Top Bar Development Length in conc wall 4

[jechols]

Do you have to apply the reinforcement location factor of 1.3 (ACI 318-02) for horizontal bars in a concrete wall which are spaced less than 12 inches on center? I have applied this factor to all the horizontal reinforcing, however two different rebar detailers have stated this factor does not apply to horizontal bars in concrete walls spaced less than 12" o.c. Any opinions would be appreciated! I work in SE USA.

 

[JAE]

According to ACI 318-02, Chapter 12, the [α] factor on "top bars" does not mention anything about an exclusion for walls. There is nothing else in chapter 14 (walls) either about this. (this factor is referred to as [Ψ]_t in 318-05)

I would say you have to use the 1.3 factor.

 

[271828]

I would apply it, although I can't prove that it's necessary.

The idea is that bars with substantial elevations of concrete below might have water/air accumulated underneath. I don't see why this would happen for a beam, but not for a wall.

Then again, the Code specifically says "member."

The research paper dealt with beams only, IIRC, so maybe nobody knows the answer for walls.

 

[JAE]

While ACI 318-02 uses the phrase "member, it's been taken out of 318-05. Even still, isn't a wall a "member" in some sense?

The concept is that during placement of concrete, there is the potential for late plastic sag and, as 271828 states, air pockets forming under the bar and thus diminishing the bond between concrete and bar. Wall, beam, whatever, the concept still applies.

The only other aspect for a wall is that sometimes the horizontal reinforcing in a wall isn't all that important other than to meet a minimum value and you might be able to use A_s required / A_s provided.

 

[Galambos]

stuff like this kills me. make the detailer give YOU the code reference. as engineers, we usually play the nice guy and do the leg work.

let's take a stand!

alright, that's over the top.

 

[miecz]

If you are asking about splice lengths, then 12.15.1 does not allow a reduction for excess reinforcement, unless you qualify for a Class A splice.

Earlier references to this provision in ACI 318, textbooks, and Code Committee 408 always included the term "top bars." My college textbook (Wang & Salmon) says it was based on limited tests with concrete depths of 12 to 18 inches that indicated the splitting resistance was lowered by 10 to 20%. I the old days, we only included this tem for bars at the top of beams and thick slabs.

Sometime between 1977 and 1995, the term "top bars" was eliminated from the code. I never saw an explanation for the change. That is, I never read that new research done on walls indicated that the provision should be applied to horizontal wall bars.

I remember reading or hearing the argument that it does not apply to wall bars spaced less than 12 inches on center. The argument is that any air bubbles migrating up that get caught on one bar certainly can't get caught on the bar above, so each bar has less than 12 inches of concrete from which to trap air. Makes sense to me.

However, the code says what it ways, so I apply the 1.3 factor to all horizontal wall bars, regardless of the spacing.

If anyone has read any research that supoorts this provision in wall reinforcing, I would be interested in reading it.

 

[271828]

"If anyone has read any research that supoorts this provision in wall reinforcing, I would be interested in reading it."

The references are 12.4 and 12.5 in ACI 318-02.

12.4 is a research report, so not sure how to get it.

12.5 is in the ACI Structural Journal, 1988. I don't have them going back that far, but surely somebody here does.

 

[civilperson]

Horizontal reinforcing with more than 12" of concrete below, (no exceptions listed). Could apply to circular ties in columns.

 

[271828]

civilperson, you mean ones without hooks, I assume?

 

[miecz]

I obtained a copy of the research report (reference 12.4) from the author and finally got around to reading it. There are some startling revelations in this report, particularly regarding superplasticized concrete, and vertical reinforcing. And yes, a position factor is meant to apply to all horizontal bars with more than 12 inches of concrete cast below, regardless of bar specing. I highly recommend e-mailing the author and asking for a copy of this report. I may take o couple of months for the author to respond, but it's worth the wait.

 

[jike]

This has to do with the amount of concrete below the bar, NOT the bar spacing. Air bubbles form underneath the bar requiring more development length. Have you ever known a bubble in a glass of water to always rise perfectly vertical? Or could it rise on a 1:12 slope?

Tell the detailer that you regard them to follow your notes irregardless of the assumptions they made in their bid.

 

[jechols]

Thanks again for the comments!

 

[civilperson]

Hooks are ineffective in compression, development length of horizontal deformed reinforcing bars in walls are under discussion. The lap length splice and end anchorage are the applicable applications, if no compression is possible in the bars under discussion, then hooks would negate the need for top 1.3 factor.

 

[JLNJ]

Are these flexural bars? I'm picturing a cantilevered or tied top and bottom wall with a nominal amount of horizontal steel.

If they are just shrink & temp then I wouldn't get too excited about using 1.0 instead of 1.3.

 

[jechols]

JLNJ,

No, the bars in this situation are in a tank wall designed for two-way flexure.

J