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Flat slab maximum span allowed?

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

RE: Flat slab maximum span allowed?

I don't think there is a maximum span, but there are certainly practical maximums for all systems, including flat slabs. How thick is your slab? Is it post-tensioned? Why do you want to use so few columns? What factors were considered in your deflection calculations?

RE: Flat slab maximum span allowed?

12m is quite a long span for a flat slab and it will be heavily reinforced, will it not be cheaper to look at
waffle slab rather that would be more effective?

RE: Flat slab maximum span allowed?

Beam and Slab, or concrete or steel joists with a slab over would be far more economical... Can you use steel? composite steel?

Dik

RE: Flat slab maximum span allowed?

(OP)
Hello all,

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?

Sorry, but if you think a 250 slab will span 12 metres with acceptable deflections, you (or your computer program) don't know what you are doing. Get some help.

RE: Flat slab maximum span allowed?

Quote (Mechanicslearner)

maximum span of 12 metres

Quote (Mechanicslearner)

thickness of slab is 250 mm

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?

My God Man, stop. Something is wrong with your model.

RE: Flat slab maximum span allowed?

Ingenuity:

"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?

Got precast hollow core where you're at? It works well for predominately one way systems. Advisable span to depth for light loads is about 45. And you can sometimes stretch that a bit.

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?

Unless your loads are high, you can easily do 12m with 300 HC slabs.

Dik

RE: Flat slab maximum span allowed?

Staad gives short term uncracked deflections. Multiply them by about 6 to get realistic long term deflections.

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?

dik, I assume that you mean 300 + structural topping.

RE: Flat slab maximum span allowed?

(OP)
Thanks for all your suggestions, Allowable deflection for flat slabs is Span/250 right? I used Etabs too it gave around 38 mm where allowable deflecttion is 48mm , so the results are not acceptable. Data M25 concrete, Fe500, slab thickness 250 mm , DL - 2.3 KN/m2 + self weight and LL - 3 KN/m2.. this is for 2 storey building

RE: Flat slab maximum span allowed?

You just seem to "forget" long-term effects (creep and shrinkage) and cracking. That would increase your deflection to about 4 or 5 times that elastic deflection... Just saying.

RE: Flat slab maximum span allowed?

What you are doing is not OK, do you even imagine what 12 m x 25 m looks like in real life? Also, you said the whole thing is supported on columns? Have you checked slab shear/breakthrough?
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?

Hokie:

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?

I recently had a similar project come across my desk that was spanning 40' in one direction and 34' in the other so a 2-way slab was requested from the client. Due to many concerns with such large spans we alternatively proposed concrete joists, intermediate column lines, and a post-tensioned slab. All of these were rejected for one reason or another until we had an owner meeting in which his concerns for cracking were clearly stated. Upon discussing the projected long-term deflection and the likelihood of serious shrinkage cracking from the ~18" slab, the decision was made to use concrete joists. I would suggest the same here.

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?

Mechanicslearner,

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?

Rapt, would you mind posting your creep coefficient, shrinkage strain, sustained load, tension reinforcement and compression reinforcement assumptions. I'm trying to match results. Thanks in advance.

RE: Flat slab maximum span allowed?

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

RE: Flat slab maximum span allowed?

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.

RE: Flat slab maximum span allowed?

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.

RE: Flat slab maximum span allowed?

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

RE: Flat slab maximum span allowed?

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.

RE: Flat slab maximum span allowed?

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

RE: Flat slab maximum span allowed?

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.

RE: Flat slab maximum span allowed?

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.

RE: Flat slab maximum span allowed?

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.

RE: Flat slab maximum span allowed?

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

RE: Flat slab maximum span allowed?

(OP)
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

RE: Flat slab maximum span allowed?

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.

RE: Flat slab maximum span allowed?

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...

RE: Flat slab maximum span allowed?

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

RE: Flat slab maximum span allowed?

Leo,

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?

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

RE: Flat slab maximum span allowed?

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.

RE: Flat slab maximum span allowed?

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

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