Thickness of Concrete pad needed

by Al Law
(Terrebonne OR)

Question:

I'm installing a water tank with an 8 foot diameter. It will weigh 20,000 pounds when full. I want to put it on a pad 10 feet by 10 feet. How thick does the slab need to be?

Answer:
Generally, there are two types of slabs for water tank, one is the footing slab and the other is the suspended slab for the roof of the water tank. The question above is assumed is asking about the footing slab which is at the bottom.

To put things in perspective, let’s figure out the psi(pounds per square inch) that 20,000 pounds of water put on the concrete footing. The first step is to calculate the area of the footing that will carry the water. For 8 feet diameter, the formula for the area is A=πr^2 where π = 3.14, radius r is 4ft ( half of 8ft). The area becomes 3.14*4*4 = 50.24 psf (pounds per square foot) multiply this result by 144 to convert to square inch yields 7234.6 square inches.

The formula to determine the pressure exerted on the footings by the water is P=F/A where F is the weight of water, A is the are calculated above. P = 20000/7234.6 = 2.8 psi which is a very small considering that concrete for this application should be 4000 psi. The 20000 pounds look big but because it is applied on a wide area, it’s does not have a huge effect on the footings.

The concrete footing thickness for this type of structure, in most cases, is governed by code. Most of water tank that I have come across are 10 to 12 inches thick and it has double layers of reinforcements, one top about an inch from the surface, and one at the bottom about 3 inches from the bottom. Because of the concrete being exposed to water, this type of structure is built with epoxy coated rebars, #4 bars @ 12” on center both ways top and bottom should be sufficient.

Hope this helps
Good Luck