differently for other reasons.) If it is making it by Chapter 17, it's probably not a problem period as far as anchorage goes. The slab rebar would have to develop the forces (of course).

Interestingly enough, the breakout part (assuming you are referring to shear towards the edge) is the part I (and others here) have worried about over the years (i.e. if Appendix D/Chap 17 is applicable). I've never worried that much about the hooks in tension....but shear near the edge.....that one has troubled me at times. (But I have heard good counter arguments about shear friction and so on.)

 

[STrctPono]

Does a standard 90 degree ACI hook meeting the Ldh requirement automatically preclude a tensile pullout failure from ACI 17.4.3?

Yes. End of story. Do not complicate it any more than that in trying to apply anchor design theory to this situation. Ldh is all you need (while of course making sure your details are sound.) This is actually quite common to get confused with development length requirements on rebar (where it is composite with the concrete on both side of the joint) and Chapter 17 anchorage design.

 

[MJC6125]

Great, thank you.

I'm guessing that you probably have slab edge top steel in your plans, even if it's not shown in this detail.

This particular case is a 2-way PT slab, so I didn't actually have top reinforcement on these free slab edges, except at the column strips and other miscellaneous reinforcement associated w/ PT detailing. Based on the responses, I'm going to add some where this detail applies. What type of hook would you prefer on the end of that top reinforcement? Standard 90 degree hook down, standard 180 degree hook down, or possibly some sort of hook that bends horizontally since I'm dealing with a thinner 8" slab? I'm thinking the 180 degree hook shown below, so it is "a bit" like supplemental anchorage reinforcement like you said.

 

[bones206]

I think it is entirely appropriate to check anchorage failure modes. Pullout strength per Chapter 17 doesn't apply here since that equation is based on testing of smooth anchors, not ribbed rebar. Development length is more appropriate for pullout capacity since the equation is based on testing of rebar. So I look at it as Ldh gets the tension from the bar into the slab, then Chapter 17 checks the ability of the slab itself to resist the tension. Similarly for shear via dowel action.

 

[KootK]

Firstly, I should mention that I disagree with some of the other fellows with regard to whether or not [development = anchorage]. I'll try not to belabor that here, however, since I get the impression that you're familiar with some of my past work on that issue. Here, I feel that the important feature of your situation is that geometric constraints will force you to turn your hook in the wrong direction: away from the strut that it is meant to contain. See the sketch below where I've rotated things to make the point more salient.

What type of hook would you prefer on the end of that top reinforcement?

I'd just go with the 90 hooks. Two reasons:

1) I find that rebar installers prefer them, pretty much universally.

2) I don't foresee a meaningful performance advantage in this situation with 180 hooks. Given that the effective depth of your "beam" is going to be a paltry 3", cover requirements pretty much guarantee that this would never survive an STM check anyhow. There just isn't enough space.

Mechanically, this would be a nice application for headed rebar. But, then, we'd have have to form a search party to go looking for your corpse and that wouldn't be much fun in Colorado this time of year.

 

[dik]

I'm missing something here... why 3" in an 8" slab? Shouldn't you use about 1-1/2" cover from the top? The 90 deg bars can be tilted to get them to fit in the slab depth.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

I'm missing something here... why 3" in an 8" slab?

It's 3" in a 6" CMU parapet (centered). Note that my sketch was rotated 90 degrees.

 

[EZBuilding]

I would recommend considering a U-bar in lieu of a hooked top bar. Parapet wind pressures can occur in both directions. In regards to the original questions, I would agree with the majority of posters and consider a hooked bar development length as effective at this condition.

 

[dik]

Thanks, KootK...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[bones206]

MJC6125 - Of all the failure modes you checked, which modes governed for shear and tension? (excluding the Chapter 17 pullout mode, which I don't believe applies to rebar)

 

[MJC6125]

The steel strength in tension was the governing failure mode in my case. Below is a snip of the check from Simpson Anchor Designer. It's done using a Gr. 55 hex head bolt that way the pullout didn't control, but I think all the other checks should be pretty consistent with the #4 hooked dowel (Gr. 60 rebar).

 

[canwesteng]

Development doesn't equal anchorage, but development does equal pullout strength, which seems to be the original question.

 

[bones206]

So the tension breakout is 1.56x the steel strength, which is a good margin. 1.2x would be the minimum ratio if you were designing for seismic (accounts for material overstrength, etc).

For shear, concrete breakout controlled. Not sure you would have known that limiting capacity without performing anchor calcs.

The results seem to validate the idea that Chapter 17 anchorage calcs are appropriate in this case.

 

[KootK]

Development doesn't equal anchorage, but development does equal pullout strength, which seems to be the original question.

1) I agree if [development = bond failure mode pullout strength and only that}. That is, effectively, the one and only thing that development guarantees.

2) I do not agree if [development = the bar, in the absence of a resisting struts / compression fields, cannot be ripped from the concrete in any fashion). This, in my view, requires checking for anchorage.

Only OP knows for sure but I feel that he was asking about #2. Given the direction that the hook is facing, it would almost have to be that.

 

[Lomarandil]

I don't even bother to check the joint explicitly for normally sized parapets. You gotta pick your battles in structural engineering... and this one isn't mine. I've never heard of one of these things failing and, if it did, my money says the parapet would just kind of sag out over the building rather than actually tearing off.

According to my colleague, parapets kill a lot of people in the seismic prone parts of South Asia... I don't know how common it is to include a top hooked bar or not (probably not)

----
just call me Lo.

 

[bones206]

Certainly unreinforced masonry, particularly parapets, are extremely vulnerable in earthquakes. Reinforced masonry parapets are much less vulnerable, but paying extra attention to detailing and ductility of the anchorage is warranted IMO.

 

[KootK]

Yeah, my approach only applies to the particular systems and detailing practices that I employ in my own work.

Part of my reluctance to get too excited about the detailed checking of these things stems from:

1) In 6" block, I suspect that your bars could wind up anywhere between the face shells on a parapet.

2) The geometry being as tight as it is, in my heart of hearts I don't have much faith in any of the checking methods that we've been discussion. I doubt that any of them would correlate well with testing. For me, this falls into the category of things for which I either:

a) Reconcile myself with "design by good detailing" or;

b) Adjust the proportions in ways that make me unpopular.

 

[slickdeals]

Lomarandil - that's because there's a lot of unreinforced brick or clay masonry.