Development length provided by U bars 1

[Drapes]

If you have an RC slab rigidly connected to an edge column, and there is little depth in the column to cater for the required development length, is it possible to use a U bar at the edge of the slab to give you sufficient development? For example, say you have a 150x150 sq column, and the tensile reinforcement coming from the your slab needs at least 500mm of development.

 

[ron9876]

A U bar can't develop in a shorter length than a simple hook. Basically same concept as a hook. Maybe the opposite side of the U bar could be considered to reduce the required length of the "tail".

 

[BAretired]

150x150 sq column?

BA

 

[Drapes]

Just a hypothetical, albeit unrealistic.

 

[dhengr]

Drapes:
You say.... “Just a hypothetical, albeit unrealistic.” It is almost as easy to think just a little bit and be semi-realistic with your hypotheticals. That way, we don’t start from the very beginning thinking that this guy has no idea what he’s talking about or doing, why should we help him? On the one hand we do understand that many times the general concept of the problem is not really a direct function of some dimensions. But, you would be surprised at how proper and realistic proportions, dimensions, loads, bar sizes, etc., influence an experienced engineer’s view of a problem and his first impression of it feasibility or stupidity; and then how he answers the question.

 

[hokie66]

Agree with dhengr, but back to the question, I disagree with ron0976. A U bar, adequately lapped with slab bars, does develop the bars.

 

[slickdeals]

I have wondered this myself. The codes allow for the use of U bars in corbels for the tensile reinforcement or let you weld perpendicular bars. Same thing in a stirrup in a beam.

The Indian codes let you make up for the development length by extending the tail of the hook.

 

[ron9876]

hokie66 I understood the question to be about using a U bar instead of a hooked bar to develop reinforcement into a column. Not sure what impact lapping with slab steel would have. It would seem that there would be the same compressive stresses at the bend in a U bar as there would be with a hook. As I said I can see where the U may reduce the length of the tail.

At corbels I have always understood that the bars had to be developed. Surely, for instance, you couldn't effectively use a #8 U bar in a 10" column and have it developed. Just my 2 cents.

 

[TXStructural]

Both ends of the U can develop, but the limitation will become the ability of the bar to take the stress and the ability of the concrete inside the bend to resist the forces. The limitation of hooks at failure (under test conditions) is frequently due to crushing of concrete inside the bend. There is room for engineering judgement in a tie situation (as you describe the use), where the U-bars enclose vertical or horizontal longitudinal reinforcement.
By ACI 318-11 11.6.8, bars intended to engage shear friction (as in corbels) are required to be fully developed on each side of the shear plane.
In the case of tension, you are required to develop the bar(s) in proportion to the demand, where there is more reinforcement than the minimum required. [ACI 318-11 12.2.5, 12.5.3(d)] This partial development (i.e., shorter development length) might be applicable in the example you suggest.

 

[hokie66]

ron9876,
I think the confusion is due to the question. The U-bar will develop the slab reinforcement. But the connection to a column is a separate issue. There was not enough information in the OP's question to know what he is trying to achieve in that regard.