What is Required for a Hook to be Considered as Engaging a Longitudinal Bar

[wijgeng]

thread507-353992

Hi, as discussed in the referenced thread above, ACI 318-11 specifies that circular ties in a compression member shall over lap by at least 6" and terminate in a standard hook that engages a longitudinal bar. My question is, what is considered "engaging" a longitudinal bar.

If there is a longitudinal bar inside of the hooks, does that one bar engage both hooks (i.e. There would be a 3" gap between the hooks and the longitudinal bar)?

What distance is acceptable between the longitudinal bar and the hook to be considered engaged? My thought would be that the longitudinal bar would need to he snug and secured in the corner of the hook. The reason being that there will be a stress concentration on the inner radius of the hook. Because steel has a much much higher strength than concrete (i.e. 60 ksi vs 4 ksi) the concrete would crush before the longitudinal bar can be engaged. Is this correct? Is this practical for construction or is it just ideal?

Thank you!

 

[KootK]

Great question. I've always wondered the same thing about beam stirrups. I don't know the answer definitively but I have a theory. Tie hooks, like standard hooks are meant to transfer their tension to the surrounding concrete, not to other bars. So concrete crushing inside the hooks ought not be a problem.

Hooks in tension have a tendency to want to spall out the concrete behind them however. I think that the main purpose in engaging longitudinal steel is to restrain this failure mode. To that end, the longitudinal bars should be near the hook knuckle but not necessary in contact with it which would be a constructibility issues. How close is close enough? I'm not sure. Hopefully someone here can point us to a standard.

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.

 

[cliff234]

That is a good question - and one that I never considered. My interpretation was that as long as the hook wraps around the longitudinal bar you were ok. But if you knew that there would be perhaps 3" of concrete between the longitudinal bar and the inside of the hook, then that would be a different story. I think the intent of the requirement is that the hook wrap around the longitudinal bar and there should not be more than an incidental amount of concrete between the two.

 

[hokie66]

I have always interpreted "engaging a longitudinal bar" to mean that the bar contacts or is in very close proximity to the hook. I think 3" is too far, and you would need two bars if the hooks are only 6" apart. In other words, I think KootK is wrong about this one.

 

[KootK]

In other words, I think KootK is wrong about this one.

It happens. I'm not clear on where we disagree however. You think that the longitudinal bars should be close to the hooks; I think that the longitudinal bars should be close to the hooks. You think that the longitudinal bars need not be in direct contact with the hooks; I think that the longitudinal bars need not be in direct contact with the hooks.



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]

I think they need to be tied together. That doesn't insure intimate contact, but that they are in "very close proximity".

 

[KootK]

No argument 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.

 

[wijgeng]

Thanks for your answers. I was under the impression that when the code indicates that a standard hook should wrap around or engage a longitudinal bar, that it should be mechanically engaged such that if the hooked bar began to pull out, the mechanical anchorage would guarantee anchorage. The two sections of the code I can think of that specify this correspond to circular ties of a compression member or web reinforcement in a beam. In both cases, development length is not necessarily required (i.e., they specify 6" and the engaged hook for circular ties in a compression member and only an engaged hook for web reinforcement). Thus, I believe ACI is expecting that the anchorage being provided mechanically via the hooked longitudinal bar rather than by distributing the stresses to the surrounding concrete.

I've attached a photo of a splice in one of the piers they are installing. You can see in the photo that they have two longitudinal bars being confined by the hooks, but the top hook is certainly not engaging the longitudinal bar (in my opinion).

Thanks again for your responses and I look forward to hearing any more insight you all may have!

 

[hokie66]

A very sloppy job of tying the cage. The longitudinals need to be tied securely inside the hook. Not much of it looks like it is tied securely anywhere, and there are loose ends of tie wire everywhere outside the cage. Lazy workers, unsupervised.

 

[wijgeng]

@hokie66

I agree. I took this photo in the field to document the quality of the work being performed. I didn't want the conversation to get distracted by the poor construction quality captured in the photo, but I do appreciate the validation

I simply wanted to upload this photo to show where they are placing the longitudinal bars in relation to the hooks. This photo might eliminate some of the ambiguity of the debate.

 

[racookpe1978]

How big is the gravel/aggregate w/r to the hook "gap"?

And, with that in mind, how well is the concrete mixture (gravel/aggregate and "mud" compacted to fill the holes and gaps caused by the rebar intersection gaps.

See, if the aggregate and "mud" is going to be small enough to get into spaces inside the hook, then there will be a positive "catch" of the hook onto the crossing rebar, right?

On the other hand, if there will be gaps and holes caused by large aggregate blocking the flow of the cement mixture, and the entire structure is not rammed in very well, then the hooks are actually weakening - not strengthening - the total assembly when it dries.

If the "hook" is so far away from the crossing rebar that there is no interaction between the two until catastrophic failure (movement of 2-3-4 inches of the hook before it is stopped) then your structure has gained nothing from the time and effort spent installing the hook

 

[hokie66]

racookpe,
The size of aggregate in normal concrete should not affect the ability to consolidate the concrete properly. We like larger size aggregates in larger structural elements, because the strength of the concrete is normally enhanced.

 

[cliff234]

In the past circular ties were lap spliced. I wonder what prompted the change to requiring hooks? The hooks certainly complicate things, because the tie detail changes depending on how many vertical bars are in the column - because the hook locations are dependent on locations of the vertical bars. Each column bar pattern would require a unique tie detail.

 

[hokie66]

cliff,
It was because of poor experience with the lapped ties. Splitting at inadequate cover, etc. See the discussion in the thread which the OP linked aboved.