Upside down Drop cap
Upside down Drop cap
(OP)
Folks,
I am having a situation where there are planter boxes (2m x 2m) right above columns. The slab is supporting landscaped area.
Can I thicken the slab into the planter box to get more punching shear capacity? In essence, it would be a upside down drop cap.
I have never seen one like it and not sure if it even works.
Your thoughts are appreciated.
I am having a situation where there are planter boxes (2m x 2m) right above columns. The slab is supporting landscaped area.
Can I thicken the slab into the planter box to get more punching shear capacity? In essence, it would be a upside down drop cap.
I have never seen one like it and not sure if it even works.
Your thoughts are appreciated.






RE: Upside down Drop cap
RE: Upside down Drop cap
RE: Upside down Drop cap
Yes, but
- punching shear check at the face of the drop panel is different, critical perimeter is D closer to the support
- PT effects for the change in section are reversed so they now hurt you. You wll need to drape tendons up into the upturn for them to be efficient.
RE: Upside down Drop cap
Actually even if I did disagree I wouldnt dare saying it on a topic like this. He would run rings around me
The main difference is not so much what happens directly ove the column as it is the implications at the perimeter of the up stand. Top reinforcement is not continuous so the critical bending point would be inside the edge not at the edge as for a downstand. Also the shear perimeter is different as rapt has stated.
RE: Upside down Drop cap
It appears my interpretation is not right based on your post. Can you comment?
RE: Upside down Drop cap
I think you need a mechanism to transfer load up into the thickened top to make use of it.
RE: Upside down Drop cap
So the top reinforcing runs continuous through the drop at the elevation it needs to be to work with the slab. Add a couple bars, bent at each end, in the top of the drop for temp/crack control and I think you`ll be all set.
I agree about the exterior perimeter. I think it would start at the corner/edge of the thickened slab and travel down @ a 45 degree towards the column, rather than traveling down to meet the corner of the column.
Couldn't you also look into something like stud rails to increase shear capacity without the upside down drop?
RE: Upside down Drop cap
the others have it right, the shear perimiter for the drop panel starts at the corner of the drop panel back at 45/30degrees depending on the code (ACI, BS). It does not reach the column face.
I would treat the top reinforcement like you would at a step in the concrete with the reinforceemnnt over the column at minimum cover to tohe top of the drop panel. The bars crank at the end of the drop panel down into the slab and need full development into the slab past the cross-over point with the slab reinforcement. And then lap with a bar at top of slab level with a full developemnt length into the drop panel past the drop panel reinforcement to provide continuous reinforcement. Otherwise punching shear and everything else does not work and you may as well leave the drop panel out completely.
RE: Upside down Drop cap
It may be easier to visualize what happens by inverting your detail. Now you have what would occur with a column bearing on a slab, with a deepened section as the footing.
RE: Upside down Drop cap
Yes, a bit more practical your way! The bars would be just about butting together anyway!
RE: Upside down Drop cap
Mike McCann
MMC Engineering
RE: Upside down Drop cap
I had a similar question previously on a pile cap. We could use vertical bars to tie the two pours together in case they are placed separately.
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RE: Upside down Drop cap
OK. Out of curiosity, what would you use to support the form and yet maintain concrete cover for the rebar? Concrete standoff blocks?
Mike McCann
MMC Engineering
RE: Upside down Drop cap
RE: Upside down Drop cap
RE: Upside down Drop cap
He didn't say how deep the upstand was likely to be, but I took it to be only 150 or 200, and the box for that is easily supported. Some tie wire is required to prevent the big-footed concretors from displacing it. Yes, you have to compact under the form.
RE: Upside down Drop cap
Sorry for not giving a lot of information.
Grid: 8.5m x 8.5m
Loading: 18 kPa + self weight
Assumed : 300mm slab with 150mm thickening above column.
column size: 800mm x 800mm (arch preferred due to parking below)
Upstand is 1100mm high, not sure at this moment as exact dimensions are being worked out.
RE: Upside down Drop cap
can you please clarify the difference between the 150mm thickening and the 1100 upstand.
Also, have you considered the implications for future modifications? What if the owner turns around in 5 years and wants the planter boxes removed? You will not have won yourself any friends if this happens.
RE: Upside down Drop cap
I think that csd made a good point about possible future movement of the planter boxes. I, too, have seen this happen.
Although it would work, you might want to rethink this.
Mike McCann
MMC Engineering
RE: Upside down Drop cap
The planter boxes are irrelevant in this discussion. The deck is a landscape area. Any thickening I have above the slab will be hidden by dirt. For all intents and purposes, I don't care whether the planter boxes exist or not.
My idea started out as a thickening into the planter box. But I realized that it did not matter because of the thickening being hidden by soil above. In the sketch I sent, assume the planter boxes do not exist.
RE: Upside down Drop cap
RE: Upside down Drop cap
My comment still stands. I have seen these landscape areas turned into cafes or similar at a later date. I have also seen engineers be too smart for their own good and create problems for future renovations.
These are soft requirements for sure but if you are just doing this to save the client a little money then you probably wont save them anything in the long run.
You are also introducing 4 additional corners to the waterproofing which is the greatest source of problems in these.
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Agree about the waterproofing. The edges of the upstand should be tapered so there are no sharp corners. And the waterproofing has to be protected from damage during installation of the landscaping materials.
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You are right about your assumption. Most of this deck is filled with 600mm soil with tree boxes over columns.
@csd72:
Good points on the waterproofing. Will certainly keep that in mind. Hokies suggestion of tapered perimeter will help.
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For your critique.
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Most landscape folks around here aren't up on their concrete design & the Architect/Owner insists "It's concrete therefore it's yours to design" for site walls & sidewalks. It seems strange (to me) not designing concrete walls supported by an elevated concrete slab.
RE: Upside down Drop cap
Looks good to me.
St20036,
I think most of us would show this type wall in a typical detail instead of in a major structural section. He has just referenced the walls here.
RE: Upside down Drop cap
The top bars in the drop cap need to "lap" with the top slab bars. They do not appear to have sufficient overlap to do this in your shetch. I would have them following the sloping concrete profile rather than a 90 degree crank to give a much longer "lap" and extending far enough into the slab to provide a full lap.
I hope when this was analysed for the PT effects and strength
- the drops were on the top as shown and not just on the bottom.
- the local effects around the drop for effective depth etc were separated from the calculations for the areas outside the drops.