Upside down Drop cap

[slickdeals]

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.

 

[ishvaaag]

Sure it works, just follow the stresses. Typically some inclined rebar could hang the slab from the thicker part.

 

[ron9876]

Just be sure you locate the potential cracks in the correct locations.

 

[rapt]

sickdeals,

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.

 

[csd72]

Agree with rapt,

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.

 

[slickdeals]

Rapt:
It appears my interpretation is not right based on your post. Can you comment?

 

[dcarr82775]

I don't think your exterior critical perimeter is right. The exterior critical perimeter would be the perimeter of the thickened portion as the crack starts at the top of slab and travels down towards the column.

I think you need a mechanism to transfer load up into the thickened top to make use of it.

 

[Once20036]

It sounds to me like the thickened slab isn't required for any flexural considerations, only for shear.
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?

 

[rapt]

Slickdeals,

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.

 

[hokie66]

Agree with rapt, except I would run the top bars continuously through at one level, and add another mat at the top of the upstand "drop", developed as he described. A 2 metre wide section is not enough to complicate the reinforcement by stopping and starting.

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.

 

[rapt]

hokie,

Yes, a bit more practical your way! The bars would be just about butting together anyway!

 

[msquared48]

I guess I would have concerns with this scenario in that two pours will likely be required here as opposed to the dropped condition. That gets into making sure the interstitial surface adequately bonds the two pours. Otherwise, you're kidding yourself.

Mike McCann
MMC Engineering

 

[slickdeals]

M^2,
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.

 

[hokie66]

No, you don't need two pours. The square form required to place the upturned section can easily be supported on the top layer of reinforcement.

 

[msquared48]

Hokie:

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

 

[hokie66]

No, you just use bar chairs off the top mat, turned upside down.

 

[csd72]

I have seen this done but not for this level of step, also, be wary of potential compaction issues at the bottom of the step.

 

[hokie66]

csd72,
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.

 

[slickdeals]

All:
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.

 

[csd72]

Slick,

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.