Foundation slab for a steel tank - joints

[Andrew88]

Hi all,

I want to design a reinforced concrete base for an open top steel tank. The tank has a diameter of 25m and height of 2.9m. I am thinking about the base that is 150mm thick, thicker around the circumference. Most of the time the tank will be empty.

The maximum pressure that will be exerted on the ground will be not more than 40 kPa and moments in X and Y directions shouldn't exceed 10kNm/m. To simplify reinforcement installation I want to go with a standard mesh A393 at the top and bottom face and circumferential/radial reinforcement in the proximity of the circumference of this slab.

As you can see I get reinforcement density in the slab of 2x393/(150x1000)=0.524%. The concrete will be poured in temperature of 10 Celsius degrees and I don't expect it to heat in the summer to more than 40 Celsius.

Here I would be grateful if you can answer my questions:
1. What is the best pouring schedule here? I though about dividing the circle into heactagonal and pouring 4 in one day and 4 on the next.
1. What is the best way to create gradient on the floor as I need a liquid to drain towards specific location? Would it be benching?
2. Would you use contraction joints here? If yes, at what spacing?
3. Would you use expansion joints? The linear expansion seems to be about 1cm if I take t0=10 and tfinal 40.

Thank a lot,
Jed

 

[Andrew88]

Okay, thanks SlideRuleEra and Agent666

I will probably go with 12mm bars at 100ctrs as believe it will keep cracks tighter but discuss it within office.

The ring beam will have different thickness due to sloped slab. The rough sketch is attached. Do you have any tips on how to provide stirrups? I want to save time on site and limit cutting and bending if possible. The ring beam is quite big, loads low, tank shallow so am not worried about shear/punching. The stirrups would be helpful to provide support for top circumferential bars...unless I specify tall chairs.

Tank manufacturer requires 200x200 clean concrete without bars around the anchor.

 

[Agent666]

I'd think something like this (with additional longitudinal bars located in the corners of the lower full width stirrup):-

Depending on the code you are working to and the detailing rules the intermediate top and bottom bars may require additional vertical ties. Keep in mind that the stirrups are also resisting any horizontal load from the tank wall and transferring the bending into the slab due to the eccentricity.

Were you intending that the longitudinal bars in the perimeter were curved around the radius, or is the beam faceted? I'd look at faceting it as it seems like an easier option to build on site (can more easily prefabricate the cages and lap them together with loose bars on site), but if the longitudinal bars are easily bent/curved to the radius you can do this.

The other issue I see with the proposed detail is your 'beam' depth is constantly changing which means the stirrups depth is constantly changing, this will be a nightmare on site. I'd personally look at a way of making the beam the same level and depth all the way round. Even if this meant a bit more concrete and more restraint, though this might be mitigated by feathering the underside of the slab into the typical slab thickness over some distance. A tank I saw once had the falls radially into the middle, and the underside of the slab was flat, this made the site preparation very easy and achieved the same detail all round the perimeter.

 

[TEDstruc]

There may be some useful guidelines for open bottom tank foundation design in API 650 or AWWA D100.

Are you using some kind of liner? How is the tank wall sealed where it meets the foundation? You should probably consider water stop at the turned up perimeter (I'm assuming the base slab is poured first and there will be a cold joint between the base slab and perimeter wall).

You could also consider a crystal forming waterproofing additive for the concrete such as xypex to seal any cracks that may develop. Not sure how it would react with sewage, but maybe xypex has some data on the subject.