Development Length

[dcvo89]

Hi,

Let's say you have a concrete beam that hangs off the side of a concrete column. For the rebars that extend from the beam and into the column, I need to calculate the development length. Based on the equation for development length (ACI 318-14 Chapter 25: Section 24.4.2.3), there are variables that relate to transverse reinforcement (Atr), tension reinforcement (s and n from the Ktr equation), Cb, and the Psi factor for casting position. Would these be based off of the geometry and reinforcement of the beam or the column?

Also, the development length would just be the length perpendicular to the wall and any hooks (if using the equation for a hooked bar) wouldn't count towards that, correct?

Thanks,

 

[MSUK90]

I feel really sorry if I disturbed the thread with my questions, will keep that in mind from the next time. Apologies to everyone who felt bad/disrespectful.
I will just reply to the comments here and will ask any further questions in a new thread.

For your first question, bend column outer bars, or provide L bars, into the beam, and splice to the top beam bars with tension splice length. However, I don't think the "column" is sized adequately to provide full restrain of the beam.
I don't understand your second question. What is the extra bar? If you want to extend this discussion, please open a new thread with additional sketches.

Thanks for your views. I will start a new thread with the other question with a picture.

Therefore theoretically the development length = 180mm+8x16mm+380mm = 688mm which is 43 times of the diameter

What? I don't think development length works that way. Only L_dhshall be considered for development.

1) With respect to the design of the beam, at these proportions I would always design the beam ignoring joint rigidity altogether. For detailing and crack control purposes, I'd install some hooked bars equivalent to at least 25% of the bottom steel area and extend those out to around 30% of the span length.

2) Were I to attempt to make this connection an ideal, rigid joint, I'd take the approach shown in the sketch below. Nobody's going to want to build that.

3) In my opinion, unless there's a column on top of the joint to provide a kick-ass clamping mechanism, I would not be calling this a rigid joint unless bar continuity was maintained around the outside corner somehow.

4) With a column on top, and full bar development into the joint, I would support calling this a rigid joint.

5) With a column on top, and partial bar development into the joint, I would not support calling this a rigid joint, even with the moment transfer capped at the moment corresponding to the tension capacity of the partially developed flexural bars. My reasoning for this stance is that the bar tension will continue to grow until one of these things happens:

a) The bar yields which can only happen with full development or;

b) The bar tears out from the joint in an anchorage / bond stress failure which is obviously unacceptable.

c) A concrete strut or shear failure occurs someplace, the timing of which cannot be predicted accurately.

In my opinion, partial bar development is the domain of situations in which the bar tension demand is self limiting to a value less than fy. I feel that it is erroneous to use partial development as a means to limit bar tension / joint moment supply.

6) Because of #5, I feel that moment redistribution and partial bar development are fundamentally incompatible. That said, I violate the code prescribed limits on moment redistribution all over the place. I'm also confident that everybody else does too whether they are cognizant of it or not.

Always love your elaborate views! Save some for my new thread.

DaveAtkins, Thans a lot for your views..

 

[KootK]

I feel really sorry if I disturbed the thread with my questions, will keep that in mind from the next time. Apologies to everyone who felt bad/disrespectful.

Don't loose any sleep over it. I'd consider your case borderline. It's not as though your topic isn't substantially related to the original. Sometimes interesting discussions just take on a life of their own.

 

[JLNJ]

When you say “hangs off the side of the column” do you mean the beam and column are not in line with one another? Does the beam pass by the column (i.e. the left face of the beam is the right face of the column?

If so, then there might be a different mechanism at work.

Most of the replies seem to be geared toward a short cantilevered beam projecting from a column, all in the same plane.

 

[hoshang]

As calculated through ACI code, the hook development length required after considering all factors comes out around 300mm which is clearly not available.

Instead of 90 degree hook, you can provide 180 degree.

 

[Agent666]

In this part of the world, irrespective of the bend angle 90, 135 or 180, the development length is the same. I thought ACI318 was the same as well.

 

[Lomarandil]

ACI is the same in that regard. Development stays the same, the only effect is the geometry of the hook itself (length of the hook tail from the bar axis, particularly if developing in a thin element)

----
just call me Lo.