Uplift at Column Location - Slab on Grade

[edomingo]

I have a column on a slab on grade w/ thickened pad (locally) and edges. Unfortunately, the thickened portion is NOT sufficient to handle the uplift load. I am not relying on the slab on grade to help with uplift currently.

How much slab on grade (beyond the thickened portion) can I assume to help with uplift?

 

[human909]

Short answer very little.

Longer answer depends on the thickness of the slab, its type and its reinforcing. The slab will likely eventually shear (as opposed to bending failure) and you'd need to calculate that limit and be confident in it.

 

[milkshakelake]

Agree with human909, I'd just add to check two-way shear. Since the slab is presumably large, one-way shear wouldn't apply like for a footing.

 

[lexpatrie]

You need to get the anchorage/concrete breakout to work first. After that, whatever you can justify in shear strength in the concrete, provided that weight isn't being used elsewhere in the same load combination. Somewhere in there, I would mark that there should be no "cold joints" in that area. Generally any of your cross bars in say, a continuous footing, would be developed so shear friction is a possibility as a failure mode you can check to demonstrate adequacy. I've tried this in the past and generally, it doesn't work. You need too much concrete this way.

https://www.strongtie.com/products/...-systems/ats/components/shallow-anchor-podium

Regards,
Brian

 

[haynewp]

I worked at a company before that used to take the slab into account in this way. You can look at the slab as plain concrete in bending to resist the uplift (assuming there is min T&S rein only), but you have to watch for joints and cracks and the condition at underside of the slab isn’t known. The underside face of the slab would be under tension at some distance away from the uplift load and the top side of the slab in tension around the region of the uplift load. Watch for exceeding the cracking moment with a FS per your Code that would initiate failure or a direct concrete shear check.

I haven’t tried to account for this extra weight in my designs for a long time now. It doesn’t seem like a very reliable method.

 

[milkshakelake]

I haven’t tried to account for this extra weight in my designs for a long time now. It doesn’t seem like a very reliable method.

I agree. I'd think that the difference is only about 1 kip or so for most 4" or 6" slabs on grade. If someone really needs that extra kip to make it work, and they've squeezed everything else to the bone, I guess it's fine. In any other case, it's engineering work for nothing.

It reminds me of considering dead load on non-bearing wood columns with hold-downs. There's a tiny bit of dead load you can add to help with the uplift, but most of the time, it doesn't change anything. A more efficient way to solve an uplift issue is to work on other parts of the structure.