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

 

[hokie66]

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?

 

[SteynvW]

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?

 

[dik]

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

Dik

 

[Mechanicslearner]

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)

 

[hokie66]

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.

 

[Ingenuity]

maximum span of 12 metres

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.

 

[dcarr82775]

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

 

[dik]

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

 

[KootK]

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.

 

[dik]

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

Dik

 

[rapt]

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.

 

[hokie66]

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

 

[Mechanicslearner]

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

 

[Guest262653]

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.

 

[greznik91]

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:

 

[dik]

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

 

[BadgerPE]

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

 

[rapt]

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!

 

[Settingsun]

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.