hooks and tails

[kfc510]

While I'm having lunch and waiting to hear from my engineer about this I thought I'd open it up for discussion with you guys. I may get a short, specific answer from my guy, but I'm now interested in the general question of rebar hooks and tails.

What do you know about specs for rebar hooks and tails? Have you ever modified a standard hook? Was there a calculated load reduction?

In my situation I have drawings for a set of vertical #4 bars from a footing up into a stem wall, with 135 degree hooks at the top over the top horizontal bar in the stem wall. The stirrups and ells were bent up based on the drawings, and have been delivered.

The issue at the project is that with standard length hooks and tails at a standard 135 degree angle, the end of the tail lacks adequate concrete coverage from the edge of the stem wall.

I could cut the tail shorter than is generally spec'ed, and I do not know what the load reduction would be. (I don't think a shortened tail would be the thing that fails first in this structure, but hey, I'm not an engineer).

Or I could bend the tails all the way around to be 180 degree end hooks. There would be enough coverage even if the bars were bent to normal 180 deg. end hook specs. (This was the obvious on-site solution- however I tried doing this to a couple hooks, and my bender won't create the normally spec'd diameter for #4 bar, which is apparently 3". My bender puts about a 1 1/2" diam. bend on the rebar, which is a much sharper turn than one should generally make with #4 bar. But, as with the shortened tails, I do not think that "weakened" 180 deg. bend would be what fails...)

Or I could call the steel company and have them send out another set of bars with the 180 degree end hook and the proper diameter. This could well cost less in the end than trying to figure out how to modify the already delivered pieces, but seems a waste.

In any case, I wish I knew more about specific end hook details and loads & load reductions.

Here's a representation of the basic end hook requirements as I understand them. (This one happens to be from Maryland, I chose it simply because it is clear.)

http://apps.roads.maryland.gov/busin...F/REB320_2.pdf

kevin

[Dick Hackbarth]

Kfc:
While the sketch that you show refers to hooks and bends in stirrups, your bars are not stirrups, they are tension bars, to counteract tension or cracking in the stem wall. But, the general hook or end bending geometry logic does not change significantly. In my old age, I am having trouble keeping up with the many, many changes in the latest different codes. They are just happening faster and at a cost that I can’t keep up with. However, I think the following is generally a reasonable engineering explanation of some of what you are wondering about. Also, your sketch does not print very well for me, so I am having some trouble reading it, but I have seen these general sketches in various other codes before. And, if I weren’t so lazy, I could find these same bending requirements in an older version of an ACI bending specs. or requirements.

The hook extension length is really a function of a min. length that the fabricator can garb with his bending equipment vs. bending and gripping forces required to make the bend, and the forces involved. And, it must fit within the concrete wall or beam. That extension length doesn’t add much to the strength of the hook, as relates to developing the rebar. Your rebar fab’er. should have seen that a 135̊ bend would not give you the proper bar cover in the stem wall thickness you are dealing with, or you put the bar in bass-ackwards, and I would go to him for a fix or correction. This might be as simple as wacking the bar with a sledge hammer, or a hydraulic press, and bending it around a proper dia. piece if pipe, and ending up with a 150̊ bend, and the proper cover. Too tight a bend can cause the rebar to crack in the bending process, and we don’t want that, thus the limits on min. bending dia. That you fit a horiz. rebar immediately under the hook/bend provides a load distributing bar to make your vert. bars act properly in tension.

In recent years, we have started to reevaluate how rebar bends work in concrete, as they try to distribute their loads or develop the tension in the rebar. There is concrete crushing, in bearing, immediately under the bent bar, so the tensioned bar can extend, rather than really being developed (restrained) in tension. We are learning that the bends in rebar are not as effective in developing the rebar as we once thought they were. And, from the structural standpoint, this is a problem in developing the rebar. This should not normally be a problem in a simple stem wall. I would be more concerned with getting good rebar cover. Show us a copy of the cross section through your wall. This would really help in understanding what you are dealing with.

[kfc510]

Good response, Dick, thanks.

My engineer did get back to me, and ok'd either lopping the tails short or bending them over to 180 degrees on the tighter radius I ended up with from my bender.

I'm not a metallurgist, but I didn't see a whole lot of flaking/stress fissure stuff happening to the #4 bar with an internal 1 1/2" diam. bend. I'm curious how much strength is lost with increasingly tighter bends of the metal.

In any case, the engineer is not looking for any verifiable load from the hooks, so I can modify them however I wish.

Like you, (and me) he was more concerned with coverage than the hook shape/length. But he didn't really have time to get into the rest of the theory behind it all, so I appreciate hearing about the concrete deformation/crushing, etc.

I can't blame the steel co., they did just fine. I placed the order over the phone- x many 10x22 stirrups w/hooks, y many 7x21 ells with end hooks. No, lol, I didn't put them in backwards. It's a small job anyway, +/- 40 ells with hooks, I could just buy new ones if it was an issue.

The tap-dancing really comes down to the irregular evolution of this job. It started out as a straight foundation job (no engineer required) then sequentially morphed into a larger structural job, which requires a very poorly defined amount of engineering. So the details are a mix of elements remaining from the original permit drawings and the things the engineer demands, and (as always) there are little discrepancies. We are kind of in permit limbo right now too, I'm meeting the engineer at the building department on Tuesday to get the revisions (hopefully) approved over-the-counter.

I'll attach the detail of where the transverse grade beam ties into the new footing. The hooks are shown at the tops of the alternating ells in the stem wall with shorter tails than normal, which is why the coverage question didn't arise before. One change that needs to be made in the grade beam is that the engineer wants the stirrups on a tighter spacing. There's also at least one other error that's been changed (the length/embedment spec of the ells into the grade beam)

[kfc510]

It's academic at this point, but here's a pic of the hooks.

To the right is the standard 135 degree hook as it came from the fabricator. The tail on it is longer than the standard minimum length from the ACI standards for #4 bar hooks (as I read the end hook/stirrup tie chart, for what that's worth), so I can cut it back an inch or two without going under the minimum, but in order to fit it in the stem wall I'd need to cut it back further than that. The tail might be 2" long (from the perpendicular radius) or something like that.

To the left is the 180 deg. hook end I did with my bender. By the ACI specs (FWIW) the diameter of the circle inside the bend should be min. 3" for #4 bar, you can see that my bender creates a bend of ~ 1 1/2". Total outside-to-outside measure on that hook is less than 3", it should be 4" for #4 bar by ACI specs. What that means in terms of reduced load, I have no idea...

Never really thought too much about hooks and tails before this question.