Dropped Balcony support

[steelnz2003]

I have a 100mm slab drop for a balcony cant liver concrete slab is ti ok to extend the lower bars of the slab to carry the negative moment of the slab
so the bars will be in positive moment inside the slab then extend to carry the balcony

 

[KootK]

Sounds like spurious detailing. I'd recommend posting a sketch of what you've got in mind. This has come up a few times recently and a lot of good info has been posted. Hopefully somebody can reproduce those threads.

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.

 

[hokie66]

Agree with KootK. 100 is too thin for a balcony slab, even before you step it.

 

[Once20036]

The way I read this, the step down from the inside to the outside is 100mm. No idea what the slab thickness is.
The OP wants the top bars for the balcony to be the bottom bars from the interior slab - bottom bars become top bars.
I`m not sure what happens to the compression at the step in the slab.

As KootK mentioned, a sketch would help.

 

[DETstru]

It would also help to know what the plan looks like. Two-way slab? One-way? PT? Beams? etc

 

[steelnz2003]

Hi all
Teh slab is 200mm attached a detail to give you an idea of what I am planning to do

 

[hokie66]

No detail attached.

 

[steelnz2003]

Sorry tried it again hope it works

 

[hokie66]

What needs to happen there is to take the external slab soffit level back into the building some distance, I suggest 600. That would avoid the weakened plane at the wall. And the little unreinforced nib at the outer edge which is shown to be formed concrete for a gutter...that will break off eventually. Poor detailing, looks architecturally driven.

 

[LittleInch]

That detail looks suspiciously like the one in this post.

http://www.eng-tips.com/viewthread.cfm?qid=415779

Having a cold joint and all tension taken by the bars looks like a poor design to me. Ok theit is a slope away but over time water will get in and rust those bars. The actual construnction might vary as well.

Continuous slab or bolt the thing into the wall.

There have been enough collapses of balconies to show that you need to take great care and get it rightin design.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[steelnz2003]

Thanks all

hokie66 yes the detail is by the arch. can not do much really


LittleInch Thanks I agree it is critical detail as no other load path

 

[hokie66]

steelnz,
You have no choice. You are in charge of the concrete design, and what the architect has drawn does not work. So saying that you "cannot do much really" is irresponsible. Architects can be made to face reality.

 

[LittleInch]

I had another look at the drawing and you cut it off a bit short - looks like the false floor end detail is very poor also and could easily collapse.

Put down your thoughts in writing to the architect and tell them to think again - far too many deaths and collapses to simply shrug your shoulders.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[oldestguy]

Even me, who did very little concrete, design sees it will fail due to almost no resistance to shearing at the "support". The title "dropped" fits this perfectly.