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Continuity and Reinforcment

Continuity and Reinforcment

Continuity and Reinforcment

(OP)
Please see the attached sketch (keep in mind it isn't drawn to scale)

This a detail which I had found which involves a slab depression.

It called for a tension lap between the top bar in the depressed portion and the bottom bar in the normal portion. I was told the reason this lap was called out was to provide continuity.

I understand the reasoning for continuity in RC structures, but I just don't see how you get it out of this lap.

Note the top bar is hooked in the non-depressed slab.

Can anyone shed some light?
 

RE: Continuity and Reinforcment

Is this an elevated slab or slab on grade?  Are these flexural bars, or are they for some other purpose?  Seismic or non-seismic?  For flexural continuity, you'd need some mechanism to transfer the tension from one tension bar to the other.  If both bars could be tension bars you'd need a mechanism for both.  If the top bars were tension bars, I'd try to have the upper slab top bar developed for the force you need in it to the left of the point where the lower slab top bar is developed for the force you need.  Similar if the bottom bars were tension bars.  (T-C-T Strut & Tie)

RE: Continuity and Reinforcment

I think details for this connection can use some modifications, but that is beyond your question. For your question, I agree that there is no obvious advantage by calling tension splice here.

RE: Continuity and Reinforcment

(OP)
Sorry I could have been clearer.

This is a structural slab, these bars are for gravity loads.

Assume that at this location the top bars are in tension.
 

RE: Continuity and Reinforcment

I think the top bars at the depression are actually lapped with the top bars in the slab to the right.  The moment from the left side can be transferred into the slab to the right, via the top bars below the depression.  The slab to the right can then resist the moment with its top bars.

DaveAtkins

RE: Continuity and Reinforcment

Yes, agree with Dave. It is to ensure that the tension forces get transferred through the depression. I don't know if you have a cantilever on the other side of the depression (a recessed cantilever). I have frequently seen Z bars used in circumstances where the depression is larger.

On a side note, does anyone know if there is a recommendation in ACI regarding how far these bars can be from each other to transfer tensile forces before requiring Z bars? Like for for columns they can be upto 6". Any help?

RE: Continuity and Reinforcment

(OP)
I agree that the actual mechanics of this will be the top steel in the depression is actually lapping with top steel in the non-depression.

I'm curious though why they call for a tension lap between the top bar in the depression and bottom bar in the normal slab.

The reason is "continuity" but I don't see it

RE: Continuity and Reinforcment

The continuity is also for the sake of preventing cracking in this section. If all bars were terminated in this are then you would have a potential weak point for shrinkage cracking.

RE: Continuity and Reinforcment

The detail as shown, if it is intended to serve as a typical detail, is poorly conceived.  Depending on the location in the slab relative to the supports, strength of the slab could be insufficient.

RE: Continuity and Reinforcment

This is tanamount to a footing step detail where the bottom bars should be directly linked via lap aplices, not a top and bottom bar - prone to cracking here.

Mike McCann
MMC Engineering

RE: Continuity and Reinforcment

See attachment for my proposal, TDL is tension development length. Try to be conservative in it, as all of your slab integrity depends on it. I proposed two options, but personally i would like to go for option-2. I am not sure of stepping domension , that's why i show T.D.L. at top too.
Although you can go for comprssion lap at top, if that dimension is relatively small compared to slab thickness.

RE: Continuity and Reinforcment

I would agree in principle with eiter of Ali07's details. I do not think they need to be 3' long however.

The detail provided initially is definitely inadequate. The top bars on the right need to lap with the top bars on the left.This is normally done with a z bar from the right as per Alio7's first option but can also use a stirrup to provide the continuity as shown by Ali07 option 2 (much more stable arrangement).

RE: Continuity and Reinforcment

I disagree with most of the posts here.  I think the original detail will work--how will cracks form?

DaveAtkins

RE: Continuity and Reinforcment

I agree with Dave that the original detail will work, if the depression is less than 6 inches, and the lap slice is measured horizontally from the ends of the top bars,(not the way its shown).  If the concrete below the top bar in the thickened section is more than 12", and is done in a single pour, then the splice length will need to be adjusted by 30% per 12.2.4.

RE: Continuity and Reinforcment

Appologize to diagree with miecz/dave.
Can u explain how the bottom bars moment at that section to be carried and transferred by bottom bars. Your bottom bars from right just end up with no continuity to be transferred to right. Don't just try to save penny and result in a failure. You are not clear of stepping in slab, if it is 12" are your argument still holds.

RE: Continuity and Reinforcment

Cut a free body diagram right in the middle of the thick part, first in one direction, then in the other.  Can you resist moment from both directions?  I think you can.

DaveAtkins

RE: Continuity and Reinforcment

ali07

Please, draw a crack on the sketch to show me where this is going to break?

RE: Continuity and Reinforcment

If this is a door step with soil below, the bottom bar does not serve much. If this is suspend slab, the original detail was poor, I will prefer ali07's solutions.  

RE: Continuity and Reinforcment

(OP)
what do the stirrups provide in ali's detail? How do they contribute to the continuity?

RE: Continuity and Reinforcment

Try to think it is suspended slab with step like stair.

It is not understandable that bottom layer which is tension is proposed to be carried by top rebar which is in compression and allow the bottom bar to discontinue.
IMHO it is not a good engineering detail.

Meicz.
If the slab step once again is about 12", does your recommendations still hold.

RE: Continuity and Reinforcment

ali07,

You are correct--the two top bars will both be in compression.  No one is saying one of them will be in tension.

What you are not understanding is that a lapped bar does not necessarily deliver tension into the bar next to it.

The slab on one side delivers the moment into the big blob of concrete in the middle, and the slab on the other side takes the moment back out of the big blob.

DaveAtkins

RE: Continuity and Reinforcment

Quote (NS4U):

Assume that at this location the top bars are in tension.

RE: Continuity and Reinforcment

Meicz.

It will not go with your assumption. If this section is at mid of 40 ft span.

Assume top bar is in tension. Can u draw the moment diagram.

So you mean the positive moment is carried by the top bar all the way. No need of bottom reinforcement?

Mind u top bar there is not cantilver from a wall.

RE: Continuity and Reinforcment

ali07

The original poster (NS4U) said that the top bars are in tension.  So, the moment diagram is a straight line of some constant moment that causes tension in the top bars.  I don't remember the op asking about a 40 foot span, a positive moment, or tension in the bottom bars.  Did I miss something?

RE: Continuity and Reinforcment

(OP)
miecz is correct. This detail is in a negative moment region. (TOP bars are in tension)

Knowing this, for strength purposes I do not believe this to be a poor or inadequate detail.

Should this be in a location of positive bending, I agree that it is a poor detail.

While I have found this debate worthwhile... No one has been able to give me an answer to me original question.

Which was:

Why is a tension lap called for in the location specified. I have been told by a co-worker that it was because of "continuity", but I do not seem to understand their definition of continuity.

I have given this some though and have a couple of theories:

1. In the thickened slab section (aka the blob), the top bar from the left side is still in a bit of tension because it is above the N.A. of the blob... For that reason you do not want to just terminate the bar. Even though this bar is not needed for strength in this region, I believe just terminating a bar in tension is bad ideas (b/c of cracking). (I'm not 100% sure why though)

2. For structural integrity it is good to have bottom bars continuous strength in your bottom bars. This tension lap could be providing that but forcing the bottom bar on the right to be fully developed inside the blob.

What are others comments on these thoughts?
 

RE: Continuity and Reinforcment

NS4U

I believe I answered your original question in my original reply:

The original detail will work, IF the depression is less than 6 inches, and the lap slice is measured from the ends of the top bars,(not the way its shown).

That is, the splice is from top bar to top bar, and the maximum distance between them must be less than 6 inches.  The splice, shown from top bar to bottom bar, is incorrect.  Besides that, it's a lousy detail as others have pointed out.

RE: Continuity and Reinforcment

NS4U:
The detail the way you presented it should provide continuity between the two (top) tension bars.  To elaborate on my first post.  The splice as it is shown is not really a splice between the bottom bar from the left and the top from the right.  The way it will work is the top bar from the right will be developed by a hook to the left of where the top bar from the left will be developed via development length.  Because of this a compressive strut will be able to form in the concrete between these points where the two bars are adequately developed.  If the top bar from the right was developed at a point to the right of where the bar from the left is developed, it would need to be a tension member transferring the force (i.e. the concrete couldn't do it and you would need some steel to make the transfer)       

RE: Continuity and Reinforcment

(OP)
Actually, you did not answer my question. My question did not pertain to the quality of the detail, nor did I ask if it was an acceptable detail.

The mechanics of concrete will of course cause the top bar in the depression to lap with the top bar in the non-depression. I fully agree.

It is my fault for not noticing that no one here picked up the fact that by calling for a tension lap between the bottom bar in the non-depressed area and the top bar in the depressed area, the detail forces top bar (from the depressed area) to extended into the thickened slab a minimum distance of a tension lap (plus the side cover). This is because, as I showed, the bottom bar cannot fit into the depressed area.  This cleverly satisfies the lap between the top bars, which is what everyone is getting hung up on.

Perhaps I should have just said this:

"why does the detail want the top bar in the depression to be very close to the bottom bar in the non depression"

Or this:

"why at the approximate elevation of the top bar in the depression does the detail want to keep the reinforcement continuous by providing a tension lap between the bottom bar"

See the attachment where... where the question is expressed visually

The original detail I provided seemed to go out of its way to avoid this gap, my question is why
 

RE: Continuity and Reinforcment

(OP)
ali, thanks for the reply. I meant to say the depression is LESS THAN 6". Sorry about this, I was rushing out the door and trying to get this posted. I agree you would get an unreinforced section if the gap was more than 6".


A couple things I should mention:

- hanger reinforcement is provided
- the hooked bar is fully developed (it's just NTS)

 

RE: Continuity and Reinforcment

If there is any possibility of uplift on the cantilever from wind load, then you would want continuous bottom reinforcing.  Otherwise, I agree--it is not necessary.

DaveAtkins

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