Slab design for large water tank 1

[m1208]

I am designing concrete pad for a large metal water tank. The water tanks is 65,000 gallons capacity and has 21 feet diameter. The total load of the tank, water and concrete is approximately 700 kips. I am using 12 inches thick by 25 feet square concrete slab pad to support the tank. For the design of concrete pad I am using 0.25 f’C to check the compression and bearing area of the concrete. My soil pressure is almost 1120 psf. The 700 kips load is very large. Does anybody know if there is a requirement for the thickness of the concrete pad or steel rebar requirements in the slab? If there is, what would be the requirements?

 

[dik]

Can you use an octagonal pad? Might save some concrete?

Do you have a geotekkie involved? May give some insight into the possible settlement. Clays can cause some larger long term deflection. The geotekkie can also provide a subgrade reaction modulus which will have an impact on the design of your slab.

Dik

 

[JedClampett]

Your slab thickness is up to you. You're basically pouring a pass through slab that has the same loads on the top and bottom. Hence, no shear or bending. The edges have slightly more load due to the shell, but that's not much. If your slab extends much beyond the tank, you'll have an unbalanced force (more pushing up than down), but that moment and shear is pretty small and easy to design for. Most tanks have a concrete ring beam and are founded on sand, so a slab isn't even necessary. An eight inch slab would work with a single mat of reinforcing or a 12 inch mat with two layers of reinforcing. In the tank business, a 21 ft. tank is actually tiny. I've done ones that are 160 feet in diameter and were supported on oiled sand.
But dik is right, as usual. The most important engineer on a tank design is the Geotechnical Engineer. If the soil has adequate capacity, you're about 80% there.

 

[TehMightyEngineer]

Agreed with dik, I'd get a geotech to get you a reasonable subgrade reaction modulus and subgrade information and then design the slab using a 2D FEA mesh. Often if there are nearby tanks the owner may already have this information. A soil subgrade modulus will help you more accurately determine the stresses in the slab and distribute the pressure on the soil much more realistically than assuming constant soil pressure. If you have access to API 650 Appendix B there's a lot of good guidance in there about (oil) tank foundations that could be used. I've also seen people recommend Differential Settlements in Steel Tanks D'Orazio and Duncan, but I've not read this paper myself.

In the past I've found that holddown of the tank and wind/seismic requirements (sloshing, overturning, etc) were the biggest controlling factors in slab design for tanks.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[dik]

I've done quite a few (and larger ones) where the slab and perimeter gradebeam are for containment.

Dik

 

[IFRs]

What about wind and seismic loads? It appears you have only considered dead load.

 

[JStephen]

For thicker slabs, assuming uniform or linear bearing on the bottom, sum vertical forces and moments at any cross-section and around the perimeter and find average shear and bending forces. Slab is usually made thick enough to handle the shear without shear reinforcement. The slab would typically be made deep enough to get below the frost line at the location as well, or to reach a designated minimum depth from the geotechnical engineer. A circular slab may be more economical to build (potentially less economical to analyze). A tank of those dimensions could require anchorage for wind and possibly seismic.

 

[m1208]

For this size of slab, is the subgrade reaction modulus so critical? Generally settlement under a slab of this size is uniform, no matter the degree of settlement.