U-Bar reinforcement hook lengths

[BAGW]

Hi,

Where can I find the requirement for a U reinforcement bar dimensions.

Dimension "a" can be standard hooked bar length. what should be dimension "b". 2 x standard hook length?

 

[BAretired]

I doubt that you will find this anywhere in the code. Dimension 'b' does not need to be more than 'a'. In fact, it could likely be a/2 because the bar is developed at both ends.

BA

 

[BridgeSmith]

Actually BA, dimension "b" doesn't have to be more than twice the minimum bend radius of the bar, since it's continuous through both bends.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[slickdeals]

In theory, "b" could be as tight as you can bend a 180 degree hook.

 

[BAretired]

BridgeSmith and slickdeals,

You may be right, but if the bend radius is 4d, I wonder if 3d is enough to develop bearing in one bar.

Bar area A = pi*d²/4
T = tension per leg = A*Fy or pi*d^2*Fy/4
Bearing area per vertical leg = 3d*d = 3d²

Bearing stress = pi*Fy/12

If Fy = 60ksi, ultimate bearing stress = 15.7ksi (seems high for normal concrete strengths)

I guess you could argue that the full strength of the bar does not need to be developed beyond the bend point, but there should be some consideration to bearing, should there not?


BA

 

[BridgeSmith]

I'm not sure what your concern is BAretired. The 'developing' the full strength of the bar is not an issue, since the bar is continuous and doesn't have to transfer any tension to the concrete through the 180 bend, or 2 90 degree bends.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[BAretired]

BridgeSmith,

A bar is not fully developed without a standard hook, so the standard hook must be taking some tension to compensate for high bearing stresses at the bend. In the case of a 'U' shaped bar, there are high bearing stresses at both bends. If the distance between the legs, 'b' is small, the bearing stress is very high.

If, as you say, the bar does not have to transfer any tension to the concrete, then why do we need standard hooks? The bar could simply be terminated at the bend point.

BA

 

[Celt83]

If you want to put numbers to it you can look at it as a curved bar node: LINK

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[BridgeSmith]

If, as you say, the bar does not have to transfer any tension to the concrete, then why do we need standard hooks?

You have to extend it a distance so the force transfers to the concrete over a sufficient distance and so that the hook can't pull out or break out and straighten. In this case, at the minimum width, it's a 180 degree hook with an extra long hook extension. It doesn't need to transfer any tension to the concrete because the bar doesn't end at the end of the hook, so the tension force stays in the bar.

The bearing stresses on the concrete along the inside of the curve are no worse than they are for a 90 degree hook. If fact, it's actually better because there is force from the curve on the other side and confinement of the concrete.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[BAretired]

BridgeSmith,

I may be misinterpreting what you are saying, but, as illustrated in the link by Celt83, the bends in the stirrup are acting like anchors for the force in the bars. They are transferring the forces from two vertical and one horizontal leg of the stirrup to the concrete. Thus, I do not understand the following statement:

It doesn't need to transfer any tension to the concrete because the bar doesn't end at the end of the hook, so the tension force stays in the bar.

I also believe that the bearing stress around the curve is very high. If the beam cage is tied such that there is a longitudinal bar placed tightly against each bend of the stirrup, it seems to me that would improve the anchorage considerably. And that is the usual practice.

BA

 

[dik]

it's confined...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BridgeSmith]

...the bends in the stirrup are acting like anchors for the force in the bars.

Exactly. The horizontal leg anchors the vertical legs, and the vertical legs anchor the horizontal leg.

They are transferring the forces from two vertical and one horizontal leg of the stirrup to the concrete.

No, the force primarily follows the bars. It only transfers force to the concrete in bearing along the inside of the curves.

I also believe that the bearing stress around the curve is very high.

No higher than it is around any normal bend in any other hooked or bent bar. If the bearing stress were a problem, the minimum specified radii for bends in reinforcing would be larger.

Again, at "b" equal to 2 times the bend radius, it's functionally no different than a 180 degree hook, except there's extra extension of the bar beyond the required hook length.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[BAretired]

No, the force primarily follows the bars. It only transfers force to the concrete in bearing along the inside of the curves.

Your first sentence is not clear. The orange colored text is exactly what I said. They (the bends or anchors), transfer the force from the bars into the concrete.

I agree with your last two sentences, and I admit that I was a little off the mark at the beginning of the thread. Thanks for clearing that up.

BA