Splice length reduction using hooked bars 1

[Batky]

Hi,

I am wondering if anybody knows whether or not it is permitted to use hooks for a reduction in lap splice length.

ACI 318-14 Cl. 25.5.2. talks about lap splice lengths for deformed bars in tension, and calculates it using the development length, but the development length mentioned pertains only to straight deformed bars. I would like to have hooks on the ends of both rebar involved in the splice to reduce the splice length.

Any experience/references on this would be greatly appreciated.

Thanks!

 

[JAE]

I've never done that before and never seen it done before. Splices are lapped straight bars and using hooks for a "splice" isn't defined in ACI and pretty sure there's not been any research on it.

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[dik]

It used to be back in the 60s... If I recall, CSA and ACI allowed 19ksi reduction in the tensile forces for use of a hook.

Dik

 

[Batky]

@JAE That's too bad there isn't any research on it, do you know if there is any research into using headed terminators in the same manner?

@dik Do you know where I can find/buy past publications of ACI? Google isn't being too helpful.

 

[slickdeals]

Yes, there is research done on lapping rebar with headed terminators.

https://ctr.utexas.edu/wp-content/uploads/pubs/0_1855_3.pdf

 

[canwesteng]

You might want to consider mechanical splices as well if you're looking to reduce splice length... if the project has the $$$

 

[TXStructural]

A couple of thoughts:
First, hooks only work in tension (so no lateral or reversal of forces).
Other Code limits notwithstanding, the orientation of the hooks would be important to avoid doubling the splitting force in the concrete created by the hooks. Two hooks occupying the same concrete will increase the crushing and splitting forces in the concrete. (Ref to ACI 318-14 R25.4.3.3). Confinement and congestion will be a issues. The overlap in a typical case lap length vs typical case ldh would be reduced from 42 inches to 17 inches. 14 inches of additional bar would be required for the bend and the required extension on a 180 bend, on each side. The savings in physical bar length would be no more than about 4.9 lbs per lap for #8 (trivial when compared to the added labor to bend, bundle, transport, handle, and precisely place bent bars). Obviously the length of the lap is not much of an issue if you are lapping bars.

 

[dik]

Batky... this is a bit of a stretch... but, I think there was a 63 edition of the ACI. I think that was the code in effect when I did my thesis on Torsion.

Dik

 

[KootK]

- I've seen it done which is not to say that it should have been done. I've seen a number of delay strip details where this strategy was employed to minimize the width of the delay strip. 180 hooks with transverse bars in the bends.

- My understanding is that the 1.3 factor on splicing is mostly about the possibility of reduced bond development where the bars might be in physical contact with one another. Rationally, bars developed via hooking would have less of that problem and therefore, all other things being equal, I'd expect them to perform better.

- In my opinion, this kind of thing largely defies rational analysis and testing must be held as the gold standard.

If it was a zillion #4 bars at 12" o/c, I'd be willing to let it go. If it were 3-#8, not a chance.

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.

 

[Ingenuity]

This may be what dik was referring:

Part extract from ACI 318-63:

'Part IV-A' is working stress:

'Part IV-B' is ultimate strength:

You can purchase historical ACI codes from 1908 to 2005 on a USB thumb drive here: Link

 

[dik]

Thanks Ingenuity... good guess, since it goes back nearly 50 years... I thought it was the 63 code since that was the only NA code with reference to torsion... and it basically stated that it had to be addressed... no prescriptive formulae. Canadian code had nothing.

Dik

 

[Batky]

Thanks for your help everyone! I think I'll head down the path of using headed terminators, but all the ideas and explanations provided were really helpful! There just isn't enough research on using hooks in this manner to confidently design, in my opinion.

 

[wannabeSE]

Terminators are used in lieu of hooks to reduce congestion. If there isn't length for a lap splice, consider a welded or mechanical splice.

 

[Toby43]

Where is the splice occurring? Regarding horizontal bars in shearwalls in seismic applications, hooking the bars at splices into the core concrete was recommended after some laps were observed to be compromised due to spalling of cover concrete after Christchurch. The lap splice recommendations were that of hooked bars with the lap length being at least that of the development length of a hooked main bar (i.e. not that length determined for a stirrup or tie)

Toby

 

[Batky]

The splice is occurring between dowels sticking out from a base slab and the vertical reinforcement in the column being placed on top of the slab. Some error in placement of the dowels cause them to be a few inches to short and the contractor has proposed to hook both the end of the dowel and the vertical reinforcement.

I am going to just tell them to install terminators on either end to due to lack of research on hooks used in lap splices.

 

[KootK]

Are you sure that you even need full tension development on your column starter bars? Often you don't. I'd think that terminators would cause all kinds of cover/congestion issues in a column. If you really gotta have more:

1) Up your f'c to lower your lap length.
2) Mechanically couple some extensions.
3) Weld some extensions.


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.