Development length for hooked bar in footing

[PostFrameSE]

I'm designing a retaining wall (inverted T design) and my question is in regards to how much of the footing I can use to develop my wall reinforcing. I'm thinking that the development length of my vertical reinforcing needs to be measured from my hook to the top layer of reinforcing in the footing.............not the top of the concrete footing correct? So, for example, if I have an 18" deep footing with a layer of reinforcing 3" from top and bottom, the most development length I can count on for my vertical bars tying into this would be 18"-3"-3" = 12" rather than 18-3" cover at bottom =15" right?

 

[JAE]

In your example - 15" is what we use.

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[PostFrameSE]

Thanks JAE. So am I overthinking this? What if I changed the scenario to the following: In an effort to limit the number of bars that the concrete guys need to place, what if I place only one mat of reinforcing at the center of the footing depth (sized appropriately with a "d" = 1/2 the footing depth for rebar design.) In this case my hook is 3" up from the bottom, my only reinforcing in the footing is 6" above that and then I have 9" of unreinforced concrete above that. Would you still use 15"? Or would you say putting only one mat of reinforcing is not wise?

 

[KootK]

In my opinion, the key here is to recognize that this is an opening/closing joint design problem rather than a development length problem. See this thread for lots more info: Link. While proportioning the footing to achieve development of the vertical stem bars often leads to favorable joint proportions, providing development length is technically neither strictly necessary (in terms of ACI Ld/Ldh) nor sufficient to ensure adequate joint design.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[DaveAtkins]

KootK, KootK, PLEASE don't muddy the waters with your theories about L-shaped bars transferring tension around corners

I agree with JAE--use 15", even if the footing is unreinforced.

DaveAtkins

 

[KootK]

KootK, KootK, PLEASE don't muddy the waters with your theories about L-shaped bars transferring tension around corners

Pfft... My "theories" are the truth damn it! I've basically become the mythological "Cassandra" 'round these parts when it comes to proper retaining wall joint design. And I intend to keep hammering this topic until folks either come around or take it upon themselves to put an end to me. It's rough out there for a concrete detailing prophet...

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

Keep it up, KootK. I don't know why DaveAtkins is dismissive of appropriate detailing in opening corners.

 

[PostFrameSE]

Thanks for all of the posts!!

 

[rapt]

I would be using 2 layers of reinforcing in the footing for the top tension in the heel and bottom tension in the toe.

Plus I would have Koot's detail for the wall bars lapping with the bottom reinforcing. I would probably increase the bend radius in the L bars to reduce crushing stresses in the bend (unlike the ACI code other codes require larger radii to develop full stress around a corner, depending on the bar size.).

 

[26sig]

What radius is "large", rapt? 2x ACI?

 

[KootK]

Oh, Dave's probably just tormenting me for the entertainment value.

While I also don't care for the single reinforcing mat approach, it makes for an interesting study case. So I studied it, for sport. It creates a rather complex development condition.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rapt]

26sig,

Australian code says 10 db.

British code has a calculation method which depends on the bar diameter and the concrete strength.

 

[DaveAtkins]

Yes, I was tormenting KootK in good fun.

Although, the ACI Code currently does NOT recognize tension "going around a corner," as far as I know. This is why I will sometimes detail a standard (12*db, I think) hook on the bottom of the wall dowels, since anything longer is not recognized, by ACI, as increasing the tension available in the vertical bar.

The above paragraph is the "letter of the law." I have been thinking about this, and I realize any of us who use hairpin bars in a slab, for a pre-engineered metal building, are allowing tension to "go around bar corners."

So...I think ACI should add something like the Australian code, which says if your bar has a large enough bend radius, you can fully develop the bar in both directions (perpendicular directions, that is).

DaveAtkins

 

[KootK]

Although, the ACI Code currently does NOT recognize tension "going around a corner," as far as I know...The above paragraph is the "letter of the law."

I believe there to be a serious misconception at play here. Here's how I see it:

1) Because of concrete crushing and bar slip issues, standard ACI bend diameter hooks longer than the ACI standard 12 db are not allowed to be assumed to improve development length.

2) Nowhere in ACI is it stated that rebar cannot transfer tension around the bend of a concrete joint. Were that the case, we'd be utterly screwed for:

2a) Roof level exterior beam column joints.

2b) Corner joints in tank structures.

2c) Retaining wall stem/footing joints.

2d) A zillion other situations that I won't bother to list...

Basically, we'd have to give up on monolithic CIP and just do precast.

3) As Dave has intimated, ACI does a piss poor job of providing explicit tools that designers can use to transfer rebar tension around corners. And that creates confusion. This is why, whenever this issue comes up, I mention non-North American provisions and the curved bar node STM method (Link). Those are the available tools that I'm aware of.





I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.