reinforcement of concrete slab 2

[greznik91]

Hello, Im wondering how do you guys solve the problem when a concrete slab has a sudden change of thickness (above a wall).
I have a cantilever slab (balcony) on the left side - 18 cm thick and the rest of the plate is 12 cm.

I have to model this plate as one element (because of cantilever part that has to be 'fixed') but im wondering what is the best solution for reinforcement detail. I dont like the first sketch very much because i think there is too much unreinforced concrete above (upper reinforcement mesh) i think it may crack?!

Im also not a fan of 2nd sketch since the reinforcement is not in one line (upper reinforcement in the cantilever part is disconnected from the rest of the slab - there is a disconnect of reinforcement at the area with the biggest bending moment.

Id really appreicate some suggestions or sketches.
TNX

 

[hokie66]

There is not much wrong with the first approach, if the 120 slab is deep enough to resist the negative moment from the cantilever. 120 is quite thin. You could add some top reinforcement in the cantilever if you want, but keep plenty of cover.

The second approach is dead wrong, as you do not have continuity of the reinforcement. But you know that.

Your situation is the opposite of a normal balcony, as you want the step the other way for weatherproofing purposes.

 

[molibden]

Why do you have this situation in the first place? It seems odd.

 

[jayrod12]

Why do you have this situation in the first place? It seems odd.

Agreed, If anything I usually expect to see the opposite where the exterior slab is lower than the interior to account for wearing membrane/waterproofing etc.

 

[ahypek]

I typically terminate the top rebar with a hook on the thicker slab and develop the top reinforcement from the balcony into the thicker slab.

 

[bhiggins]

This is what I would do, but I wouldn't rely on the top reinforcement at the face of column unless you can fully develop the hook.

 

[ahypek]

I agree with bhiggins detail.

 

[rapt]

bhiggins,

That is NOT a good detail. While it will have sufficient strength, it would be much better to fully develop the hook. There is nothing stopping a shallow angle crack at the step from basically causing delamination .

 

[bhiggins]

Improved detail attached? Less builder friendly though. I don't see a way to fully develop the hook to catch the crack so alternative measures must be taken.

 

[KootK]

The sketch below shows what I might do. Very similar to BHiggins' latest. In terms of strict strut and tie methodology, there are a couple of holes. I consider it acceptable for a lightly reinforced slab with distributed flexure though. And it's easy to build.

Structurally, for a ~2.5" step, I think that OP's original option one would be fine. My only concern there is durability. With ~3.5" cover on the top bars, I might be concerned with top side crack widths should the balcony slab ever actually crack. I'm curious to know what others think of that supposition.

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.

 

[OldBldgGuy]

I'm lost, as are others, on why this occurs in the first place, but I also fail to understand why you would attempt to use any of the concrete above the interior top surface since the weak point is still the transition. Design a 120mm (or whatever it takes) slab right through and assume it's carrying an extra 60mm of concrete topping outside.

 

[bowlingdanish]

I'm lost, as are others, on why this occurs in the first place

I assume it is an external/internal junction, and internally there is a lower slab level to allow finishes to be installed. Also assuming it's carrying a 60mm unreinforced topping is not a good idea in my opinion. It might be OK for strength, but a balcony slab should have reinforcement near the top face to control cracking.

I agree with KootKs detail. Or, for larger steps, a Z bar so the vertical face of the step can fully develop.

 

[kipfoot]

I'd go with topping per OldBldgGuy. I think it's best for ease of construction and future maintenance.

 

[dik]

I usually use the third one, but use a hooked bar to develop it. KootK's detail works, but, is slightly more costly.

Dik

 

[rapt]

bhiggins,

The sloping bar - yes, as per Koot's detail, with a bit more slope than yours.

The open U,s no, development is still inadequate, and would be painful to install.