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?

 

[jt12]

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)

 

[kslee1000]

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.

 

[NS4U]

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.

 

[DaveAtkins]

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

 

[slickdeals]

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?

 

[NS4U]

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

 

[csd72]

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.

 

[hokie66]

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.

 

[msquared48]

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

 

[ali07]

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.

 

[rapt]

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).

 

[DaveAtkins]

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

DaveAtkins

 

[miecz]

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.

 

[ali07]

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.

 

[DaveAtkins]

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

 

[miecz]

ali07

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

 

[kslee1000]

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.

 

[NS4U]

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

 

[ali07]

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.