Cantilever slab (blacony)

[mar2805]

Hello!
Im interested if you agree with the following.
Increasing the thickness of the cantilever part of the slab "CASE B" (left side from the support), from 20cm to 25cm, will probably reduce the need for reinforcement on the right side.
But since this is an continous slab, the problem remains on the right side from the support since the moment will remain "almost" the same.
Only way to deal with an long cantilever problem is to increase the thickness on the left side, and on the right side, to 25 cm.
Posttensioning is not a option here.

Interested to hear your observations.
Thank you

 

[HTURKAK]

No....Moreover The design moment shall increase for the dead load increase ..



Apparently , the right side slab , is a single span and the hogging moment at the support ( is for equilibrium ) is dictated by cantilever portion.


Not sure if this is the only option.. you may try tapering of the cantilever to reduce the DL( say 125 mm at tip and 200 mm at the support ) ..If the supports are RC beam or RC wall, you may increase the cantilever thk. but keeping the right side the same..

Will you post more details for the spans, support cond's.. ?

 

[mar2805]

Heres an plan view and a section of the building.
Masonry load bearing walls, 25cm thick, with an RC slab, on top of it.
Slab is an 2 way load bearing, wich has some positive impact on the cantilever part.
3 story building. Same floorplan for all stories. All dimensions in the drawing provided are in centimeters.

Tappering wich you suggested is not a option due to aesthetic reasons.

What aboth making an ribbed RC slab just on the cantilever part of the slab?
Im a bit skeptical aboth this solution, but maybe have some ideas on how to deal with it.


Another option to provide T beams on the faar left and right side, and also in the middle of the balcony. But would like to try first without them.

 

[HTURKAK]

Ribbed slab with clay brick infill etc. could be an option.. 3.0 m cantilever is substantial amount. ..If high seismic region, in general the codes do not allow..

If prestressing is not an option, I will suggest the slab thick. at cantilever section 150 mm or better 120 mm. Provide perimeter beams so, the cantilevers are the beams rather than slab..

 

[mar2805]

Ribbed slab with clay brick infill .

I would not use the brick infill.
Just pure ribbed RC slab.
There will be substantinal compressions stresses on the support of the canteliver. I dont think brick is a wise solution there.

 

[dik]

Can you go to 150 at the end?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[mar2805]

Yes I could, but the investor doesnt like the idea od the balcony "ceiling" being tappered.

 

[mar2805]

Your opinion on this solution.
Anyone done something like this?

The balcony part beeing an ribbed slab.
Ribs are 15/30cm and the slab part is 10cm. Ribs are spaced 50cm CL.
The volume of the balcony (beeing first an full 20cm thick slab) is reduced from 8,15m3 to 6,40m3.
I see a problem with shear above the support on the ribbed part of the slab beeing only 10cm thick (just the slab part)....but have to check it first.

 

[molibden]

You can cover the bottom of the slab in a way it will look flat.

Deflection is probably an issue. You can precamber formwork.

 

[HTURKAK]

What is the depth of the lintel ?? how the compression force of the 15/30 rib will be transferred to the 20 cm slab? Will you post the rebar detail ?

For 3.0 m cantilever, the deflection could be issue.. suggest you run analysis and calculate long term deflection..

 

[molibden]

Creep deflection with cracked concrete section will govern the design.

 

[mar2805]

Hello!
I had to increase the rib depth to 35cm to satisfy the creep deflection check with cracked concrete section, and also the compressive stresses on the bottom of the ribbs above the support (wall).
Question regarding the anchorage lenghts of the top bars.
Would you provide the full anchorage lenght at the end of the canteliever or just one hook going down to the bottom of the rib?
If I provide the lenght that the code is asking me (Eurocode), I need to have 2 bends like the red line above the picture, couse the rib height just isnt enough.

Thank you.