Using existing rc column to support flat slab 1

[sybie99]

I have a situation where a concrete column is already built. I now need a flat slab at mid height of this column, to be supported by this column. In effect the column should be cast to slab soffit, the slab then cast, then column above cast. The slab was added later after column was already built to a higher level.
Now, I know of a number of engineers who have simply designed the slab as though the column supports it, and then dowelled bars through the column and epoxied in. So for example, say you need 16mm bars @ 200mm centres top steel in the slab at the column location, and you have a 650mm x 650mm square column, you would drill 3 holes right through the column, place the bars through, fill holes with suitable epoxy and build slab. You would also drill through holes for the bottom steel.

Has anyone done this, are there risks involved? I would think shear, but the bars will transfer the shear through direct bearing in the column.

 

[hokie66]

That is not an appropriate fix. Just knock the column down and recast it. No big deal.

 

[sybie99]

Column can not be knocked down, it already supports a roof slab overhead. Hokie, if you say it is not appropriate, what would be the concern, technically speaking?

 

[hokie66]

Punching shear.

 

[sybie99]

Hokie bear with me, I'm not trying to disprove what you are saying. But this has been done in practice, maybe the ultimate loads were never applied and that is why it is working. My thinking: You have sufficient bars going through column in 2 directions, holes filled with an epoxy stronger than concrete. Now, if your bars can transfer the shear at column face, face shear is not an issue. Then bearing of bar on concrete comes into play, but its a "through bar", same as a through bolt, and if the holed is properly filled bearing of the bar on the column concrete will be over a long enough bar section not to exceed the bearing strength of the concrete.

So face shear is dealt with, punching shear will not be an issue if sufficient bars are provided through and around the column.

The way I see it, is that all you lose is the shear strength of concrete itself, which is minimal, such as construction joints/cold joints in slab where almost all shear is transferred through the rebar.

 

[jayrod12]

Why not fasten angles to the column to provide solid bearing surface for the slab. Then drill and epoxy dowels into the column.

 

[sybie99]

jayrod, could work but unsightly in an area where ideally clear face of column must be visible, no ceiling to hide bracket

 

[jayrod12]

Stupid architectural concerns get in the way of easy solutions again. Blast!

 

[jayrod12]

How about turning the vertical leg of the angles up to cast them into the slab? Then the only thing you would see is the horizontal leg of the angles. It's much less obtrusive.

 

[sybie99]

yes, again could work, depending on load to transfer, but I would think I could use bolts going through column with angles on all 4 sides, maybe 4 bolts, 2 each face passing through column fixing angles brackets at opposite sides.

 

[jayrod12]

You could drill all the way through the column. But if you didn't need to I'm sure the contractor would appreciate not having to. Sometimes it's hard to drill clear through due to slight variations in column rebar locations, rebar laps etc.

My experience is they would much prefer to do some for of adhesive anchor for each connection however it's up to you in the end. This is just how I would do it.

 

[hokie66]

Construction joints are placed where the shear is minimal, not as in your case where the shear is maximum.

As you say this has been done in practice, perhaps you should consult with the engineers who did it. I haven't seen it done, and wouldn't do it.

If the column is large, you could remove enough concrete to achieve a bearing surface around the column, then use your drilled through bar solution.

 

[Ingenuity]

Here is a link [SHEAR COLLARS] to a paper by two WJE engineers for an existing parking garage slab that had punching shear issues...not the same case as you cite, but some similarities with respect to punching shear around pre-existing columns.

"hokie66" is going to dislike the reference - it refers to shear-friction!

 

[MrHershey]

You would need to design your epoxy anchors to be able to transmit 100% of the load to the column since you have no bearing to rely on. One way to this if your epoxy doesn't work would be to drill oversized holes all the way through the column, pass rebar totally through the column (and develop on either side) and then inject back with nonshrink grout. Punching shear for the slab itself could be addressed with stud rail reinforcing. I don't see a reason why they wouldn't still be able to work in this situation.

Would generally prefer some sort of bearing connection here to help with redundancy but the above (sans stud rails) is used all the time for connections of concrete slabs to elevator cores, etc. where the vertical element will go up ahead of the slabs.

 

[hokie66]

MarkHirschi,
Connection of slabs to cores is typically done with rebates which allow the slabs to bear on the walls, not just rely on dowels.

 

[hokie66]

Ingenuity,
I have been involved in a similar punching shear reinforcement exercise, but we used steel collars, and column enlargement full height where space permitted. We rejected the idea of concrete collars.

 

[Ingenuity]

hokie66, yes, I know what you mean as far a concrete collars - 'leap of faith' type stuff.

We have used bolt-on steel brackets, but fire resistance usually has to be addressed.

 

[hokie66]

Yes, I think that is usually handled with fire rated gypsum board assemblies.

 

[sybie99]

I have a 450 x 450 column, and must transfer 320kN. I will create a bearing area 30mm deep for the 300mm depth of the slab that must connect on all 4 sides. Just by this bearing I will have sufficient capacity to transfer the load. Then, holes for the required top rebar (4Y16's) will be drilled right through column. Bars inserted the protrude with required lap length. This "notch" in the column as well as the cavity around the through-bars will be grouted up with non shrink grout.

of course the required punching links will be placed in the slab around the column.

Does anyone see anything wrong with this solution?

 

[CELinOttawa]

In my opinion I would not be happy with 30mm bearing. I often neglect an inch (25mm) as ineffective just for site issues, etc. The few times I have been forced to take advantage of all bearing, I have armoured the edge with angle iron on both seat and slab, with a thin elastomeric pad between.

I would be more comfortable with a circular column where I could put in annular reinf. to hold this on... I don't like this at all with re-entrant corners. You need more bearing.