Column Rebar 4

[kronosconcrete]

I'm designing a slab on grade (post tensioned) on fractured granite base that will support columns for a three story residence. The rebar will be the minimum 1%. Slab flatness is very important. How can I embed into the slab to come to the top of the slab so vibratory screeding isn't interfered with. Then I need to tie in the rebar above the slab for the columns.

Thanks for any ideas.

 

[Ron]

If you are supporting 3 stories on a slab, I would suggest you thicken the slab at the columns as the shear and moments may be rather large and top steel won't help you much.

This technique is used often in the pre-engineered (I love that term...as if other structures were not "pre-engineered"!) metal building industry.

Ron

 

[phuduhudu]

Do I understand that you want to be able to screed over a flat surface before you continue with the column so that columns don't get in the way of your screeding?

First of all I agree with Ron that you may need slab thickenings. To keep the columns starter bars below screed level you could cast a small pocket at column locations and then use threaded rebar couplers in the pocket to continue your column rebar. Otherwise how about a small pocket again but with a steel base plate with rebar welded to it so that essentially it is a steel column base (though watch it if you need to take moments in this). Carl Bauer

 

[dik]

To thicken the slab in the vicinity of the columns with post tensioning may be counter productive for moments (assuming the thickening portion goes down to yield a flat slab. For a 3 storey building on a granitic material, albeit fractured, are your footings large enough to require a full post-tensioned slab for bearing?

I've not ever used them for column rebar, but I often use 'Z' shaped dowels for slabs. They are embedded in a keyway and straightened after the keyway filler is stripped out. The bars sort of look like:

. ___________
. |_____

and straighten to look like:

. |
. |
. |
. |
. |
. |_____

(The browser formatting will likely really mess this up)

If you have small bars, you can try something like this.


Also, there's a corrugated galvanized tubing material (like a small diameter culvert) that can be inserted and the increased diameter enhances bond. The column dowel rebar could then be post-installed using an expanding or non-shrink type grout (used this for the odd precast project).

 

[phuduhudu]

Yes I missed the bit about post tensioned in which case the thickenings wouldn't be that useful but why not a normal reinforced slab with thickenings under the columns? I don't think I have ever seen a post tensioned slab on grade.

The problem with the keyway idea above might be that the bent down bars extend into the area of slab and you seem to be concerned about the finish on this. The other thing you could do is to drill in bars. In fact if you can accept pin based columns you will only need a little bit of steel for base shears. You could also do it with a pocket which you cast your column into so there is no steel across the joint. Carl Bauer

 

[dik]

The keyways would mess up the slab, so looks like the post-applied is becoming more attractive.

I often spec 'Dywidag' threadbars for steel AB's where there is a significant tensile force. These have a proprietary nut and coupler accessory that could also be used... connections tend to be a tad pricey. I don't know about the weldability of these, either, but could do up an assembly using these.

The idea of using a steel plate with headed studs or rebar, and welding the rebar dowels to the plate may be the most economical. Again, should check that rebar is weldable.