splicing and overlab of bottom slab rebars

[lolobau]

Hi

I was always wondering what is the right way for the slab bottom reinforcement.
In general we splice it over the beams with the 50xdia splicing length

but is it really necessary?

If you support it just on the edge (see attachment)
you also don't have more anchorage length available then the wall width below

the green marked hook is actually also not needed

what is you recommendation regarding the solution A, B, C?

 

[KootK]

In North America, such interior bottom bars will extend 6" into the supports. No hooks, no splices. Of course, this assumes that you'll be installing properly designed top steel as well.

At the edge it depends if you're calling the joint fixed or pinned. If fixed, same as above. If pinned, you need to satisfy code provisions for the anchorage of positive steel at supports. The quick and dirty ways of doing this are to a) develop the bars past the face of the support or b) terminate the bars in standard hooks past the face of support. There's a detailed method allowing partial development as well but I'll leave it to you to look that up.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[lolobau]

Hallo KootK

thanks for the fast respond

but strength wise version A would be ok, or? since this area of slab should be anyway in compression. It would be interesting to see it in FE

 

[lolobau]

what about this

 

[KootK]

All of the versions that you've shown do far would be adequate.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[lolobau]

but it would be waste of reinforcement
and it will clash with the beam rebars
so actually version A is most economic and practical

but everyone is doing version B

 

[lolobau]

but what do the codes say to it?

 

[TehMightyEngineer]

Depending on your moment diagram you may require additional reinforcement length to develop the bar by the time it hits the steepest part of the moment diagram. ACI 318 has a check for this.

However, given a typical slab/beam geometry I would expect "option A" to be the most typical layout used. Worst case is something between option A and B.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[rapt]

An overlap of bottom reinforcement is always prudent to allow for things like load patterns, support settlement etc. And at columns it gives a good increase in punching shear action especially ductility (it is a requirement of the Canadian code for this). It is also useful for robustness to allow for the loss of some supporting members (a requirement of BS, Euro, Australian and probably ACI codes).

 

[LQQD]

Are you considering, on splice A, some stiffnes reduction due to plstification and cracking of the slab above intermediate supports, negative moment zone ? This will lower you moment diagram and require to start anchoring nearer to the support.

 

[lolobau]

Punching will not be a problem there.
It is a wide beam and not a column
the calculations show clearly a compression zone here at the supports
which is in almost most structures the case

some engineers are even taking the bottom bars up to the top layer (bended up)

 

[KootK]

- it is a waste of rebar

- none of the details that you've shown are likely to result in any serious congestion problems. Less congestion is always better though.

- I'm surprised that your "everyone" uses B. My "everyone" definitely uses something closer to A. In fact, every firm that I've worked for has had a typical for one way slabs similar to A.

- the ACI code spells out something similar to A for one way slabs as the minimum bar extension requirement. It doesn't eliminate the need to satisfy other code provisions but it's enough the vast majority of the time.

- negative moment redistribution per code provisions (about 20% max) will move your inflection points closer to the supports but, for uniform loads, I can't imagine a practical case where that effect would be significant enough to trigger positive steel at support checks.

- in a one way slab with beams like this, I disagree with the prudence of lapping bottom bars. Punching shear improvement isn't needed in a one way system and no load pattern, other than reversal to uplift, would necessitate support continuity in the bottom steel. Lapping bottom bars through beam cages is a pain and I don't feel that it's justified by such nominal/questionable benefits.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

The bar bend trick doesn't make sense as shear improvement in a one way slab. It can be used as an efficiency measure if the bar's use is being alternated between positive and negative reinforcement. In North America, the long standing view has been that material savings with this method don't justify the fabrication and installation hassles. I a market where labour is less costly, the economics of that might flip.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TXStructural]

Do NOT truss-bend bars (that is, bend them from top layer to bottom layer.) It is NEVER practical in North America. It is costly (time consuming) to bend, dangerous to handle (the bundled bars get caught on each other), and difficult if not impossible to place within tolerances.

I generally prefer to lap all bars in slabs to avoid confusion and assure continuity, especially if there is a load reversal. The added cost of steel and labor to lap these bars is negligible if you find any use in doing so, such as reducing the risk of serviceability issues. In a slab layout, congestion at laps is not a problem because the slab bars lap side-by-side, the laps do not have to touch, and the bars are far enough apart that space is seldom an issue. From a placing standpoint, it may be easier to place bars that stop near face of support (or 6 inches into support) in wide beams.

 

[lolobau]

@Koot: I agree with you "....Lapping bottom bars through beam cages is a pain ...."
that's why I would like to move away from it but as TXStructural is saying he also overlaps them, you see this is what I was referring to (the most people) do it although it might be a waste of rebar. What does the ACI says about it?

lolobau

 

[TXStructural]

ACI 318 (and other building codes) are minimum code and not best practice.
Adherence to the minimum only means the performance should be adequate, not that the structure will perform optimally.

ACI 318-14 7.7.3.8 covers termination of reinforcement for one-way slabs, including requirements about carrying 1/4 of bottom bars at least 6" into integral supports.
Other minimum provisions of this section are fairly clear for each condition.

 

[KootK]

What does the ACI says about it?

We've already answered this: ACI does not require bottom bars to be lapped at the supports of one way slabs unless there is an actual moment demand that would make such lapping necessary.

that's why I would like to move away from it but as TXStructural is saying he also overlaps them, you see this is what I was referring to (the most people) do it although it might be a waste of rebar.

All engineers have their own ways of doing things. And most of those conservative "quirks" will be supported by some sound logic. However, if you take a poll on an issue like this and incorporate everyone's quirk into your future designs, your designs will wind up ridiculously overbuilt.

You know what the code requires and you've gathered some diverse opinions here. It's time to do what all good engineers are required to do sooner of later: exercise your own judgment and move forward with whatever you feel is best.

It's essential to recognize that it's not prudent to do everything that would make a structure stronger, more resilient, or more serviceable unless the required changes would be cost neutral. Our job as structural engineers is not to produce indestructible structures. Any layperson with deep pockets and a high school physics class under his belt could do that. Our job is to create adequate structures in which installed costs are minimized.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.