What is Required for a Hook to be Considered as Engaging a Longitudinal Bar
What is Required for a Hook to be Considered as Engaging a Longitudinal Bar
(OP)
thread507-353992: #4 Tie
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!
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!






RE: What is Required for a Hook to be Considered as Engaging a Longitudinal Bar
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.
RE: What is Required for a Hook to be Considered as Engaging a Longitudinal Bar
RE: What is Required for a Hook to be Considered as Engaging a Longitudinal Bar
RE: What is Required for a Hook to be Considered as Engaging a Longitudinal Bar
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.
RE: What is Required for a Hook to be Considered as Engaging a Longitudinal Bar
RE: What is Required for a Hook to be Considered as Engaging a Longitudinal Bar
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.
RE: What is Required for a Hook to be Considered as Engaging a Longitudinal Bar
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!
RE: What is Required for a Hook to be Considered as Engaging a Longitudinal Bar
RE: What is Required for a Hook to be Considered as Engaging a Longitudinal Bar
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.
RE: What is Required for a Hook to be Considered as Engaging a Longitudinal Bar
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
RE: What is Required for a Hook to be Considered as Engaging a Longitudinal Bar
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.
RE: What is Required for a Hook to be Considered as Engaging a Longitudinal Bar
RE: What is Required for a Hook to be Considered as Engaging a Longitudinal Bar
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.