Need help with placement of 50,000 lbsbulk tank on concrete slab

[CabecaChata]

We currently have a HDPE vertical bulk tank that holds a surfactant. This tank is capable of holding 6,000 gallons. However, we have never filled this tank with more than half of its capacity because we are not sure if the concrete slab in which this tank is placed will support the weight. We would like to fill this tank up to 5,000 gallons, or 54,000 lbs. This bulk tank diameter is 8.5 feet, with a flat base area of 8,171 sq in. With 5,000 gallons, the product would exerts a pressure of 6.6 lbs per sq in onto this concrete slab. The concrete slab is 5 in thick without any rebar reinforcement. My question is: Will the slab hold such weight without cracking or settling? What is the theoretical pressure capacity of this slab? Are there any other factors that would affect the stability of this slab-other than pressure?

Note: This tank is vertical, and properly leveled. The weight of the tank is unknonw. However, it is calculated to be less than 1,000 lbs.

Thank you for your time and professional opinion on this subject.

Werner
Chemist

 

[chichuck]

Werner,

Yes, indeed, there are other factors. Probably the most important one is the soil conditions under the slab. They will control the amount of settlement you get under load. That settlement in turn will determine if you have problems with the slab cracking, etc. Even with no rebar, a 5" slab might be sufficient for the loads you indicated, which translate to 0.95 ksf, are not really very high for "common" soil conditions. You need to investigate, probably by consulting with a geotechnical engineer, to confirm what soil conditions actually are.

Based on the iformation you provide, 6000 gal capacity, dia = 8.5', I'd say your tank must be about 14-15' high. I've not run any numbers on it, but wind might yield a large enough overturning to increase that bearing pressure some. I'd also suspect that wind on an empty tank might blow it over or away. I presume that the tank must be anchored to the foundation.


Regards,

chichuck

 

[GeoPaveTraffic]

Based on the information provided, it is very likely that the slab would take the weight of the tank completly full. The resulting average pressure would be about 1,150 psf under the tank at the top of the concrete slab. If the tank has a relatively thick steel bottom then the slab would pretty much feel this average pressure, the tank has a thin steel bottom then the slab under the center of the thank would feel a higher pressure. Either way the soil beneath the concrete slab will feel less pressure than the top of the concrete slab. 1,150 psf is not much pressure for almost all soil conditions.

If you want to be sure, hire a geotechnical and maybe a structural engineer to evaluate the conditions. Otherwise monitor the tank while you load it. Does the tank move? Does the slab move? If not you good to go, if it moves a little, keep watching, if it starts to move a lot, stop loading it.

 

[JAE]

GeoPaveTraffic....I would hold out against your first paragraph...first off you can't say that anything is very likely as we have no clue as to the underlying subgrade. If the slab was sitting on wet quicksand, would you still say its highly likely that its OK? I don't think so - I know this is an ridiculous extreme - but chichuch is right - the underlying subgrade is very important to this question.

Also, the thickness of the bottom plate doesn't do much to alter the pressure distribution under the tank. In fact, I would argue the opposite - that a thicker steel plate would re-distribute the weight into hot spots, bridging over low spots or localized settled areas and create higher pressures around those low areas. A thin plate is just a bladder - the weight of the liquid directed onto the slab and the pressure is simply the height of the liquid times the density.

 

[GeoPaveTraffic]

JAE,

I agree that ANYTHING could be under the slab, but if the tank has been in use for awhile at 1/2 full without problems, then it is unlikely there is very soft material under the slab. If the material under the slab is anything but very soft then 1,150 psf is not a significant pressure.

As for the bottom of the tank. You are of course correct that a thin bottom means that the contract pressure is the same everywhere. However, once you get the soil structure interface, the effect of the tank load under the center of the tank is greater if the structure is flexible. This is more of an issue the larger the tank, but true for a small tank as well.

 

[structengineer]

Instead of depending on posts from the internet, you should probably hire a structural engineer to analyze your situation and possibly hire a geotechnical engineer to investigate the underlying soils below the slab.

 

[CabecaChata]

Gentleman, I thank you very much for your help. For all the examinations we have done on our plan floor, we have not detected any settling or cracking. However, I am not an engineer or somebody who has extensive structure engineering knowledge and experience. Your technical opinion coincide with what we have seen with our floors. These tanks are inside our building for over 10 years,and they not have a steel floor. These tanks sit level on the concrete floor. We have just not filled them to their capacity because it was not needed. However, our production has reached a point where we need to increase our holding capacity, and we just want to ensure we will not have a tank failure due to floor structural problems.

Again, thank you Gentleman. Your help in trully appreciated.

Sincerely,

Werner
Chief Chemist