Water Tanks on Existing Slab

[someengineer]

Hello,

I was wondering if anyone could please provide me some advice or suggestions.

Our client would like to install 2 flat bottom water tanks (approx. 3m in diameter and 3m high) into an existing building. There is an existing slab on grade within the building (200mm thick) and the client is pushing for us to build up the existing slab to form a pad foundation. Typically we would break out the existing slab and pour an isolated footing for the tank, however I am wondering what the risks may be if we were just to build up the existing slab since the load is relatively small.

I would imagine that there may be increased cracking and settlement around the tank location. Can anyone provide any insight into whether or not this is advisable?

Thanks!

 

[KootK]

If your client is aware of the potential for localized cracking and settlement, then I would probably go for it. It's a savings to do it that way and it's really the client's prerogative to decide if the serviceability issues outweigh those savings. For the sake of liability, however, I'd be inclined to document your concerns and the client's response.

One thing that's always bothered me about adding loads to slabs on grade is assessing the bearing capacity of the soil. I'm not sure that slab on grade substrates get geotechnically blessed for real bearing loads the way that regular foundations do. One would think that the granular fill and careful compaction would be an improvement upon native soil conditions. I'll be curious to hear what others think on this front.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[someengineer]

I'm trying to get my hands on any geotechnical reports from site so I can have a look at the slab subgrade and assess the bearing pressure.

I will be sure to document my concerns as you suggest.

Thanks for your help!

 

[Archie264]

Wouldn't punching shear likely be the governing criteria? If not, and/or if you wanted to isolate the slab in any case, could you saw-cut around the area of the proposed tank? Cracks won't form across a joint. And if you still needed a pad above that could you simply design a full-depth pad case on the concrete, effectively treating the concrete like very-well compacted fill?

 

[XR250]

200 mm slab = 8" thick
3m = about 650 psf

I don't think i would worry about it, but I would still run with Kootk's suggestions.

 

[jayrod12]

I wouldn't likely be too concerned unless the cracking is a severe issue with the client, then I would do what Archie said and sawcut a box around the equipment.

We've got sandtraps here that would support 750psf allowable.

 

[hokie66]

I would just tell him to do it. 30 kPa on a 200 thick pavement is nothing to worry about.

 

[someengineer]

I was thinking about sawcutting as a way to control cracking as well, however would there not be the possibility of a water ingress issue if they were to sawcut and the tank leaked? I suppose that either way you will have cracks, but I would imagine that the cracks wouldn't be as wide as the saw cut.

 

[Archie264]

Hokie has spokie. Use his suggestion.

 

[jayrod12]

Yes the cracks would likely be narrower than the saw cut. But they would not follow a nice pattern and likely look worse.

Just caulk the sawcut after.

 

[KootK]

I didn't originally catch that the slab on grade was 200 mm. I agree with everyone else that you're surely okay here. I'm not so sure that saw cutting would be effective unless you plan to saw cut pretty deep into the slab on grade. Based on what I envision the slab moment profile being, the slab cuts would need to be some distance away from the pad in a location that would be difficult to predict accurately.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.