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Tension Anchor Bolt Reinf - Can you count on both legs U-bar?

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Philip C.

Structural
Jul 28, 2021
1
Referring to Fig RD.5.2.9 of ACI 318-11. It shows that Anchor Reinforcement must be developed on both sides of the breakout plane. Assume you have plenty of dev. underneath the breakout cone, and provide Ldh above the breakout cone, do you have to be concerned with Lh above the breakout cone for each leg?

For example if you are using #6 U-bar, they require a Lh of 12". Do these legs need to be spaced 24"? Or by using a U-bar are the just assumed to be fully developed for Lh? See the attached image for what I'm talking about.

Tension_Reinf_jpyzqm.png
 
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I assume Lh is ( the tail development length ) Ld ..

For #6 bar , the tail development length ( ld ) should be around 25 in.
The reinforcement legs shall be spaced ≤ 0.5 hef

I think the figure shown at the code is self explanatory..
anchor_reinf_fw9pny.jpg
 
I believe OP meant the horizontal portion of the U. So OP is asking if he needs to treat U as two 90 degree hooks, each having 12" long horizontal section on top.
 
If you were using standard hooks instead of a U-bar, the horizontal dimension for each of the hooks would be lext + [bend diameter]/2 + db/2 = 12db + 6db/2 + db/2 = 11.6"

So you are wondering if you need to double that dimension (2*11.6" = 23.2") so that the U-bars are not both using the same horizontal portion of the bar for their development. I couldn't find a location in the code that directly addresses this situation, but the commentary for ACI 318-14, R25.4.3.1 states that:

"Study of failures of hooked bars indicate that splitting of the concrete cover in the plane of the hook is the primary cause of failure and that splitting originates at the inside of the hook where local stress concentrations are very high."

I would think that since the issue is the inside corner of the bend splitting that the two bends would be far enough apart that they would not influence each other, and this would not be any weaker than just having two overlapping standard hooks instead of the U-bar. Additionally, if you're able to locate a horizontal bar perpendicular to the plane of the bends (like as shown in the diagram), this starts to look like stirrup reinforcing where wrapping the bars around the longitudinal reinforcing helps to prevent splitting.

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Alternatively, you can use 180 degree hooks on top instead of a U-bar, and if development length is an issue, go with a larger number of smaller bars if space allows. You can have more than 1 reinforcing bar per anchor rod as long as they are not farther than 0.5hef from the anchor.
 
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