## 6 Failures in Silos: Main Causes and Modes
For the purpose of this study, a single curved surface slider was proposed (designed and optimized based on the horizontal stiffness and the friction parameters) and the seismic performance was compared after and before the retrofit interventions. Whereas the original system suffers from stress concentration, elastic deformation, and yielding in the supporting structure, it was noted that all the response parameters were positively reduced after retrofitting. The different observations proved the advantages of the suggested system in terms of inelastic deformation, global horizontal shear, inter-storey drift, isolator displacement, and residual displacement. Another cause of failure of silos is given by the mechanical phenomenon known as thermal ratcheting. This effect could develop in the metal silos, since steel is more sensitive to the temperature fluctuation than the RC. In general, for a possible rise in internal/external temperatures, steel walls of the silo expand allowing the stored material to settle. However, when the temperatures drop, silo walls are subjected to a contract (or a shrink), which is in contrast with the settled particles that cannot move if a discharge phase is not taking place. Thus, the expansion becomes irreversible and thermal stresses accumulate, which in turn amplifies the tensile stresses in the wall. This phenomenon is repeated over many and many thermal cycles and, eventually, this ends up with a failure [70]. This phenomenon could occur due to any cyclic swelling and shrinking conditions (e.g., moisture fluctuating) applied to the stored material, as shown in [132]. One famous example of this kind of failure is reported when describing the collapse accident of a new brand steel silo containing thousands of tons of fly ash, shown in Figure 9, and located in southwestern United States [116]. The experts attributed the accident to the thermal ratcheting phenomenon, which was not considered in the design. Despite this, most of the main international standards account for the thermal ratcheting effect, especially the European, American, and Australian ones [71; 99; 109]. Regarding this topic, the scientific literature provides different detailed stress models to account for the phenomenon of thermal ratcheting, often opting for FE methods [6]. On the other hand, a discrete element method (DEM) along with experimental tests approaches was defined by [PERSON] et al. [133]. This study investigated the effect of this phenomenon on the shell walls of tanks used in thermal energy storage systems (as essential parts in power plants). The investigated silos typically contain a thermal storage medium of solid material (e.g., steel, sand, gravel, or rock). The functionality of this kind of silos makes it exposed to differential expansion between filling and walls. DEM was used to simulate the stored material and two different thermal approaches were considered. Specifically, homogenous heating and vertical gradient heating along the wall's height were investigated as typical thermal configurations. The study revealed that higher stresses develop in silo walls in case of thermal gradient along the height. Moreover, the significant effect of slenderness ratio, the internal friction, and the solids-wall friction on thermal stresses was affirmed through the simulation of DEM. An increased radial stress was recorded and it was equal to three times the initial one after performing 108 cycles. In the same context, a statistical description of the pressure on the wall of silos storing hot material was introduced by [PERSON] and [PERSON] [134]. The study employed an experimental and statistical approach to quantify the total load given by the combination of thermal loads and static loads on the walls of RC silo. Still, in the context of extreme events threaten silos, an explosive atmosphere could be created inside the silo depending on the nature of the stored substances. The risk of fires and explosions is presented inside the silo [135].