Hooked Reinforcing Bars

1. Introduction

When I think of `hooked bars' I think of two things: 1) hooked rebar dowels that join a foundation wall to a footing, and 2) hooked bars necessary to develop serious bond, tension, say, at the base of the stem of a retaining wall. The idea behind both, really, is the relatively shallow distance the bar has to embed into the footing. There are other uses of hooked bars, but the two above are what we will concentrate on here.

2. Foundation Wall to Footing

The context of the first situation is Residential construction, though it is not necessarily limited to such. And whether the wall is tall, or short, this part of the discussion is limited to a wall where the connection to the footing will be a `pin' connection (not rigid), and if the wall acts as a retaining wall (resisting heavy or light lateral forces) ... it acts as a one-way vertical slab/beam. Whether we consider the wall a slab or a (really wide simple) beam, we are required to extend the reinforcement into the support at least 6 in. See ACI 318 12.11.1 for beams, and ACI 318 Fig. 13.3.8 for slabs. Neither of these here require that the rebar be `hooked'. They merely require `positive connection' and are not assumed to carry significantly huge tension loads. Since the minimum embedment into, in this case, the footing, is 6 in., and since footings are generally cast on soil (or other bearing material), in which case a minimum concrete cover of 3 in. is required, I generally detail at least 9 in. footing thickness. In some residential applications I may detail a footing thickness of 8 in., but I do so recognizing something has to `give', and thus specify 5 in. embedment. I only do this when I know the reinforcement will not be carrying significant loads (of any kind). And I do it recognizing that I am probably `breaking' the Code, and that I might get challenged by the Building Official. And while I have emphasized that `we always obey the Code' - this is one place where I sometimes try to sneak around. But, other times I will stand to principle and demand the 9 in. thickness, which isn't all that bad, as it it easily obtained with 10 in. nominal forms.

But, having ventured into the realm of trying to `get around' code issues ... it is the trend around here for building departments to require that these dowels be hooked. Even for relatively short stem walls. And, so, when I feel brave, I detail straight bars. But, if the Building Official requires they be hooked, he/she has the last word, and I flow with it. In any of these cases where I try to `sneak around' the code, I am only doing it is applications that are NOT over my head, and that, if something goes wrong, a building won't come crashing down.

Note that these dowels are cast-in-place during the construction of the footing. And then formwork for the wall is placed around them during the later construction of the wall. These bars thus extend some minimum splice length up into the wall, or, though more cumbersome to handle, may indeed be the vertical wall bars themselves. See the lesson on reinforcement details for a one-way basement retaining wall, here.

If I end up detailing a hooked bar, I will detail the minimum length standard hook in accordance with ACI 318 12.5.

Note: in these applications where the dowel is not necessarily being depended on for tension, it is often depended upon for `shear' (resisting the bottom of the wall pushing in from lateral soil force).

3. Standard Hooks in Tension

Now let's talk about the other kind of hook ... a hook that is necessary to develop serious tension. And let's consider a cantilever retaining wall. A cantilever retaining wall is way different than the one- and two-way walls we have been considering. In these one- and two-way walls the bending moments in the walls were assumed to vanish at the wall bottoms ... the so called `pin' connection. At the bottom of the cantilever wall the bending moment is the greatest and thus the demand for development of tension bond concrete-to-steel. If the cantilever wall has a `key' in the footing, the key can accommodate the necessary straight bar development length. A key might be necessary, for example, to keep the whole wall from sliding. If a key is required for this or some other purposes, I will also use it to develop the bars. I generally do not detail a key just for bar development, though; they require additional excavation and concrete. So, let's say we don't have a key, but still need to develop vertical reinforcement in the stem of a cantilever wall. We'll use standard hooks. I'll present the Code equations, and then an example.

4. Equations, etc.

ACI 318 12.5 gives us the `stuff' for hooked bars. Simplified information is in your Ambrose Text, Pages 428 - 429.

... l _{d h} = (0.02 β γ f_y /√f '_c) d_b ...

where,

... l _{d h} = development length required for the hooked bar ... in our example starting from the `joint' at the bottom of the wall ... see Figure 13.22 in the Ambrose text or Commentary Fig. R12.5 in ACI 318.

... β is our epoxy-coating factor, 1.2 for epoxy-coated bars, otherwise 1.0,

... γ is the lightweight concrete factor, 1.3 for lightweight concrete, otherwise 1.0,

... f_y and f '_c ...are our specified steel and concrete strengths, and

... d_b is the bar diameter.

Additional adjustments ...

From the Code (12.5.3) ...

(a) For No. 11 bar and smaller hooks with side cover (normal to plane of hook) not less than 2-1/2 in., and for 90 deg hook with cover on bar extension beyond hook not less than 2 in ... 0.7.

... and ...

(d) Where anchorage or development for f_y is not specifically required, ... (As required/ As provided).

For a lot of applications we'll be able to use (a) above, and around here, also (d). The (d) thing deals with seismic requirements where, even if there is excess reinforcement, it needs to be fully developed anyway.

NOTE: in no case may the development length be less than 8d_b, nor less than 6 in.

5. Example

Consider a cantilever retaining wall with # 6 Gr. 60 vertical bars. A convenient selection of bar size and spacing has resulted in the need to develop 88% of the bar capacity. The footing does not have a key so the space for straight bars is limited. Let's investigate hooked bars. Use f'c = 3000 psi.

First let's put 60,000 psi and 3000 psi into our equation ... we run into these numbers often so it's worth doing our calculations in stages and coming up with some potentially more useful (or streamlined) equations. And, our situation does not involve epoxy-coated or lightweight concrete.

... l _{d h} = [ 0.02 (1.0)(1.0) 60,000 psi /√3000 psi) d_b = 21.9 d_b ... (no epoxy coating, normal weight).

And, if we satisfy (a) ...

... l _{d h} = (0.7) 21.9 d_b = ...

... l _{d h} = 15.3 d_b ... (90 deg hook, with adequate cover, etc.).

For # 6 bar ...

... l _{d h} = 15.3 (6/8) = 11.5 in.

In this example we had excess reinforcement, so we could multiply the 11.5 by 0.88 to get ...

... l _{d h} (adjusted) = 11.5 (0.88) = 10.1 in. ... round up to 11 in.

So, we could detail the # 6 bars embedding 11 in. into the footing, and with 3 in. of cover beyond, the min. footing thickness would be 14 in. Yeah, BABY!

If we wanted to cut the embedment length down a bit, we could ...

1. try a smaller diameter bar
   
2. provide more excess reinforcement
   

Note that the 11.5 in. is pretty close to the 11.9 in. in the example in the Text. The difference is that the Table value of 17 is way rounded up, and then the author multiplies it by the 0.7. Note, also, that the author's example does not include the 0.88 thin.

6. Conclusion

In this lesson I have exposed a couple areas in the `code' that I sometimes try to get around. Let me stress that I only try and cheat the code a bit in applications where the forces involved are very minimal and all the stuff is below us, probably never to be seen (foundation stuff). Where reinforced concrete is carrying significant loads, or where it is spanning overhead, or retaining significant backfill, I never try and cheat the code. The code is there for good reason. And if I am ever challenged by a Building Official, on the `below ground' stuff, I don't argue, I just go ahead and demand the hooks, and another inch or so of footing, as needed.

7. References

Design of a One-Way Reinforced Concrete Basement Retaining Wall, Jeff Filler, Associated Content.

Basement Retaining Wall Details Continued, Jeff Filler, Associated Content.

Development Length or Bond, Jeff Filler, Associated Content.

Building Code Requirements for Structural Concrete, ACI 318, American Concrete Institute, P.O. Box 9094, Farmington hills, Michigan, 48333.

Simplified Engineering for Architects and Builders, Ambrose, J. and P. Tripeny, 10^th edition, John Wiley & Sons, Hoboken, New Jersey.