slab bottom bars: how to overlab at best? 1

[lolobau]

Hi

I was wondering what is the best way to slice the slab bottom bars?
See attachment option A, B, C

My colleague says option A would be the best because you have then the double amount of steel in the middle of the slab (where you most loads are)
somehow I can't believe it. It just feels wrong to splice the reabrs in your Mmax area

Who can advice me here?

Regards

Lolobau

 

[lolobau]

http://files.engineering.com/getfile.aspx?folder=23adbc62-4176-49e7-8c5d-9e

 

[Once20036]

If option B is possible, that would be my first choice.
If the spans are too long for that, I would opt for option A and stagger the splices such that no more than 50% of the bars are being spliced at any given point along the length.

I wouldn't accept option A. Sure, you have twice as much steel, but you don't have the benefit of continuity - each bar end immediately adjacent to the maxiumum moment.

 

[lolobau]

Hallo Once

yes of course option B must be the preferred option

I really doubt if A is allowed? what does a double steel area give you if it doesn't have any anchorage?

http://files.engineering.com/downlo...6-49e7-8c5d-9e57738ed9bf&file=slab_bottom.jpg

 

[KootK]

My colleague says option A would be the best because you have then the double amount of steel in the middle of the slab (where you most loads are)
somehow I can't believe it. It just feels wrong to splice the reabrs in your Mmax area

Your colleague is horribly misguided. As you have suggested, it is not the quantity of rebar at a given section that matters but the quantity of anchored/developed rebar that matters. No matter where you lap, nowhere along the span of your slab will you have more than 1x the provided amount of anchored steel present.

Under most circumstances, you can place laps anywhere without violating code. However, most engineers will locate them away from maximum moment locations in acknowledgement of the fact that a bar splice is always a bit less reliable than no bar splice. As we discussed in your other thread, I would not splice these bars at all and simply allow them to be discontinuous at the supports. Baring that, I would locate the splices as near to the supports as possible.

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.

 

[Lion06]

I would never use option A and our typical details explicitly preclude splicing bottom bars at/near midspan. When young engineers are reviewing rebar shop drawings, splice locations are one of the first things I ask them to keep an eye out for.

 

[Illbay]

You may splice the bars ANYWHERE YOU WISH. If your splice lengths are correct, the strength therefrom are equivalent to (actually, they are a bit greater than) continuous reinforcement.

But you cannot say "oh, the splices are BETTER because you have double the area..." Area of reinforcement is only one variable in a very complex equation where flexural strength of reinforced concrete is concerned.

You may use lap splices as prescribed by the ACI 318 code anywhere, not because it's "double the area" but because the tension force is transferred between the bars over the lap length.

I like simplifying assumptions - they make an engineer's life tolerable. But oversimplification is misleading and not helpful.

 

[Illbay]

Lion06: "I would never use option A and our typical details explicitly preclude splicing bottom bars at/near midspan."
​

Why? Show me where in your applicable design and building codes you are prohibited from "splicing bottom bars at/near midspan."

Lion06: "When young engineers are reviewing rebar shop drawings, splice locations are one of the first things I ask them to keep an eye out for."​

Then you are teaching them your personal preference, not engineering science.

 

[Lion06]

I will preface this as being accurate for gravity framing. The ACI detailing manual and every typical detail I've ever seen on every set of drawings for a concrete building show bottom bars spliced over the supports and top bars spliced at midspan. The fact that there is nothing prohibiting it in the code is not relevant to me. Although ACI doesn't preclude it, they do state in the commentary that splices should be made away from points of maximum tensile stress, plus it's just good detailing (IMHO) to not splice bars at the location of maximum demand.

 

[IDS]

One reason for not splicing bars at points of maximum moment is that spliced bars are likely to be much less robust than the equivalent continuous bar. If the cover concrete is lost, either due to overloading or corrosion, a spliced bar will lose most of its capacity, and will have a brittle failure. A continuous bar will provide almost full strength, and will still be ductile.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[racookpe1978]

So, why are mechanical or welded splices not more widely used in favor of twist-tie rebar wired splices?

Seems that a longer splice with fillet welds (or tight clamping bolts through the splice) would always be more reliable than twisting wire around the two rebars.

 

[rwpe]

I agree with Lion06....Make your splices lap over the support, assuming of course, that this is a horizontal deck and the loadings are typical in that you have negative moments over the supports and positive moment at the bottom of your slab.

 

[lolobau]

first of all thank to all your constructive feedback. It helps me enormous further

let me still ask some details

@Lion: "...and our typical details explicitly preclude splicing bottom bars at/near midspan..." where can I find it? is there an document online?

ok so the answer is "no to detail A" thanks a lot, I also think this way but just wanted to make sure because of the double area of steel could be also a valid point

I think this is the golden rule (Lion06)
[highlight #CE5C00]bottom bars spliced over the supports and top bars spliced at midspan[/highlight]

I just wish I could find it black on white in the code

 

[hokie66]

Late to this thread, but reading it, one thought did come to mind. The best position for the lap depends on the length of the lap. If the lap is nominal or maybe less than nominal, put it at the support. But if you have a very long lap, longer than required to develop the bar, the suggestion that there are double bars at midspan has validity.

 

[lolobau]

@hokie66: yes you right but why should I have such an overlenght slicing?
then it would be basically like extra bars in the span middle where by only 50% goes to the support
this is then different scenario

 

[hokie66]

Yes, I agree. But still possible and sometimes an efficient use of the material.

 

[lolobau]

found some good images which also underline this idea