Slab on grade or structural slab? 1

[milkshakelake]

I have an 8 story concrete building with no cellar. The footings are 6' to 7' below grade (varies). Can the 1st floor slab be a simple 4" slab on grade? Or does it have to be a structural slab, like 10"?

My main concern is column slenderness. I'm afraid that if I use a 4" slab on grade, it might be able to laterally pin interior columns but not edge columns, thus making edge columns slender. Some of the columns will abut neighboring foundation walls, so they won't have soil confinement on one side, which leads to slenderness in that direction. The columns are all 12"x24".

 

[Greenalleycat]

Not sure of your question really. I think the answer is that it can be either? Subject to geotechnical conditions being the governing criteria and you haven't provided insight into that. Assuming a typical 200mm (or whatever depth) site strip for organics and soft spots, you could then put your layer of hardfill down and a 4" SOG and go about your day. I don't see what benefit you would get by making that an 8" suspended slab, given it's sitting on dirt, in this scenario.

If you're instead saying that your governing criteria is that you need your slab to provide lateral restraint to the column then I guess it becomes a matter of defining the strength requirements to achieve that.
I imagine your code will have some requirement for % axial force in column required laterally to provide restraint, in which case you stick that into your slab and see how thick it needs to be before it stops buckling out-of-plane..?

 

[milkshakelake]

@Greenalleycat It's the second concern. The geotechnical considerations are fine for either option.

True, I think the requirement is something like 2% of vertical force or 10 kips, whichever is greater. In that case, I either design the slab for this force (which will be like 56 kips at places) or design the column as unrestrained, with a larger height and ignoring the slab restraint. Thanks for the tips!

 

[Greenalleycat]

I was curious as to what our code (NZS3101) says about this, so here's the only relevant code snip I can find
I guess to be conservative you could assume that this value could occur in either tension or compression so you need tension tie steel and sufficient thickness/reinforcement in the slab for buckling

 

[milkshakelake]

Oh yeah, that's a much higher value than the 2% we have here in hamburger and freedom country, or at least in New York. Anyway, I did check the tall columns for slenderness and it's still working, so I'm choosing to ignore the 1st floor slab on grade.

 

[KootK]

Ignoring the first floor SOG is my first choice. If needed, I've sometimes put piers under the columns. That costs money of course.

 

[milkshakelake]

@KootK I considered the piers but the slenderness is working out so I didn't go that far. I'd probably do it if it were like 10' below grade.

 

[ryaneng]

I either design the slab for this force (which will be like 56 kips at places)

Forgive my naivety, but are you suggesting that you're designing a 12x24 column for 56kips/0.02 = 2800 kips?

Or is that the resultant force you would have in your slab from the combination of a few columns?

 

[BridgeSmith]

I don't know what the code provisions would say, but I would say basically if the slab supports walls or columns, or is connected to the walls or columns in a way that could transfer moment, shear, or bearing, it's a structural slab. If it's just 'floating' on on grade inside the foundation, it could probably be a slab on grade.

 

[milkshakelake]

@ryaneng Oops, my math was way off. It's actually around 7.2 kips. Rookie mistake. Good catch!

@BridgeSmith When it's fully supported without needing soil, even if it is on soil, I consider it a structural slab. Since it's supported on soil without a way to transfer moment (though it could take some lateral shear), I consider it a slab on grade.

 

[BridgeSmith]

@BridgeSmith When it's fully supported without needing soil, even if it is on soil, I consider it a structural slab.

I agree. I meant to convey that in my comment, but apparently it was poorly worded.

 

[HTURKAK]

Apparently there are two different properties adjacent to the plot which could be excavated deeper than this one .

I would prefer 2 ft mat ...If the footing depth 6' to 7' , for frost depth , i would choose the bottom of mat 6 ' then use selected fill and use SOG 4".


My opinion..









Not to know is bad;
not to wish to know is worse.

NIGERIAN PROVERB

 

[milkshakelake]

@HTURKAK I think 2' mat would be overkill for an 8 story building. If the adjacent lot wanted to develop, they would underpin. It's their responsibility to ensure that this building is safe.