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In reinforced concrete design, there are some cases that engineers have to design beams which rebar doesn't develop its complete length or hook length according to codes. It is because of restrictions of architecture. The solution from some engineers considers that the connection beam-column in structural analysis is not rigid but free of moments.

Until now, I didn't find research to endorses it. Maybe you can comment about experience or investigations.

The mandatory solution is to increase the column length or decrease de rebar diameter. Is there another solution?

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    $\begingroup$ adds hooks/bends to the rebar to shorten development length. Little foggy on this one but you code may allow for a development length reduction if the As provided is greater than the AS required. Use higher strength concrete. Not that I have seen it in column beam connects but you could also use plates on the end of the rebar. $\endgroup$
    – Forward Ed
    Feb 11 at 15:18
  • $\begingroup$ "...there are some cases in practice that engineers find beams which rebar doesn't develop its complete length according to codes." Are those afterward findings or discoveries at installation? Please be more specific, your question is quite vague. $\endgroup$
    – r13
    Feb 11 at 15:45
  • $\begingroup$ @r13 I changed my argument. I mean when engineer is designing a beam. $\endgroup$
    – Isai
    Feb 11 at 15:51
  • $\begingroup$ I'm having trouble understanding what you mean. Is the column so short that you can't bend the beam's rebar along the column's height? That is allowed in every code I'm aware of. Could you please edit your question with a sketch or diagram of your problem? $\endgroup$
    – Wasabi
    Feb 11 at 17:53

2 Answers 2

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"The solution from some engineers considers that the connection beam-column in structural analysis is not rigid but free of moments."

The solution/considered is both incorrect and dangerous. A beam to column/wall joint may be designed as a pinned support (due to the presence of multiple degrees of structural indeterminacy in the framing system), however, it still must support the gravity loads which will cause shear force at the beam-column/wall interface. Then, the minimum development length should be designed in accordance with the shear friction reinforcement criteria to ensure the two elements stay in close contact. Under such conditions, small rotation may occur and be tolerated, but in no case, the pull out of the shear reinforcing bars shall be permitted as the potential for the bars being sheared to yield is too great a risk.

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A beam, or a wall, usually is not a good candidate to carry shear friction, because the bars are essentially in tension, and the required development length may be well beyond the width of the beam/wall. The solutions could be

  • providing more smaller bars at the interface (with a shear key is preferred),

  • using higher strength concrete or steel,

  • utilizing the "excess reinforcement provision", that is providing more bars of the same size so the demand is less than the capacity ($As_{req} \lt As_{act}$)

  • using hooks (not preferred, as the hooked bars will increase the stiffness of the joint and draw bending)

  • rearrange the structural elements to reduce the load if member sizes can't be altered.

Depending on the situation, the design code provides a few ways to minimize the development length. But in no time and in no case, the bottom line of the code should be violated unless full-scale tests that meet material testing protocols/standards are conducted, which is the way the code develops the governing equations of the development length.

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  • $\begingroup$ I didn’t know that hooks increase stiffness and then draw bending (bending of the beam?). I know that inelastic deformation of the joints origin the lateral displacement of structures around 20%, according to Priestley and Paulay, 1992. Please, cite some paper or reference. $\endgroup$
    – Isai
    Feb 11 at 21:08
  • $\begingroup$ The hooked bar has a much higher pull-out resistance than straight bar due to the tendency of slippage of the straight run and the much greater compression developed around the bent corner. Also, tests have shown that a 135 degrees hook is much strongerthan the 90 degrees hook for the verysame reason. $\endgroup$
    – r13
    Feb 11 at 21:45
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If we need a flexible joint having underdeveloped bound in short embedment is not the right way. It is actually a time-bomb waiting to go off and crash the beam explosively.

All the rebar regardless of what is their task have to be fully developed.

If there are needs for a flexible joint there are ways to design a flexible joint, but never an underdeveloped bar. In concrete, by design, failure starts in rebar, not in concrete. If there is a lack of space a bar can bend or form into a hook to develop.

Because of the ductility of the steel, it deflects by a large amount, signaling imminent failure, before the collapse. This gives ample time to occupants for evacuation.

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  • $\begingroup$ I agree with you. I wanted to know if there were a solid argument about solving the problem in the model as I was taught. I’ve never read in the literature about the commented solution. $\endgroup$
    – Isai
    Feb 11 at 21:13

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