Thick RC slab to avoid beams?

[mats12]

GREETINGS

I have heavily loaded RC slab (design load is 25 kN/m2). Slab is also additionaly loaded by two point loads (steel columns that are anchored in a slab). One acting on a free edge and the other one not that far away.

The span of a free edge is 4,80 m. The contractor suggested to make a part of RC slab 300 mm thick (istead of 200) to avoid formwork for beams(I suggested 2 RC beams instead that are partly integrated in a slab and support steel columns - point loads).

Any experience with this kind of stuff.

I already controled RC slab for capacity and deflections and it seems OK, but it feels off to me.

Whats your opionion?

 

[SlideRuleEra]

Simply supported flat slabs work for heavy loads, but a span-to-depth ratio of 16:1 (4800mm/300mm) is about the maximum practical. Also, the geometry keeps the slab from being truly simply supported... free edge span is 4.8m, but interior of span is a little longer (5.0m) and to a limited extent a two-way slab. Heavy point loads near the center and edge are of concern, too.
Overall, agree with you, I would not use a 300mm thick flat slab.

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[JAE]

I think the back-span of the cantilevered slab you have should be 300 mm. Gives you a better depth to reinforcement.

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[JAE]

But if you are incorporating an internal beam in the outer edge of the 300 mm slab, then perhaps the 300 mm slab is spanning from that edge-internal beam back to the interior wall and the backspan issue in my images above doesn't matter so much.

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[rapt]

I always find drawing to scale helps visualize better!

The slab carrying the point loads is much stiffer in the other direction, with a 5m span and a 3.8m back span, compared to a 3.5m cantilever with a 3m back span.

I would think that about 80% (guess) or more of the load would be carried in that direction and the remainder by the cantilever. You could do analysis in both directions to see how much deflection you get in both directions to see where the loads are wanting to go if you do not have FEM software.

It is really a 2way slab and as JAE suggested needs a "column strip" as the support at the free edge.

 

[hokie66]

Agree with rapt. This will act more as a two span slab than as a propped cantilever.

I would use a slab of the required thickness over the entire area, and think it might be more than 300. 25 kPa is a lot of load. Must be heavy vehicles or such, as that amounts to 2.5 metres of water.

 

[JAE]

Well the two-way action behavior also depends a lot on the layout and amount of reinforcement in each direction. If you don’t reinforce the edge much then the slab will crack and behave more like a cantilever.

The 200 mm thickness is OK as long as you appropriately reinforce the edge.

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[SlideRuleEra]

The way I read the OP's question it is about accepting Contractor's proposal to use a 300mm flat slab instead of the OP's 200mm slab that includes a longitudinal T-beam under each of the two point loads:

If the OP's T-beam under the applied 130kN (29.2 kip) point column load is deleted for any reason, better check (and address) punching shear.
If T-beam under 25kN (5.6 kip) applied point load at the edge is deleted, check it for punching shear too. (Lighter load but less critical perimeter).

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[rapt]

JAE,

Yes, it depends how you reinforce it. I am suggesting to reinforce it as it wants to act! Not force it to act otherwise in some yield-line fashion meaning it has to crack and deflect more in some areas to redistribute moments to where the reinforcement has been supplied. You could reinforce it as a pure cantilever, but it does not really want to be one.

 

[clarke1973]

Haven't read through all the replies so I'm assuming you have received the right advice by now, but attached is a useful guide for determining appropriate floor slab systems. It's my go to for scheme design.

 

[hokie66]

I'm not going to propose a detailed design for the OP, but my quick thumbnail calculation shows that providing enough reinforcement along that edge in a 200 slab would result in the slab being over-reinforced. It should work as a 300 slab throughout.

 

[TehMightyEngineer]

clarke: That's a nice graph; I have a few similar ones but nothing that concise.

You wouldn't happen to have a clearer copy would you?

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)
American Concrete Industries

https://americanconcrete.com/

 

[clarke1973]

Unfortunately I don't have a clearer copy, but it is from The Ove Arup Structural Scheme Design Manual. You should be able to source a copy online

https://www.scribd.com/doc/50817571/STRUCTURAL-SCHEME-DESIGN-GUIDE-BY-ARUP

 

[hokie66]

Returning to this slab, I worry that some may have glossed over the magnitude of the loading. The OP says his superimposed uniform load is 25 kPa, which is more than 500 psf. And there are two substantisl point loads of 130 kN and 25 kN (about 29 kips and 6 kips respectively). The OP didn't elaborate as to what kind of loading this is, but it is heavy.