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?t Location Factor For Two-Way Spanning RC Walls

?t Location Factor For Two-Way Spanning RC Walls

?t Location Factor For Two-Way Spanning RC Walls

(OP)
I'm trying to compute minimum reinforcement splice lengths for a RC wall with vertical and horizontal rebar. When calculating the splice length of the horizontal rebar, ACI 318-11 section 12.2.4(a) states the following:
"Where horizontal reinforcement is placed such that more than 12 in. of fresh concrete is cast below the development length or splice, ψt=1.3. For other situations, ψt=1.0"

My intuition tells me that this is meant to apply to horizontally spanning RC slabs and beams, not RC walls, however ACI contains no such exclusion or explicit instruction.

In Concrete Mechanics & Design 6th Edition, the following reason is stated for the factor:

"Horizontal reinforcement with more than 12 in. of fresh concrete below it at the time the bar is embedded in concrete is referred to as top reinforcement. During the placement of the concrete, water and mortar migrate vertically upward through the concrete, collecting on the underside of reinforcing bars. If the depth below the bar exceeds 12 in., sufficient mortar will collect to weaken the bond significantly. This applies to the top reinforcement in beams with depths greater than 12in. and to horizontal steel in walls cast in lifts greater than 12in. The factor was reduced from 1.4 to 1.3 in 1989, as a result of tests in [8-5]."

Reading this, it seems like my intuition may have been wrong. I still feel like this gathering of mortar would be much more pronounced under horizontal bars in a horizontal slab than horizontal bars in a two-way RC wall.

However, development length methodology in a now defunct (as of 2008) military manual (TM-5-1300) treats this factor differently.
TM-5-1300 Section 4-21.4 Development Length for Reinforcement in Tension:
"For top reinforcement, that is, horizontal reinforcement so placed that more than 12 inches of concrete is cast in the slab below the reinforcement, the basic development length must be multiplied by 1.40. This provision applies to horizontal slabs only. Walls with multiple runs of horizontal bars plus vertical bars are not affected by this provision."

A more recent version of this code (UFC 3-340-02 (2008)) simply refers to ACI for development length calculations.

So... Who's right? I feel that I'd be justified in using a ψt factor of 1.0 for RC walls based on my understanding of the spirit of the factor.

RE: ?t Location Factor For Two-Way Spanning RC Walls

1) I've always assumed horizontal wall bars to be top bars.
2) All firms that I've worked with have assumed horizontal wall bars to be wall bars.
3) Not sure what to make of the TM-5 recommendations.

So yeah, I vote for horizontal wall bars to be top bars.

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.

RE: ?t Location Factor For Two-Way Spanning RC Walls

I also vote for horizontal wall bars to be considered "top bars" if there is more than 12 inches of fresh concrete placed below. It's empirical, so I don't really feel it's valid to use an older methodology.

RE: ?t Location Factor For Two-Way Spanning RC Walls

(OP)
Thanks for the replies. I'm not inclined to agree that anything changed empirically other than the factor was reduced from 1.4 to 1.3. It still seems to me that the only difference is that TM-5 interpreted that the factor does not apply to walls, while ACI doesn't explicitly mention the extent of use of the factor. Maybe the only way to truly know is to dig up the research that determined this factor.

In lieu of that, I guess standard practice is going to dictate these #9 bars will have to lap 10ft.

Edit: While skimming through various articles for top-bar effects, I couldn't find anything in particular to justify TM-5's interpretation.

RE: ?t Location Factor For Two-Way Spanning RC Walls

Funny, I just got called on this today. I always assumed top bars in an elevated 12 inch thick slab to be, you know, top bars. We marked up a drawing and required that a lap be increased. The contractor responded that there's less than 12 inches of concrete below the bar, so the "other splice" definition applied.
I've done this for a long time and never been challenged on this. But the contractor had the definition on his side. I had to accept it.

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