Including Mechanical Splice (Coupler) in Development Length

[bumbler]

Hi all,

I am trying to come up with a solution for a misplaced pile and I'm struggling to find information on whether the development length of rebar includes or excludes the length of the coupler for a mechanical splice. Our foundation system is augercast piles embedded into grade beams. The piles were placed 2' higher than intended and the contractor has suggested cutting the headed reinforcement for the piles ~2' and using couplers to splice the headed bars. The bars are #7s so their development length plus the depth of the head is ~15". The length of the coupler they spec'd is 13" long. I think that this would change the development length of the bar, but I can't find any information on whether this is true or not. I checked ACI 439.3R-07 and ACI 318-14. See the attached sketch for reference.

Thanks!

 

[KootK]

1) I'm sure that the coupler would help develop the bars somewhat but I've no idea how to justify that rigorously. I'm imagining the kind with set screws.

2) Per #1, I would say that the coupler does not contribute to the required development length.

3) In some ways, I feel that the coupler might even compromise the portion of your bar development that is associated with bearing against the bar head.

 

[bumbler]

1) That's a correct assumption, the proposed coupler is Zap Screwlok by Barsplice.
2) Understood, that's what I was leaning towards.
3) I agree, and it makes me think of the overall contribution of the bearing against the bar head to the development length versus the contribution of the straight bar. Just doing a quick check for f'c = 4,000 psi, Ld = 41.5" for a straight bar and Ldt = 13.3" for a headed bar. Do you know if a breakdown of the contribution of the bar head bearing and the straight portion of the headed bar to development length is written down anywhere? It seems like there hasn't been much research on headed bar development considering the changes in ACI 318-19.

Thanks for the responses so far!

 

[TLHS]

I feel like if you have enough clear space between the head and the coupler so that the breakout cone can 'escape' into the surrounding concrete, the coupler likely wouldn't significantly affect the capacity of a headed bar much. If there's a lower energy breakout mechanism where the concrete fractures in a cylinder between the head and the coupler and then there's a pullout failure of the bar, then that'd be the problem. I feel like you probably don't need a lot of space to preclude that mechanism, though. Something equivalent to a couple times the diameter for the head or the coupler, whichever is bigger, seems like it could get the job done.

If you're worried about how bond between the bar and the concrete might affect the expected development length of a headed bar, you could maybe do an anchorage style check for breakout based on the head. I haven't looked at testing, but I bet most of the head sizing on the bars was originally based on developing everything in the head.

 

[TLHS]

If you're worried about it, you can use a steel embed to make sure you're nice and developed.

 

[KootK]

Do you know if a breakdown of the contribution of the bar head bearing and the straight portion of the headed bar to development length is written down anywhere?

I don't know where/if that exists in writing although I know that it's been a fairly common thing to do in the past. It is somewhat complicated by the fact that the test limit for these things is slip rather than raw capacity.