## 4 The Contained Material Properties, Behavior, and the Imposed Loads
### Discharging Patterns  
Ideal silos must ensure a regular solid flowing compatible with the intended patterns specified by the design that avoid the discharging problematic phenomenon. Consequently, the desired flow rate and the intended operation of the silos are guaranteed. Depending on the grain-wall friction characteristics and the flatness of the hopper wall (silo bottom hopper) [71], two main flow patterns could be distinguished when bulk solids are discharged (gravity flow) from the bottom of the silo. The first main type of flow pattern is mass flow (as shown in Figure 6a) [71], where the whole mass of the material moves downward whenever the outlet is opened (given that arching does not happen). The second main type of flow pattern is represented by the funnel flow (also known as core flow or pipe flow), where the material flows from the top to the outlet through a funnel built by means of thematerial itself. In case of funnel flow, it is possible to develop stagnant zones (symmetric or asymmetric dead zones) in the upper silo part making pipe flow (as shown Figure 6b), or in the lower silo part making mixed flow (as shown in Figure 6c). Moreover, depending on the material properties, outlet position and number of mobilized outlets, eccentric pipe flow (as shown in Figure 6d) could develop as one of the funnel flow types, with eccentric channel forming near to the wall and exerting unsymmetrical pressure [67].  
In the case of mass flow, the material at the center of the silo moves downward with the highest velocity, while the material near to the walls moves with the lowest velocity due to the friction or to the flatness of the hopper wall [102]. For overly frictional or extreme flat hopper walls, flow velocity of the material vanishes near to walls and the mass flow pattern converts to be funnel and this gives an interpretation on how the flow pattern could vary during the silo lifetime, depending on the material and grain-wall frictional interaction characteristics. When the discharge process is proceeding, there is an increase in pressure imposed on silo walls. This increase is considered by the different international standards, which vary in the complexity and the accuracy. For example, depending on the storage capacity, the geometry, the possible filling/discharging eccentricities, EN 1991-4 suggests different approaches to calculate the additional stresses exerted through discharging.  
With this regard, [PERSON] et al. [34] presented a new dynamic model (employing Drucker-Prager plasticity model) for silo discharge simulation, considering mixed flow and mass flow silos. The study also investigated the variation impact of the wall friction and outlet radius. The outcomes of the study reported that the flat-bottom silos with mixed flow present overpressure values lower than the ones obtained in the case of hopper silos with mass flow pattern. Moreover, the overpressures mainly occurred in the lower part of the flat-bottom silo (14 m high and 2 m radius). The study also indicated that the discharge pressure increases as the size of the outlet increase and the grain-wall friction coefficient decreases.