A partially underground water storage tank is 49.5 ft in diameter and 18.0 ft high. If the ground water is 7.15 ft above the base, what is the upward force of water pressure lifting the tank?

• A. 800,000 lb
• B. 900,000 lb
• C. 858,000 lb
• D. 1,000,000 lb

Answer: (C)

Upward hydrostatic force comes from the water column directly above the tank’s bottom. Use pressure p = γh with γ (weight density of water) ≈ 62.4 lb/ft^3 and h = 7.15 ft, since groundwater is 7.15 ft above the base.

p = 62.4 × 7.15 ≈ 446.16 lb/ft^2.

The bottom area of the tank is A = πr^2. The diameter is 49.5 ft, so r = 24.75 ft and A ≈ π × (24.75)^2 ≈ 1923 ft^2.

Lift force F = p × A ≈ 446.16 × 1922.9 ≈ 8.58 × 10^5 lb, i.e., about 858,000 lb.

Since the groundwater height is below the top of the tank, the pressure on the sides is horizontal and does not contribute to vertical lifting, so the bottom pressure times bottom area is the net upward force.