Flat slab maximum span allowed?

[Mechanicslearner]

Hello,

I am designing flat slab for G+2 college building. The area is 12 x 25 metres and is it possible to design flat slab by providing column only at edges but not in the middle? I designed using staad pro by giving columns at edges and deflection is within allowable limits (DL + LL) so maximum span of 12 metres is allowed for flat slabs? please let me know

 

[dik]

With edge beams, you could run it as a 2-way flat plate... that would likely give you the minimum slab thickness; two 12m x 12.5m panels and you may get by with 250 or 300 slabs. I'll take a gander at the code... haven't done a 2-way plate in a couple of decades... don't know what the changes are.

Dik

 

[hokie66]

dik,
Please don't encourage him. He should listen to rapt. And so should you, if you think a flat plate is appropriate for those spans.

 

[rapt]

dik,

If it has edge beams it is a 25*12 two way slab. But at 25 * 12 it is greater than 2:1, so the slab at the centre of the 25m is one way. You cannot tell it that it will be anything else. I doubt that treating it as a flat slab with 2 spans one direction and one in the other will help much anyway. I would not even do it PT at 300 thick. For a 12m single span PT probably about 330mm thick.

steveh49,

I did not save the run. Standard Australian creep and shrinkage figures with 32MPa concrete (about 4600psi). No compression reinforcement, only the tension reinforcement required for strength.

 

[dik]

Not to encourage him, but with stiff beams all around, the thickness of the slab between can be the clear span perimeter of the slab divided by 140. Slab thickness can be 340mm assuming 400 wide beams. There are some other requirements to ensure perimeter beam stiffness. See annex b of CSA A23.3-14. I didn't remember what the factor was, but, if the slab were continuous in all directions the thickness could be clear perimeter / 16o, or 300mm. I wouldn't do it, but, it's allowable.

Dik

 

[hokie66]

Never heard of that perimeter criteria, but span/depth ratio is a poor substitute for deflection calculations done with consideration of all the factors listed by rapt above. At one time, I was of the opinion that deflection calculations for concrete were just black magic, but I have reformed...a bit.

 

[dik]

Hokie: there are even tables for greatly reduced +ve and -ve moments using two way action... it's likely a carry over from several decades back... and, I haven't used it for a couple of decades. If I recall, it used to be the clear span perimeter divided by 180 and 200 respectively... just too far back and too many things have nudged it out of my memory.

Dik

 

[hokie66]

Right. Time to enter the 21st century, I think. Over the years, many flat plates and flat slabs have suffered from excess deflection, partly because that sort of rule of thumb was used. We know a lot more about the serviceability problems of flat plates now.

 

[rapt]

dik,

But the perimeter is 25 + 25 + 12 + 12 = 74m / 140 = 528mm! Less a little for the beam width reduction in span lengths. You would only get 340 if you have a beam across the middle of the slab as well as the edge beams, reducing it to 12 * 12.5m panels. Rereading the question, not sure what he wants. Everyone had assumed only beams around the edges.

Those factors in CSA for the 2:1 case are assuming a LOT of moment redistribution (about 30%). Where it is redistributing to in a single rectangular panel I am not sure. But you cannot allow for the redistribution in normal deflection calculations, as redistribution creates extra deflection. Deflection calculations need to be based on the elastic moment/stress distribution or need to calculate the extra deflection caused by redistribution.

 

[hokie66]

The OP never mentioned any beams, so I assumed he didn't know they were needed. I just hope he will get some help, or he will build a hammock.

 

[dik]

rapt:
Two issues... It's possible to clear span the structure by the introduction of interior beams and reduce the slab thickness in the process. Breaking the 25m span in half by using a beam and creating two 12mx12.5m panels to deal with the slab thickness... It's not the full 12X25, but adding a beam mid length...

The 25m could further be broken into three panels of 8.33m, or even four, and the slab thickness could be further reduced. The original proposal suggested by the OP was unworkable.

Hokie:
I thought we had established that the original 12mx25m panel was unworkable with a 300 slab. I was just providing a solution to reduce the slab thickness by reducing the panel width... I didn't have a code handy at the time and was unable to do anything but guess the resulting slab thickness required.

If challenged for a solution that required reinforced concrete (RC), I'd likely have a column in the four corners and one (if allowed, at mid span of the long dimension) and likely break the 25m surface into 4 panels and treat it as a 2-way slab or maybe even use a 'waffle slab'. If the structure did not require to be RC, I'd likely use structural steel as my first choice.

Lateral support is a whole other problem... in particular in a seismic zone.

Dik

 

[Mechanicslearner]

The flat slab span is 5 m x 12 m not 25 m x 12 m ... I am placing columns along 25 m span. I am also placing column along 12 m span in the middle. So flat slab area is 6m x 5m .. and slab thickness 0.2 to 0.25 m

 

[jayrod12]

That is not what you described in your original post. Those are much more reasonable spans and it is likely you could get something to work however you are still running at L/30 which you will still want to keep a close eye on long term deflections.

 

[Leo Baldwin]

Don't ban me here you guys.
I work for a manufacturer that makes forms for different depth concrete joists (24" o/c each joist being 5" wide - one way span).
We just did a community Saferoom (casino) that spans 40 feet @ 100psf LL and 100PSF Net Uplift.
The concrete depth including 6" slab was 20".
Lots of #8 rebar...

Not advertising anything - just agreeing with some of you that flat slabs should be a thing of the past...

 

[jayrod12]

That doesn't seem too unreasonable Leo. That's a L/d=24 right around where it would be expected.

 

[rapt]

Leo,

I do not recall anyone suggesting that flat slabs should be a thing of the past. Just antiquated and outdated design methods.

 

[dik]

Are you talking a two-way slab with edge beams? These can be an architectural feature and can be cost effective, although not common, they are not archaic.

Dik

 

[bhiggins]

Recently had a 4-column concrete structure and contemplated a flat slab. I ultimately decided to break apart the spans and do a frame due to the large thickness of concrete required.

I figured you would need at-least 2 edge beams in the long span, and if you are forming 2 beams, might as well form 5 for a stiffer structure.

 

[dik]

bh: a 6" slab may have worked with those proportions... with design moments from Tables B1 nand B2. treating it as a 2-way slab with stiffened edges.

Dik