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Two way slab proper detailing
3

Two way slab proper detailing

Two way slab proper detailing

(OP)


I just want to clarify what is the proper detailing of a bent up rebars for the two way slab
.

and I want to know the sections of the aci code that supports it.

the slab is 5 m x 5 m with a perimeter support beams with a width of 250 mm
the plan specs for the bars @ long direction is 12 mm diam @ 140 mm O.C BENT-UP ALTERNATE @ L/4
FROM FACE OF SUPPORT.

The following is my question :
1.) for the continuous slab, is it okey if I will not continue the bottom bars to the support as illustrated from bellow?
2.) for the hanger bars bars bars marked as yellow , is it required to continue from the top bar of one support?
3.) is it required for the hanger bars to be lapped(standard lapped) in it's termination to the bent up bars
4.) do we really need to bend bars with in the column strip?
6.) if the next slab spacing requires 100 mm O.C. how can we continue the bars from the 140 mm spacing? as I could see that the " bent up alternate bars " would be really hard to apply in this situation .

"also please correct my detailing if you would see something out of standards"

RE: Two way slab proper detailing

Cranking bars like that is old fashioned, and not practical anymore. Too labor intensive. Using straight bars, with only cogs at the perimeter, should answer your questions.

RE: Two way slab proper detailing

(OP)
Hello hokie66 , I know that it is old fashion . But there are old designers that cant move on . Please enlighten me if you know. . .

RE: Two way slab proper detailing

I am an old designer, started practicing as a design engineer in 1970, and never did it that way, as even then it was impractical. So I can't help you.

RE: Two way slab proper detailing

(OP)
Hello dik.. I was refering to two way slab but I drawn one direction only for simplicity . .I saw your link images..but it only refers to cut bars. Do you have any bent up bats?

RE: Two way slab proper detailing

(OP)
Thank you for your time hokie66.

Can I ask you an opinion if my thoughts are somewhat logical.

Inorder for the bent bars to continue in its direction.. can I develop the bars at the continious support as shown in the figure. . .

RE: Two way slab proper detailing

No bent up bars... they were occasionally encountered with existing work circa 1970, but, have never used them.

Dik

RE: Two way slab proper detailing

Quote (Penpendrum)

4.) do we really need to bend bars within the column strip?
You need less positive moment bars within the column strip than middle strip. So, you may alternate bend bars in the column strip. See distribution of positive moment bars among column and middle strips in textbooks.

Quote (Penpendrum)

6.) if the next slab spacing requires 100 mm O.C. how can we continue the bars from the 140 mm spacing? as I could see that the " bent up alternate bars " would be really hard to apply in this situation .
You may use 14mm diam. @ 140mm in the next slab instead of using 12mm @ 100mm.

RE: Two way slab proper detailing

"You need less positive moment bars within the column strip than middle strip" is WRONG. Where did that come from?

RE: Two way slab proper detailing

Quote (hokie66)

"You need less positive moment bars within the column strip than middle strip" is WRONG. Where did that come from?
See page 510 CHAPTER 16 Two-Way Slabs, Direct Design Method (Design of
Reinforced Concrete by Jack C. McCormac)
If you sketch in the approximate deflected shape of a panel, you will see that a larger
portion of the positive moment is carried by the middle strip than by the column strip, and
vice versa for the negative moments

RE: Two way slab proper detailing

I don't have that reference.

But perhaps I have misunderstood the OP's situation. If this is just a single bay two way slab supported on all four sides, hoshang is correct about the bending being greater in the middle. I thought we were discussing a two way flat slab or flat plate supported on a series of columns, because the pictorial representations show continuity.

To use the terminology "column strip" and "middle strip" in an edge supported slab confuses the issues, IMHO.

RE: Two way slab proper detailing

Quote (hokie66)

If this is just a single bay two way slab supported on all four sides, hoshang is correct about the bending being greater in the middle. I thought we were discussing a two way flat slab or flat plate supported on a series of columns, because the pictorial representations show continuity.
We are not talking about the middle of the direction moments found (l1), rather we are talking about middle strip in the transverse direction (l2).

Quote (hokie66)

To use the terminology "column strip" and "middle strip" in an edge supported slab confuses the issues, IMHO.
We studied column strip and middle strip in (reinforced concrete design) course in our graduate. Column strip is a strip of slab having a width on each side of the center-line equal to one fourth the smaller of the panel dimensions l1 and l2. Such a strip includes column- line beams, if present.
A middle strip is a design strip bounded by two column strips.
When we find the positive and negative moments in a section, we divide these moments by the width of these column and middle strips to distribute our positive and negative bars across these strips.

RE: Two way slab proper detailing

That definition of the strips is the same as mine. And the column strip moments are always greater than the middle strip moments, therefore the reinforcement, both top and bottom, is greater in the column strip.

In a two way system, flat slab or flat plate, the same is true in both directions, as the loads must be carried in both directions.

Perhaps if someone else is reading this who has the McCormac text, you could clear up the confusion.

RE: Two way slab proper detailing

Quote (penpendrum)

But there are old designers that cant move on . Please enlighten me if you know. . .

I'm an old codger like Hokie, and, maybe the OP... I suspect Hokie, like myself, keeps up with current technology... I'm a bit of a techno junkie, aka, packrat. Hokie's pretty much on the mark when he notes the bent bar approach is uncommon; it was uncommon 45 years back when I first started. Other than existing buildings in the late 60's, I've never seen it used, or, been involved with a project using the method. I've seen it used with concrete joist systems of a hundred years back.

By being involved with this site, I try to keep up with current 'thought' and implement some of the material as I see fit. The OP may be a bit 'dated' but by reviewing the information in these forums may be able to reach a comfort zone with new approaches. I'm dated because I don't work in SI, well... it's always a matter of converting it to Imperial for my comfort level.

Dik

RE: Two way slab proper detailing

(OP)


Thankyou all For Participating . . . my thread

Hello Hoshang , hokie66 , and Dik
I was asking about 2 way slab . . I've just drawn the one direction of the two way slab for
simplification of the topic . .

By the mean of column strip I was talking of the bars parallel in its direction in which I believe hoshang is correct . .

I just want to know how do we continue the bent up bars ( where do we splice them ) for the continuous slab . . .

is it okay to develop the bars like this in the figure above (due to the limited commercial length available in the market) ?

* note that I did not draw the top cut bars on purpose to avoid confusion in my drawing . .

or it is required to alternate the bars like in the figure I mention at the start of this thread?

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