designing cantilever part of RC slab - help

[n3jc]

Hello, Im hoping for some help.
English is not my first language so sorry in advance foor poor language.

My upper floor is larger than a floor bellow so I have to deal with a cantilever part of a slab that is 200 mm thick. Cantilever is only 1 m long, but it is loaded in the end by a wall (mansonry + RC vertical ties) that is supporting a part of the roof structure.

I have calculated that design force in a RC column/vertical tie is 65 kN.

I have controlled punching shear at the edge of a slab and shear strenght of a slab at a wall bellow.

Im wondering - for control of shear - can I consider that slab is 1000 m wide and 200 mm high cross section?
Can I consider that single/point load (from column at the end of cantilever slab) is transfered at 30 degree so my effective width for slab control is 1200 mm (for conservative reasons I took 1000 mm).

 

[jayrod12]

If it were me I'd probably be using 800 mm wide rectangle. The width of column plus 2 times the slab thickness.

 

[gmd255]

Is it common to 'solve' problems like this with 'just' a thicker slab?
Do slabs have such a large shear capacity?

Not really my field but I was always wondering how do they solve situations like this.

 

[n3jc]

Is this reinforcement detail allright?

I was thinking about using rebars (red line) as bending moment reinforcement above a support (wall) and at the end as shear reinforcement (bent at 45 degrees).

Thoughts?

 

[jayrod12]

Even though theoretically that rebar passes the shear plane, I'd ignore any contribution to the shear resistance and just thicken the slab as required to get the shear resistance required.

I'm fairly certain we have hashed this out in a couple of threads within the last few months.

 

[rapt]

I do not like that reinforcing detail at all. You need to continue the top reinforcement to the end of the cantilever and provide a hook/cog at the end.

With the slab thickness and loading that you have, I doubt that you would need shear reinforcing. But if you do, either thicken the slab or add ties. Your detail would not be adequate in my opinion.

 

[n3jc]

tnx for reply but you havent really explained why do you think that?
as far as I can see the top rebars are anchored and can have two functions:
- as bending moment tension reinforcement at the top of a slab above a wall
- as a shear reinforcement (bent part) that is also anchored

 

[rapt]

You have tension moment in the top of the slab over the whole length of the cantilever. But at some arbitrary point you turn down your top face reinforcement so that there is none.

To develop shear capacity as well as flexural capacity you need tension face reinforcement. You have none.

You bars are doing nothing after they kink downwards other than creating a difficult reinforcing bar bend requirement.