Capacity of rebar developement with less than a standard hook 1

[Prestressed Guy]

I have a situation where I need to use a hooked rebar but do not have sufficient room for a standard hook. Is there a way to calc the capacity with less than Lhk? I am using #3 bar but can only fit 4" hook.

 

[JAE]

Here's some options on hooks:
1. You can always rotate the bar along its main axis to orient the hook at something other that vertical or horizontal - sometimes that allows a hook to fit.
2. You can also use 180 deg. hooks
3. There was a discussion on another thread here recently where others suggested mechanical heads on ends of rebar to create anchorage. I haven't done that before but might be a possibility.



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[Prestressed Guy]

Thanks for the response JAE.

In my case I do not need the full capacity of the #3 bars and the hooks have already been improperly installed with the short hook. I am trying to find out if I need the needed strength. (The contractor sent me a photo which showed the short hooks.)

 

[SlideRuleEra]

Haydenwse - Since you can't follow JAE's suggestions, I would interpolate:

CRSI Standard 90^o Hook for #3 is 6" long. The existing rebar hook is 4" long.

Therefore assume approximate hook capacity = 4"/6" or 67% of standard hook capacity. That is probably a conservative value.

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[DaveAtkins]

I agree with SlideRuleEra--ACI allows development length to be reduced based on the required force in the bar, unless full development is needed.

DaveAtkins

 

[MrHershey]

I've also used appendix D J-bolt provisions. Usually for justifying less than 6" embedment in minor situations rather than a non-standard hook with a shorter than standard tail, but may be able to look at it for non-standard hooks as well. Especially since your load is pretty small.

Typically only do it when I'm adding a 4" housekeeping pad or similar to an existing slab in a military building where they want to see a calculation for absolutely everything.

 

[Prestressed Guy]

Thanks all for the comments.

This is a Calif. jail so it needs justification for everything.

 

[KootK]

What's the situation in terms of the detail that's being considered? I'd be more inclined to demonstrate that you never needed a hook in the first place. Hooked bars depend disproportionately on the hook itself so prorating is difficult to do with much confidence. The hook resistance to straightening out and becoming utterly useless is very much a function of the tail length. You want as much lever arm holding that tail in as possible. The J-bolt business does strike me as clever and, one would expect, conservative.

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.

 

[Deker]

The length of a #3 stirrup hook is 4"...does the bend diameter they provided comply with a stirrup hook? If you can anchor the hooks around a longitudinal bar you may be able to justify it that way.

 

[Prestressed Guy]

The connection is for out-of-plane anchorage of a CMU wall to a hollow core floor. The hook goes into an edge of plank blockout. I have anchors at 4' with 10' trib of fully grouted 8" CMU at about 70psf mass so total mass to dowel is 2800lbs. Fp = 0.308 so 862 per dowel.

Minimum structural integrity requires 1500 plf but I will get additional tie in at the topping slab for that. This connection is primarily for erection stability.

 

[KootK]

No getting around that then. Technically speaking, this an anchorage problem and not a development problem. The J-Bolt approach is actually more appropriate than the original design. Let that be your justification.

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.

 

[atrizzy]

To answer the original question, and in addition to MrHershey's reasonable procedure, I would also consider treating the bar as a straight bar, adding the length of the hook to the primary embedment length, and conservatively reduce the capacity by the ratio of your embedded length to the required straight bar embedment.

 

[cliff234]

A standard 90 degree stirrup hook is 4". If you install a continuous bar inside the re-entrant corner of the hook, you will get the full tension capacity of the hooked bar (per ACI 318).

 

[slickdeals]

@cliff - ACI is clear about that for corbels where they say you're developed if you weld a bar at the end to anchor it. This is in ACI 318-05 11.9.6.
Where is the language regarding the development of a short embedded hook if you provide a longitudinal bar on the inside corner?

I know that you can get away with a stirrup hook compared to a standard hook if you have a bar in the inside corner. ACI 318-05 12.13.2.2.