Punching Shear Reinforcement @ Two Way Flat Plate Slab
Punching Shear Reinforcement @ Two Way Flat Plate Slab
(OP)
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.
If any one has experience with something like this and could help that would be greatly appreciated.






RE: Punching Shear Reinforcement @ Two Way Flat Plate Slab
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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RE: Punching Shear Reinforcement @ Two Way Flat Plate Slab
RE: Punching Shear Reinforcement @ Two Way Flat Plate Slab
RE: Punching Shear Reinforcement @ Two Way Flat Plate 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.
RE: Punching Shear Reinforcement @ Two Way Flat Plate Slab
RE: Punching Shear Reinforcement @ Two Way Flat Plate Slab
RE: Punching Shear Reinforcement @ Two Way Flat Plate Slab
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.
RE: Punching Shear Reinforcement @ Two Way Flat Plate Slab
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.