Using existing rc column to support flat slab
Using existing rc column to support flat slab
(OP)
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.
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.






RE: Using existing rc column to support flat slab
RE: Using existing rc column to support flat slab
RE: Using existing rc column to support flat slab
RE: Using existing rc column to support flat slab
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.
RE: Using existing rc column to support flat slab
RE: Using existing rc column to support flat slab
RE: Using existing rc column to support flat slab
RE: Using existing rc column to support flat slab
RE: Using existing rc column to support flat slab
RE: Using existing rc column to support flat slab
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.
RE: Using existing rc column to support flat slab
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.
RE: Using existing rc column to support flat slab
"hokie66" is going to dislike the reference - it refers to shear-friction!
RE: Using existing rc column to support flat slab
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.
RE: Using existing rc column to support flat slab
Connection of slabs to cores is typically done with rebates which allow the slabs to bear on the walls, not just rely on dowels.
RE: Using existing rc column to support flat slab
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.
RE: Using existing rc column to support flat slab
We have used bolt-on steel brackets, but fire resistance usually has to be addressed.
RE: Using existing rc column to support flat slab
RE: Using existing rc column to support flat slab
of course the required punching links will be placed in the slab around the column.
Does anyone see anything wrong with this solution?
RE: Using existing rc column to support flat slab
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.
RE: Using existing rc column to support flat slab
Personally my bearing seats are always 150mm minimum, with reinforcement, and an elastomeric pad. If you are designing this as composite, that's a different matter entirely, but in this case you discount bearing completely and design for the shear depth required. In that case my money's on Hokkie66 - Punching shear failure.
RE: Using existing rc column to support flat slab
It may even be against code to rely on dowels to transfer shear force in a suspended slab. You have no redundancy you are just relying on what would normally be integrity reinforcement.
RE: Using existing rc column to support flat slab
RE: Using existing rc column to support flat slab
If it isn't, the client's real problem is the Architect.