Would use of a corner bar in standard hook shorten the development length? 3

[thejonster]

I'd love to reduce the dev length for my retaining wall bars in the footing making a more efficient use of concrete using a longit bar in the corner of my standard hook. This would be analogous to a stirrup hook, which works in beam shear with small dimensions, right? A force is a force. It's not allowed by ACI318 for standard hooks, but perhaps a conversation should be started to make that a new reduction factor

I'm usually good at finding pertinent discussions, but can't find anything in the forum on this

 

[SWComposites]

A sketch would probably help.

 

[bugbus]

I think it could be justified in theory, but I had a few thoughts on it.

For a retaining wall, I assume you're talking about L-shaped bars (90 degree bends). In the Australian codes (and I assumed in North America as you alluded to in your comment), a stirrup cannot be considered anchored by a 90 degree bend if the bend is in the outermost layer of reinforcement. You would need either a 135 or 180 degree bend.

I think that reducing the embedment of the wall bars in the footing (presumably by making the footing thinner overall) would begin to impact on the joint's ability to achieve its intended bending capacity. My impression is that these kinds of moment-resisting joints are already under-designed and not enough attention is given to strut-and-tie modelling, controlling splitting along the diagonal strut, etc. In other words, the development length of the wall bars is just one piece of the pie, but probably not what will govern the ultimate capacity of the joint.

 

[Trippelsewe]

My understanding is that the problem relates to crushing within the radius of the bend. In theory a transverse bar probably helps with this by increasing the bearing surface, but the only code allowance I am aware of is to use 'slow' bends with a certain radius. These bends are not always easy to do since pins are set with a certain radius.

AS3600:2018
13.1.2.5 Development length around a curve
Tensile stress may be considered to be developed around a curve if the internal diameter of
the curve is 10db or greater.

 

[thejonster]

Here's a sketch of the bar I'm referring to, I'd like to minimize the development length 'ld' of a standard hook using a longitudinal bar in the corner

 

[phamENG]

No. That would not reduce required development length. You'd have to develop it with the other bar for the other bar to help, which just puts you back where you started.

 

[thejonster]

Thanks pham, that was my suspicion.. no go for a standard hook
But aside from what the code allows for a standard hook, could I develop it like a stirrup hook? If so what's the biggest stirrup bar dia permitted? If not, why not?

I think it should be added as a factor for std hook dev length in ACI 318, can we just vote for that here?

 

[KootK]

This isn't actually a development length problem. It's a "transmit the bar tension around the corner" problem which is usually less onerous in terms of footing depth. See this thread for a ridiculously exhaustive discussion of the issue and a bunch of strategies that you might use: Link

The increase for stirrup anchorage wrapped around a longitudinal bar has to do with controlling the amount of concrete splitting expected to occur parallel to the stirrup. The longitudinal bar very often is not even in contact with the stirrup hook so we don't rely on it for mechanical bearing.

 

[thejonster]

That is quite the thread, I found the joint testing research you referenced compelling, and makes me glad I asked the question.

Nothing I saw that thread discussed using transverse reinforcement as confinement, or controlling the amount of concrete splitting which is the failure mechanism of hooks, and increasing the joint strength.

I would call the corner bar a good idea and would allow for a shallower footing, though difficult to quantify, and not worth the fee I'm being paid for a basement stair well design.

Do you think corner bars are worth exploring to save on footing depth? This would increase efficiency and safety for countless retaining walls. Can anything in code on stirrup development be used to quantify the increase in joint strength?

 

[KootK]

Nothing I saw that thread discussed using transverse reinforcement as confinement, or controlling the amount of concrete splitting which is the failure mechanism of hooks, and increasing the joint strength.

The point that I was really trying to make was that, since this is not strictly a development length problem, I see little benefit in trying to massage the development length calculation in order to reduce footing thickness. If you really want to reduce footing thickness, throw out the development length thing entirely and use another method to justify the performance of the joint.

In the context of retaining walls, I feel that the attention that is paid to development length is really just a rule of thumb that tends to generate reasonable joint proportions (and it does). So, in shaving down the faux development length, you are really just shaving down the reasonable proportions. I do not feel that justifying a reduced development length actually creates an adequate joint in any direct manner.

All that said, I do see the logic behind what you're trying to do. And I feel that whatever anchorage improvement accrues to stirrups would probably also apply here so long as the limitations outlined for the stirrups are adhered to. I just feel that the focus on the anchorage is an attempt to solve a problem that isn't really the problem.

 

[SWComposites]

This would increase efficiency and safety for countless retaining walls. > reducing footing depth has NO effect on "safety" (whatever that was supposed to mean). And trying to save a small amount of concrete will have a very small effect on overall project cost.

 

[thejonster]

KootK, I haven't used Strut and Tie Method as exhaustively as the thread delves into, is there any confinement factor for struts and nodes that someone researching this could benefit from? If not, it seems like a shortcoming of the 2d STM system.

SWC, It's not a small amount of concrete, I have to double the concrete depth to develop the vertical reinforcement, or as KootK states, get the tension around the corner designing the joint. Concrete has a lot of strength in shear and bending so the footing section could be as shallow as the wall is thick otherwise. Miles of retaining walls in transportation wouldn't see that as a small effect, that's a lot less soil to move as well.

Why not replace 8" of concrete with 1 reinforcement bar if that can control the rebar bend failure mechanism?
Aerospace design looks for those kind of savings too, right? that snark though.. confinement increases safety, reducing the footing depth can increase efficiency, they're pals and work together, like US

 

[KootK]

KootK, I haven't used Strut and Tie Method as exhaustively as the thread delves into, is there any confinement factor for struts and nodes that someone researching this could benefit from?

1) STM isn't the only design method proposed in that thread. Far from it. STM is just a useful way to discuss the problem. There are efficiency tables based on testing and that is probably what I would be inclined to use for something like this.

2) The longitudinal bar used for stirrups isn't there for confinement. Confinement is the improvement of concrete compression stress resistance by way of transverse restraint. With the stirrups, the longitudinal bar is there to restrain splitting in the plane of the bar being anchored. It's there to improve concrete tension.

 

[KootK]

It's a "transmit the bar tension around the corner" problem...

Getting deeper into the weeds, it's actually a hybrid as I've shown below.

It's "around the corner" with respect to the toe and partial depth bar anchorage with respect to the heel.

 

[thejonster]

I appreciate the clarification, and the test results are compelling. I'm glad to see U70 performs as designed, as I've seen that used most commonly.

Have you used a corner bar like we're discussing to improve the efficiency of a joint? (or has anyone else?)

 

[KootK]

Have you used a corner bar like we're discussing to improve the efficiency of a joint? (or has anyone else?)

I will virtually always have the bar as I consider a matter of good detailing practice. That said, I do not feel that it significantly improves performance so I've never deliberately tried to use if for that purpose.

What size are your vertical wall bars? The stirrup thing is only good for 15M and smaller.

 

[thejonster]

I'm using #6 @ 8" for an 11 ft wall (19mm), good to know.

I won't be sharpening the pencil on this project, 16" deep is fine for a 10 ft wall. But wanted to poke the bear and see if anyone else found a better way to detail that joint.

 

[cliff234]

thejonster - If you want to reduce "ld", then use smaller bars.

 

[rapt]

The main benefit of a corner bar is to reduce the compression stress concrete in the curve of the main bar, thus allowing tighter bend radius.

This is useful for stirrups and other applications where bend radii are a problem in detailing such as corbels.

Eurocode and the FIB Model Code have rules that allow the determination of bend radius for different situations if you want to look into it.

Agree with Koot that there is really no need to consider it in your application, though you should always have a transverse bar in the curve as good practice.

 

[KootK]

But wanted to poke the bear and see if anyone else found a better way to detail that joint.

Sort of. That, fully acknowledging that, whenever I think that I've found a loophole, it almost always turns out that I haven't and that there is a "reason everybody does it the way the do". I'm afraid that all of the other engineers are / were pretty sharp cookies and they've not left many valid openings for better mouse traps.

The first thing that I'm usually trying to get rid of is the key. If you're stuck with the key already, however, it does afford an opportunity as I've shown below. This will make for a pretty good joint. You might get a modest reduction in footing depth which will be at least partially offset by the need for a bit more rebar.