×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

reinforcement of concrete slab
2

reinforcement of concrete slab

reinforcement of concrete slab

(OP)
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



RE: reinforcement of concrete slab

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.

RE: reinforcement of concrete slab

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

RE: reinforcement of concrete slab

Quote (molibden)

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.

RE: reinforcement of concrete slab

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.

RE: reinforcement of concrete slab

I agree with bhiggins detail.

RE: reinforcement of concrete slab

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 .

RE: reinforcement of concrete slab

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.

RE: reinforcement of concrete slab

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.

RE: reinforcement of concrete slab

Quote (OldBldgGuy)

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.

RE: reinforcement of concrete slab

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

Dik

RE: reinforcement of concrete slab

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources