Retaining Wall - Flexural Reinforcement from Stem Into Footing 21

[CWEngineer]

I am trying to get some clarification regarding the flexural reinforcement of the stem of a retaining wall into the footing.

Does the flexural reinforcement in the stem of a wall, need to be developed into the toe, such as show in Figure 1 of the attached document. Or is providing a standard hook (12db), sufficient, such as that show on Figure 2 of the attached document? If providing a standard hook is sufficient, can the hook be turned towards the heel?

Thanks in advance

 

[JedClampett]

I vote for a standard hook. Make sure you have top and bottom reinforcing in the slab.

 

[hokie66]

The bar needs to be developed. A standard hook may or may not do that, depending on the footing depth, bar size, etc. As per Jed, you need top bars in the heel of the footing.

 

[KootK]

Neither a hook nor development is necessarily sufficient here in my opinion. At the connection of the stem to the footing, you're creating a moment connection, not just developing bars. Conventionally, the wall bars are made to lap with the toe bottom steel to transfer whatever tension needs to wind up in the toe bottom bars. Often there is no lap as the stem bars are simply bent into the toe bottom bars. There are other ways to make the connection work but, to my knowledge, none is as simple as just developing the stem bars.

You definitely do not want to turn the "hook" towards the heel. It should extend into the toe so that the bursting stresses emanating from the bend will be restrained by a concrete strut.

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.

 

[Teguci]

The vertical bars need to be vertically developed on both sides of the maximum moment plane (top of the footing). With a standard hook, you need to provide a footing depth of Ldh plus your cover (12 db is not Ldh). I highly recommend reviewing the CRSI design manual for this type of reinforcing detail.

 

[DaveAtkins]

I disagree with KootK, and agree with Teguci. Sketch 1 is no better than sketch 1. Extending the stem bar, after it bends into the toe, beyond the hook extension required by Code does NOTHING to increase the development of the stem bar. The tension in the stem bar cannot go around the corner into the toe, and transfer into the toe bars by lapping.

DaveAtkins

 

[KootK]

I highly recommend reviewing the CRSI design manual for this type of reinforcing detail.

So do I. In virtually all of their details, they show the "hook" extending to the toe of the footing as I recommend (see below). If desired, one could lap the horizontal leg of the hook with separate bottom bars.

Extending the stem bar, after it bends into the toe, beyond the hook extension required by Code does NOTHING to increase the development of the stem bar.

This is a misconception that pervades north american practice. In Britain, for example, it's common to develop bars around corners with the extensions much further than the 12db. One simply needs to check that bearing stresses inside the hook bend are not exceeded. This will likely pan out to be the way that we handle this in north american practice: Link

The tension in the stem bar cannot go around the corner into the toe, and transfer into the toe bars by lapping.

I disagree and contend that this is exactly what needs to happen.

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.

 

[Teguci]

Huh, I thought CRSI clarified this.

There is nothing wrong with extending the hook (I usually detailed a 6" toe and saw no point to adding another mat of rebar). However, if the hooked bar is not developed a distance of Ldh into the footing, then it is not developed per chapter 12 of ACI 318. I don't know what the British standards are for hooks and radii. It makes sense that larger radii will decrease the splitting stress on the inside of the hook but the OP specifically mentions 12 db and "standard" hooks (ie not non-standard hooks).

As an example, if we were detailing with fy = 60 ksi and f'c = 4 ksi, #6 bars would need more than a 12" deep footing to be developed in.

 

[KootK]

but the OP specifically mentions 12 db and "standard" hooks (ie not non-standard hooks).

That really speaks to my point here. This isn't a development problem in my opinion. As such "standard hooks" really have little bearing. The stem dowel that has an elbow in it is not primarily developed rebar. Rather it is rebar transferring moment around a high demand "closing/opening" joint. There's a significant difference there.

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.

 

[KootK]

This kind of stuff:

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]

KootK's points are well made, and I agree. It is not just a development issue, it is a moment joint.

 

[mike20793]

This may be a silly question, but does monolothic pour versus separate pours have anything to do with the crack location shown in the figures above?

 

[KootK]

This may be a silly question, but does monolothic pour versus separate pours have anything to do with the crack location shown in the figures above?

.

Not to my knowledge. Obviously, if there are construction joints, then those joints might deserve direct shear attention of some kind (shear friction, dowels etc).

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]

mike20793,
Yes, there are differences. Constructions joints are by definition "cracks". Monolithic construction in the same configuration may or may not crack. Reinforcement is to control cracking, not eliminate it.

 

[Teguci]

In practical terms, rebar doesn't transfer moment, just tension. For the stem, I think we can agree that, to be useful, the reinforcement needs to be developed for its full tensile capacity (a nice yielding failure should the wall ever be overstressed). Therefore, to resist, we need the bar to be fully developed into the footing. ACI 318 tells us that we can get full development with development length, a standard 90 or 180 degree hook or headed/mechanical devices. In the commentary, ACI notes that hooks fail by splitting the concrete on the inside of the bend, followed by slipping of the bar. If we lengthen the tail of the standard hook, does that preclude this type of failure? Not according to the forces on a hooked bar at failure which shows shear transfer on the bar stopping about 4 or 5 bar diameters from the hook with the rest of the tail acting in bearing. We might get a nominal amount of additional capacity, but nothing allowed by code.

As I see it, for a shallow embedment length, the compressive capacity of the concrete will be exceeded before the yielding of the bar. The concrete will split in the plane of the hook, the bar will slip and the wall will fail. The split will be in line with the compressive strut in this "closing" joint as is expected under a strut and tie model. Because the bar has a full development length into the footing, at some point the radius will elongate and halt the compressive failure of the concrete.

For walls taller than 1-story, I usually showed an additional bar on the inside of the hook. Tests showed that this additional bar adds about 10% to hook capacity. Footing concrete is cheap and, for moment capacity, directly trades off with rebar, don't skimp.

 

[mike20793]

Not to my knowledge. Obviously, if there are construction joints, then those joints might deserve direct shear attention of some kind (shear friction, dowels etc).

Thanks. This is what I thought, just didn't know how a construction joint would affect the propagation of the diagonal crack.

 

[DaveAtkins]

It is not just a development issue, it is a moment joint.

I am not sure what you mean, hokie66. As Teguci stated, per ACI, you cannot transfer tension around a corner in a moment joint. The bars must be separately developed in each member at the moment joint.

DaveAtkins

 

[KootK]

I feel as though I've done a pretty good job of demonstrating that you can transfer tension around a rebar corner. In countries outside North America, codes direct designers to do this by checking bend stresses and adjusting radii accordingly. In North America, this can be done using the curved bar STM model.

the bars must be separately developed in each member at the moment joint

This exactly what you don't want to do. This case would be represented by the left hand sketch in figure 13-27 above (60% calculated capacity).

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]

Well, I am not sure what Teguci means, so I suppose that makes us even. The Figure 14-1 which KootK provided above shows how I think retaining walls should be reinforced.

 

[KootK]

The fact that we use the hook extension as the toe flexural reinforcement means that we're pulling on both the horizontal and vertical legs of the bent bar. How would that be statically possible if rebar tension could not be transferred around the bend?

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.