Hoop stress is the resulting stress, usually tension, that is developed in a circular structure (e.g., a tank) from the uniform lateral load imparted by a gas or liquid, either by gravity, induced pressure, or both. In a simpler form...if you fill a vertical tank with a liquid, it creates an outward pressure against the tank wall proportional to the unit weight of the liquid and its depth. This outward pressure creates a tendency to bend an already circular shape, thus the "bending" is transposed to lateral tension within the tank wall.