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Punching Shear Reinforcement @ Two Way Flat Plate Slab

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Stenbrook

Structural
Dec 5, 2014
125
I have a client that has bought an existing structure with a two way flat plate slab conventionally reinforced with drop caps at the columns. The required loading for the new occupancy has resulted in a higher punching shear load then the current capacity. As of now, it will be very difficult if not impossible to use a collar tie because of the large bars and spacing in the columns and the owner has no desire for us to widen the column all the way down to the foundation. So, for now we have come up with a widened column detail that only extends 2'-0" down from the bottom of the slab. The question is this, how much load can I count on the existing slab to resist and how much should I design this widened column and epoxied rebar into the column for? If I design the widened section for the entire load, the design will require 40 doweled in bars at each column and with over 25 columns on the job, I would like to try and reduce that if possible.

If any one has experience with something like this and could help that would be greatly appreciated.
 
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Are they "drop panels" (vs. drop caps?)
Just making sure of your terminology. Typically the word "cap" refers to a smaller column capitol on top.
A drop panel refers to a wider section of slab used to increase negative moment bending and punching shear resistance.

A sketch might help too.

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Yes they are drop panels. Sorry for the confusion. It is just a thickened portion of slab around the column to increase the punching shear and moment capacity as you said.
 
At the face of the column you have the existing condition which is worth something, so if you design your remediation to be composite with the existing conditions you only need to dowel for that portion of load the geometry says goes to the new bit. If you are using is as a drop panel that is a sizeable bit of concrete. If using as a shear cap it is more manageable.
 
1) You've got a bunch of DL punching shear stress locked into your slab.

2) The concrete collar will have more give to it than the original column.

3) Depending on the concrete that you use for the collar, it may tend to shrink away from the underside of the slab slightly.

For these reason, I would design the collar for 100% of the punching shear load. I feel that you'll more or less have to fail the existing punching shear perimeter before the collar will really kick in.

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.
 
I think I disagree with dcarr and agree with KootK. When faced with this issue, I have always designed the bracket/collar to take the whole load, as the shear perimeter is outside the new bracket.
 
My opinion is to design for 100 % of the load as well to be the most conservative. However, just to play devils advocate a little bit, if I were to design it compositely in the hopes of being able to get a little bit out of the existing slab, could I consider doweling into the existing slab to prevent gaps from occurring from temp/shrinkage or possibly grouting any gaps should they form. Are those appropriate or is there some reason that they would cause big issues?
 
Grouting the gaps would probably be unrealistic as the gaps would be very small. I suppose that you could make an intentional gap that could be grouted later but that's getting pretty weird.

The doweling option was something that I considered as well. It's a chore to do overhead dowel installations but it can be done.

While both of these approaches may help, it's very difficult to know by how much they help. And neither fully addresses item #2 that I mentioned above. The partial collar will slip vertically a bit before it fully engages the existing column. I worry that slip might mean failure of the original punching perimeter before the new one kicks in.

It's worth remembering that most of your punching resistance in a rectangular column is concentrated around the corners. In the interior of the column section, the column may actually be providing a downwards reaction to the slab.

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.
 
I too would design it for 100% if I was doing it, but that doesn't mean it couldn't be done. I agree you have the DL locked in already. The LL is the problem child. If you made it composite and paid attention to detailing (roughening/dowels/maybe even fancy epoxy bonding agents/pre-placed aggregate) I do not see why it wouldn't work. I have seen it done by some very fancy pants US forensic engineers with several other parties agreeing it was a valid repair. I felt there were cheaper ways, but that didn't matter. Unless it actually fails we will never know the validity.

The old ASD work was so straightforward. You have a max stress, you hit it and you were done. The ultimate strength world makes things much more open to interpretation and judgement. What failure mechanism will occur, that is ultimately what matters.

If you want to go 100%, you can cut and chip a bearing ledge into the column and take some load out through direct bearing.
 
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