slab bottom bars: how to overlab at best?
slab bottom bars: how to overlab at best?
(OP)
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
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






RE: slab bottom bars: how to overlab at best?
[img http://files.engineering.com/getfile.aspx?folder=2...]
RE: slab bottom bars: how to overlab at best?
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.
RE: slab bottom bars: how to overlab at best?
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/download.aspx?folder=...
RE: slab bottom bars: how to overlab at best?
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.
RE: slab bottom bars: how to overlab at best?
RE: slab bottom bars: how to overlab at best?
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.
RE: slab bottom bars: how to overlab at best?
Why? Show me where in your applicable design and building codes you are prohibited from "splicing bottom bars at/near midspan."
Then you are teaching them your personal preference, not engineering science.
RE: slab bottom bars: how to overlab at best?
RE: slab bottom bars: how to overlab at best?
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: slab bottom bars: how to overlab at best?
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.
RE: slab bottom bars: how to overlab at best?
RE: slab bottom bars: how to overlab at best?
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)
bottom bars spliced over the supports and top bars spliced at midspan
I just wish I could find it black on white in the code
RE: slab bottom bars: how to overlab at best?
RE: slab bottom bars: how to overlab at best?
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
RE: slab bottom bars: how to overlab at best?
RE: slab bottom bars: how to overlab at best?
found some good images which also underline this idea