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Column Blockout

Column Blockout

Column Blockout

For a slab on grade, can someone please explain to me what a column blockout is?


- What it's made of
- Size/Shape
- Is used on all column types (ie, HSS, W, etc..)
- How it is construction with the slab
- Is it only used in slab on grade construction
- Anything else you can think of

I believe the purpose of the block out is to isolate the column from the slab on grade, such that the slab on grade doesn't act a diaphragm, is this correct?


RE: Column Blockout

It is to isolate it from the SOG, but not necessarily becasue of any diaphragm action, more of an issue of settlement,  You don't want the slab to crack at that column at the footing beneath it settles.
At least that's what I've learned.

RE: Column Blockout

The blockout does a couple of things.
1.  The column & footing is more likely to settle than the slab is. It allows the settlement to stay local.
2.  The slab will shrink and control joints can't pass through the column.  This gives those cracks a place to go around the column and maintain the aesthetic.

RE: Column Blockout

Thanks for the replies.

Can anyone add to large these blockouts are? What is filler material is?

RE: Column Blockout

maybe a 2x2 diamond or 2'-6" x 2'-6".  There is typically an isolation joint material around the outer edge of this blockout and then the blockout is filled with concrete.

RE: Column Blockout

The blockout should be 3-4 inches on all sides larger than your baseplate, but make sure the baseplate (rectangular) fits in the blockout (diamond).
Filler material is usually 1/4" or 1/2" compressible filler. to separate the two pieces of concrete.

RE: Column Blockout

what is the reasoning for the diamond?

RE: Column Blockout

You typically have control joints coming to the points of the diamonds.  This shorter segment of concrete coupled with the control joint ensures that cracking will occur where you want it to.  A rectangular blockout would require a diagonal control joint or else you risk random cracking.

RE: Column Blockout

Thanks... great explanations


RE: Column Blockout

as struturalEIT pointed out, inset corners are great crack inducers in ground slabs - best to avoid them by using the diamond.

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