Bridging happens when a stable arch is formed above the outlet and the flow of the bulk solids is interrupted. To avoid bridging, we need to calculate the critical diameter of the opening of the hopper. The opening is a function of the density of the bulk solids, the shape of the silo, the wall friction, the internal friction and the cohesion of the product. The hopper has to be designed with an opening greater than this critical diameter to prevent bridging.

Figure: Bridging of particles.