Hollowcore and Balconies

[Galambos]

I am reviewing a cantilevered CIP concrete balcony bearing on a steel WF and attached to a hollowcore plank. the engineer has detailed the top reinf steel at the base of the cantilevered balcony to drop to the bottom layer about 12" from the face. He has also placed a hooked bar in the top layer that does not get developed into the building.

has anyone ever come across a similar detail? im trying to figure out its origin, because it looks really scary to me.

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CIP Balcony I Hollowcore (building side)
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[271828]

If (and that's a big IF) I understand you correctly, the top steel becomes bottom steel very quickly, probably while still in the negative moment region. If so, that's just wrong.

Could the CIP slab be spanning the other way too, so he's not really counting on negative moment strength there?

 

[Galambos]

271828,

thanks for the response. yes, you are correct, the top steel transitions to bottom steel quickly. and no, the balcony is a pure one-way cantilever.

needless to say, this drew some folks attention.

 

[JAE]

Galambos - where does the top steel "drop"?

The "correct" detail would have top steel hooked at the end of the cantilever, and then extending to the right (in your picture) back into the topping slab over the hollow core - developed PAST the point where it's no longer required to resist negative moment.

Bottom bars in the cantilever, if even deemed necessary, should be separate bars that could extend to, and just over, the wide flange.


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* bottom bar
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[Galambos]

JAE,

i agree, to me, it looks like a very nice hinge at the base of a cantilever.

here goes nothing...

top bend occurs 'pictiorally' at 12" from beam CL, although the transition is not dimensioned. the top bar gets developed into a 2.5" nonstructural topping slab.

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and the middle layer hook bar....
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[JAE]

So there's a middle bar. Do you think the designer was relying on the middle bars for the main reinforcing of the cantilever (i.e. a smaller "d") and perhaps doing this to add cover to an exposed balcony?

I'd do whatever you can to question this and even go so far as to perform my own calculations to back up my concerns.

 

[ChipB]

Galambos,
RUN!!!
Is it built or are these questions coming up during construction?

The whole thing doesn't sound good. Middle bar isn't going to help as it appears hooked at WF beam CL. Lap going into the topping sounds really bad. I think you'd barely have enough cover if you put the bars directly on top of the hollow core. How's that going to develop? Besides that, I'm sure no epoxy bonding agent would have been used between the topping and hollow core. AND aren't you supposed to have a slight drop in elevation when you go from inside to outside?

 

[hokie66]

The cantilever design which you are reviewing and as you describe it is clearly and grossly wrong, as I think you already knew. Are you asking for advice as to how to proceed now? It would help to know your role and your relationship to the responsible design engineer.

 

[archeng59]

the reinforcing does not appear to be detailed properly for the cantilevered portion.

 

[ChipB]

Thanks Hokie66. I got lost in the details. Couldn't see the forest for the trees.

Yes. It is important to know your relationship with the EOR. I have gone to the EOR with my concerns during construction and either had things clarified, or corrected. Much rather handle it that way. If it is constructed as shown, it would allow him/her to design a repair and hopefully avoid legal issues. Their E&O insurance might have to pick up the tab somewhat.

Much easier to be friendly and helpful. There isn't a single one of us who hasn't made a mistake in the past.

 

[Galambos]

chipB, i left out the step from the building to the balcony as i was recalling the detail sitting on my office desk from my home.

let's call my role 'peer reviewer.'

i dont think developing into the slab is a problem if they can set the bars with enough cover. the slab might be thin, but it is very wide and very deep, similar to using hairpins in a metal building. also, correct me if i am wrong, but the difference between a structural and non structural slab is the bond which is necessary for the composite action with the planks. given that portland cement concrete is used, structural or nonstructural makes little difference in this situation so long the appropriate factors are accounted for.

it appears that the intent was to develop the base moment with the bent bar over the first 12" or so and then develop the remaining moment in this region with the hooked bar. i dont think i buy it though, but i cant put my finger on what is wrong. any help?

 

[hokie66]

The topping slab may have been considered nonstructural, that is non-composite, for bending of the hollow core. But it certainly is required to be structural for the cantilever to work. There seem to be a few things wrong here:

1) The bars in the topping should project into the backspan as negative reinforcement, not just development length. There should be some shrinkage reinforcement perpendicular to these bars in the topping.
2) The bars should turn down at the setdown, then turn horizontally near the bottom of the cantilever for at least a standard hook length.
3) The lower bars should be kept near the top of the cores and grouted into the cores a sufficient length to develop the bars and lap with the bars above in the topping. The hook shown doesn't do the job. This is the worst part of what you have drawn.

Another concern with this detail is that the dissimilar materials from inside to out and the top bar being forced low to suit the core will probably result in cracking and water penetration.

 

[csd72]

Galambos,

You mentioned a step? Maybe this step was bigger originally and the detail was just reused without proper ammendment.

This is a simple detailing issue.

1. check the stress development at the top bar set down (treat as a hook)
2. check the development of the bars over the beam.

If both bars develop full strength at both positions then you are most likely okay. If not then it is definately not okay.

I would ask them to check the detail anyway just to cover yourself.