Interior Column Isolation

[Antnyt23]

Curious to see some expert opinions on this detail (see attached).

I believe with the diamond block cut isolates footing from the slab-on-grade which should be adequate for crack control. Does anyone see any issues that could come about from the slab-on-grade sitting directly on the footing or do you believe there should be a gap of compacted fill between the bottom of slab-on-grade to the top of the spread footing? Any other possible issues?

Wasn't sure if the fact that slab-on-grade takes minimal load vs. the footing taking a much great load would cause issues due to the slab sitting directly on the spread footing.

Any opinions would be great.

 

[DETstru]

I've never seen that detail done this way in the US. The top of footing is always held down about 6" or more below the bottom of slab.

I would suspect you could be more prone to a crack developing along the footing perimeter if a heavy load passed over it.

On the other hand, in my limited European exposure, they don't do diamonds (or circles) for column isolation. They put the isolation material directly on the column and pour the slab to it.

 

[Teguci]

1. You will want to create a slip plane between the concrete elements. Heavy plastic sheeting should do.
2. I am used to seeing anchor bolts in footings being placed over the bottom mat of reinforcement to transfer compression.
3. the slab control joints should intersect with the points of the diamond block cut.
4. I would expect some differential settlement between the slab and footing (heavy load on well prepared soil vs light load on SOG). In this case, the footing might wind up "holding up: some of the slab.

 

[Antnyt23]

DETstru,

Yeah similarly I am use to the footing being lower as mentioned. This detail was brought up just as discussion and I was curious on others opinions on issues.

Tequci,

I agree with you on the heavy load vs light definitely think it would greatly increase the chance of issues under direct contact vs having the footing held down. 6" like DET mentioned.

I also agree about the anchor bolts didn't even notice that. I always do the same reinforcing below the anchor bolts.

 

[KootK]

My thoughts:

1) I've never really bought into the argument that placing some fill between the footing and the slab on grade reduces slab on grade "hard spots". The six inches of compacted granular material that you'll likely get between the SOG and the footing won't be much more compressible than solid concrete would have been.

2) I sympathize with having a horizontal slip plane of some kind between the footing and the SOG to reduce drag on the SOG when it shrinks. That said, I've seen many buildings in Canada and the US that were built similar to OP's detail without incident.

I also agree about the anchor bolts didn't even notice that. I always do the same reinforcing below the anchor bolts.

I don't agree on this one. I think that the bolts are typically placed above the rebar simply because they're easier to install that way. Consider:

1) In a grouted assembly, compression should be travelling through the grout, not the bolts. This is especially true when leveling nuts are omitted.
2) A J-bolt would be an atrocious choice for resisting compression.
3) Were the bolts in compression, underside breakout would be only marginally improved by having the bolts above the rebar. And, to my knowledge, we've no way of quantifying that improvment.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Antnyt23]

Should have specified I agree on the location of the bolts being above the reinforcing not necessarily for the purposes stated above. My main reason for the anchor bolts always being placed above the reinforcing is generally due to the clear cover. Do not want any steel outside those limits and reinforcing steel tends to be at the limits to increase depth for flexural strength.

Good points koot all make sense.

Good to hear that there hasn't been problems with similar. My biggest curiosity was due to the differential loading between column and slab.

I appreciate he feedback pretty simple standard details are used just like to get a better understanding of all the aspects that could have negative effects.