Top Bar Development Length in conc wall
Top Bar Development Length in conc wall
(OP)
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.






RE: Top Bar Development Length in conc wall
I would say you have to use the 1.3 factor.
RE: Top Bar Development Length in conc wall
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.
RE: Top Bar Development Length in conc wall
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 As required / As provided.
RE: Top Bar Development Length in conc wall
let's take a stand!
alright, that's over the top.
RE: Top Bar Development Length in conc wall
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.
RE: Top Bar Development Length in conc wall
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.
RE: Top Bar Development Length in conc wall
RE: Top Bar Development Length in conc wall
RE: Top Bar Development Length in conc wall
RE: Top Bar Development Length in conc wall
Tell the detailer that you regard them to follow your notes irregardless of the assumptions they made in their bid.
RE: Top Bar Development Length in conc wall
RE: Top Bar Development Length in conc wall
RE: Top Bar Development Length in conc wall
If they are just shrink & temp then I wouldn't get too excited about using 1.0 instead of 1.3.
RE: Top Bar Development Length in conc wall
No, the bars in this situation are in a tank wall designed for two-way flexure.
J