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[vikram.jeet]

Er Ramu ji ,,

The shown bracket arrangement from column seems OK, in case section has been structurally sufficient .
But there is need to tie the brackets on both sides with a steel beam which is sufficient to take balcony loads + Parapet/ Railing + some extra for few planters.
Also the fastners , fastening the brackets with columns must be adequate as per design .

[sohel]

Hello Mr Ramu,
I would not prefer to use steel sections below balcony that you have use in the image. I will look ugly.
Again recasting the cantilever slab with higher dia reinforcement like 10 or 12mm will not help.Reason is that the section-150mm is not adequate as simple deflection rule has not been followed. Atleast basic value of deflection for cantilever isspan/7and after some modification ratio for tension and compression, it will give you deflection and taking permissible limit of deflection, you will find a section size higher than 150mm. Please calculate it from IS:456-2000.
My suggestion is that, If you can extend end beams upto to the cantilever ends, then it will be slab supported on three side and in that case 150mm thick slab will be sufficient. In that case, you can remove top cover of concrete and recast.
If yo can talk to architect and let him see it in 3D views. If he allows for this, it will be better than using Knee below the cantilever.

Reason of Cracks:
1 .Actually there is two sunken portion and one staircase just to the cantilever.Through all of which reinforcement can not be contiued to the balcony.
2. Section is not adequate
3. Theoretically reinforcment(8mm dia) is ok but normally 8mm dia reinforcment is avoided in cantilever section as it has less stiffeness.

[vikram.jeet]

Using Steel brackets :
Aesthetic reasons as discussed by Er Sohel are very much pertinent,, and ugliness needs to be camouflaged , if possible , architect of project consulted .

Deflection must be checked ,, Deflection check to be done using load factor 1.0 . The reinf of 8mm @ 150mm crs as provided by u is for Load factor 1.5 .

Thus % steel for Deflection check needs to hypothetical ie ( actual% / 1.5 ).

Just wanted to see deflection check :
Eff depth = 15 - 1.5 - 0.5 = 13 cm
% Ast = 3.33 / 13 = 0.256 %

% Ast for deflection check ( hypothetical) ~0.256/1.5 = 0.17 %

As per IS 456
Themultiplier for tension reinf = 1.4

Assuming half reinf bars as bott mesh
Comp reinf = 0.08% at LF = 1.0
乘数for comp reinf = 1.05

Hence Defl factor = 7* 1.4 * 1.05 = 10.3

Span/ eff depth = 10.3
Eff depth = 1.52/10.3 = 0.147

Overall depthneeded = 14.7 +2.0 = 16.75 cm About 6.5 "

The remark by Er Sohel is very right ,, Depth is deficient from deflection point

[ramu_se]

Thank you Sohel Sir & Vikram Sir.

The crack is not developed further, they have applied repair material to the cracks.

The project owner has no issue with providing a steel section since that can be camouflaged as Vikram sir said.

Sohel Sir,

1. Since the slab is cast, can we extend the beams below the slab from column support?
2. The beam will be cantilevered from the column, is this ok?

Thanks & Regards,
Ramu矿.

[vikram.jeet]

Will await reply fm Er Sohel

Just to say on my own :

You can provide cantilevered rcc beam on either side and tie them with a common rcc beam in front ,, underside the existing rcc cant slab .with all beams of same size .
Use rebaring with chemical Grouting from reputed rebaring Agency making holes in columns for top and bottom bars of beams . Chemical grout must be of proven quality .

Do not see results of crack not widening as assurance of structural safety on a later date when any overcrowding or otherwise create mishap.

[spsvasan]

[ramu_se]

Dear Vikram Sir,

If we provide, cantilevered RCC beams on either side and tie them with a common RCC beam in front, under the existing cantilever slab, then the cantilever slab will get support on all four sides.

How would the bending behavior of the slab be?

If it bends in a shorter direction, we need to provide bottom reinforcements.
Since the existing cantilever slab does not have bottom bars, how to rectify this?

Thanks & Regards,
Ramu矿.

[vikram.jeet]

Since there is no bottom reinf in existing slab :

Please make a check on four edges supported slab :
Three edges simple support and one edge on room side continuous,, refer Formulas for stress snd strains by Young,, and see BM 's along short direction
At top for -ve M along room side
At bottom +ve M about mid span

For top - ve moment at edge on room side ,, your provided top reinf would be ok even with less effective depth

For +ve M at bottom - since no reinf exist,, pl check it as pcc slab for tensile stress = Mbott / bd^2

Even considering one meter width along short span,,
Loading in residential sla w =

w = 0.15*2500 + 150(flooring) + 20(ceiling finish) + 200 LL
w = 745 kg/m2

For a propped cantilever under ur load
-ve moment at support = w*L^2 /8

+ve moment near mid span = wL^2/16

Just checking stress a bottom in pcc M20

Mbott =745*1.40^2/16= 92 kgm /m

Tensile stress = 92*100/(100*15^2/6) = - 2.4 kg/cm2

But as per Working of bott moment from Book FFSS by young, this value will be further less

然而试图提供一个我sh at bottom with rich 1:3 cement :coarse sand mortar using epoxy treatment .Pegg new mesh at bottom to existing underside of slab .