Continuity and Reinforcment

[NS4U]

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?

 

[DaveAtkins]

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

 

[miecz]

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

 

[ali07]

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.

 

[miecz]

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?

 

[NS4U]

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?

 

[miecz]

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.

 

[jt12]

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)

 

[NS4U]

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

 

[ali07]

I don't understand why you want that lap. Even if you lap it. Your section between top and bottom bars are in tension and can shear off.

 

[NS4U]

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)

 

[DaveAtkins]

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