Flat slab maximum span allowed?
Flat slab maximum span allowed?
(OP)
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
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






RE: Flat slab maximum span allowed?
RE: Flat slab maximum span allowed?
waffle slab rather that would be more effective?
RE: Flat slab maximum span allowed?
Dik
RE: Flat slab maximum span allowed?
Thanks for your replies.
I am using Fe500 for reinforcement and thickness of slab is 250 mm and I am using few columns for more space. I considered live load with floor load and dead load with (self weight and PCC)
RE: Flat slab maximum span allowed?
RE: Flat slab maximum span allowed?
So single span of 12m and 250 mm thick - L/D is 48!
Not going to work as RC. Even if post-tensioned it will be struggling to work. It will NOT be economic in most markets, and realistic deflections will be problematic, especially if brittle partitions/finishes are to be accommodated.
RE: Flat slab maximum span allowed?
RE: Flat slab maximum span allowed?
"realistic deflections will be problematic, especially if brittle partitions/finishes are to be accommodated." With all the strand... the upward camber will likely be permanent <G>... deflection may not be an issue...
Dik
RE: Flat slab maximum span allowed?
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: Flat slab maximum span allowed?
Dik
RE: Flat slab maximum span allowed?
I will not go into my usual rant about understanding the design area and the software you are using.
RE: Flat slab maximum span allowed?
RE: Flat slab maximum span allowed?
RE: Flat slab maximum span allowed?
RE: Flat slab maximum span allowed?
I guarantee you that deflections will be way larger than they should be/are allowed. Also If there are gonna be other brittle materials/walls on the top of a slab they will crack because of large slab deflection.
Can you post a plane view of your slab/columns? What you are doing is wrong, common sense should tell you that.
Most likely you ll need to do some beams that are partly integrated into slab (and they wont be that small in dimensions) and they span in short direction (12 m).
Something like that:
RE: Flat slab maximum span allowed?
Yup... strange though, I thought I had responded but for some reason it didn't show up... I've noticed that with a few posts lately. Either my mind is going... or there's a bit of a sieve.
Dik
RE: Flat slab maximum span allowed?
As alluded to by many above, please take some time and read the literature associated with your software, as well as your required design code to understand the limitations of your software as it pertains to the requirements of your design code!
I should also thank several members of this community for sending me down the right path...
http://www.eng-tips.com/viewthread.cfm?qid=427872
RE: Flat slab maximum span allowed?
to give you an example of the difference in possible deflection calculations for RC slabs from different software,
if I run a 12m span one way slab single span (I assume you have edge beams so it is not a flat slab, and at 25*12 it is actually a one way slab) with 600mm *600mm 4000m high columns above and below in RAPT, the deflection is about 216mm including for cracking, shrinkage and creep effects. So L / 55!
If I do the same run assuming uncracked short term deflections, the deflection is 29.4mm. Similar to what you are getting.
The latter is what a program like Staad, Etabs etc that do not consider cracking or long term effects will give to you.
The former, is what you will get in about 30 years after you build it.
Learn what is required for the calculations you are trying to do. And check that the software you are using does it. Just because a program gives you a deflection value, does not mean it is a design deflection value. Analysis programs that do design calculations often give only short term uncracked deflections for concrete members. These are meaningless for design. It is your job as an engineer to understand this!
RE: Flat slab maximum span allowed?
RE: Flat slab maximum span allowed?
Dik
RE: Flat slab maximum span allowed?
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.
RE: Flat slab maximum span allowed?
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.
RE: Flat slab maximum span allowed?
Dik
RE: Flat slab maximum span allowed?
RE: Flat slab maximum span allowed?
Dik
RE: Flat slab maximum span allowed?
RE: Flat slab maximum span allowed?
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.
RE: Flat slab maximum span allowed?
RE: Flat slab maximum span allowed?
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
RE: Flat slab maximum span allowed?
RE: Flat slab maximum span allowed?
RE: Flat slab maximum span allowed?
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...
RE: Flat slab maximum span allowed?
RE: Flat slab maximum span allowed?
I do not recall anyone suggesting that flat slabs should be a thing of the past. Just antiquated and outdated design methods.
RE: Flat slab maximum span allowed?
Dik
RE: Flat slab maximum span allowed?
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.
RE: Flat slab maximum span allowed?
Dik