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.

 

[CELinOttawa]

P.S. At 30mm you're basically seating this on the column's cover... First bit of impact, first concentrated load causing a stress riser, or any construction error and you're screwed. Frankly if by some miracle it does hold, the least little iota of rust in the column's main bars or stirrups and your "bearing" is going to snap away in a devastating brittle fracture.

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.

 

[amecENG]

Can you just increase the size of the lower column? Say add 6-8" all around.

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.

 

[Robbiee]

How about do it this way. Please see the attached sketch. Steel bearing brackets hidden in the slab fastened with threaded rods extended to allow the installation of 2-way shear reinforcing if required.

 

[CELinOttawa]

I really like Robbiee's detail; Very similar to some of the suggestions above... But if this is a purely Architectural concern stopping the simpler solutions, this may not be "acceptable" either.

If it isn't, the client's real problem is the Architect.