Slab on Grade at Interior Columns

[RFreund]

I am curious to here from others on how they usually detail this. I have seen it both ways but more often the top option rather than the bottom. It seems that the bottom row of details would be more work. We have a project where the geotech is (sorta) recommending the bottom row of details so that the slab does not bear on the footing or pier. I am wondering though if the top of footing is a -1'-0" how much does the granular subbase really compress, why not just pour on top of the footing?


Thanks.

EIT
www.HowToEngineer.com

 

[Ron]

The top plan detail is correct and the bottom section details are correct. I would not use a thickened slab turn down at the column.
Yes, isolate the slab from the top of footing.

The bottom plan detail results in offset joints that will crack in odd places.

 

[TLHS]

Yeah, I don't think I've ever seen the top section detail.

 

[jike]

I would either encase the base plate and bolts in concrete or apply two coatings of bitumen if exposed to soil. Soil and water can be corrosive to exposed steel.

 

[BAretired]

For the typical interior column detail, I have always used the upper detail, but the lower detail would be okay too. Doesn't really matter.

For the other two details, I have used the constant slab thickness shown in the lower details. Why do you think the bottom details would be more work? I would think the opposite.

Where there is a large gap between the top of footing and underside of slab, there could be a concern about adequately compacting the region around the pier, so backfill would have to be done carefully. Granular backfill or low strength concrete (Fillcrete) might be an option.

BA

 

[steellion]

I've always used the diamond shape shown on the top details. The sections imply a turned-down slab, but that's not really what you have. They are just going to come back in and fill the diamond isolation joint with concrete. That keeps it structurally separate from the footing.

 

[RFreund]

Thanks for the comments. It seems as though the diamond shape of the top plan detail and the bottom sections which keeps the slab from bearing directly on the concrete win out.

At interior columns - do you size the diamond so that it covers the base plate?

At exterior columns - do you keep the same size as the pier? If yes how do you detail the control joint?'

Thanks again!

EIT

http://www.HowToEngineer.com

 

[EngineeringEric]

The size i use for interior is typically 2'x2' but will increase if base plate is larger.
the exterior ones i typically use the same size as interior.

Honestly they are standard details which go on every set of plans my firm make that involve steel columns. I check but really don't think much about.

 

[hokie66]

I can't say that I have always used either detail, because I have done it all sorts of ways. But I prefer to make slab joints away from the columns. That way, you can saw straight through, and you avoid having to come back and fill in. And you allow the slab to shrink toward the column rather than away.