## 6 Failures in Silos: Main Causes and Modes
In general, silos that experienced earthquakes suffer from additional stresses combined with the ones induced by the stored material, mechanical equipment, and different sources. Slender silos are more vulnerable to horizontal forces, as they could be subjected to overturning due to high seismic inertial forces, especially when the anchoring or the foundation fails. What makes this effect more problematic, is the massive weight of the stored material, which increases the weight of the entire structure. As is shown in Section 3, the effective seismic mass of the stored material is an active research zone, and it is still a matter of dispute among researchers. An accident was reported about the full-collapse of a slender silo among group of 4 steel silos after the seismic swarm occurred in the Emilia-Romagna, Northern Italy, in 2012. In addition, visible buckling damages in the highly stressed areas were observed on the other three silos of the group [93]. Many strategies were introduced by the literature regarding the seismic risk mitigation of elevated [35] and flat-bottom ground supported silos or tanks [130]. For instance, [PERSON] et al. [130] presented a study in which the seismic vibration-induced damage of ground supported fuel storage tanks was investigated. However, similar strategies could be extended for silos storing solid materials. In this study, a new type of seismic isolation was adopted to mitigate the seismic risk, which was based on a finite locally resonant metamaterial concept. In this scope, four meta-foundations characterized by different layers and column heights were designed, exploiting properties of metamaterials, and combining them with classical seismic isolation concepts by using the traditional construction materials (e.g., steel, concrete, and wire ropes). The study was made in accordance with the Italian standards and considering the response spectrum for an active seismic site located in Sicily, Italy (peak ground acceleration, PGA, of 0.56 g for safe shutdown earthquakes and soil type B). Two tanks were evaluated by means of a performance index (PI), and an energy dissipation index (EDI). Time history analyses showed that base shear was reduced by 10-15% for slender tank with one-layer meta-foundation. Nevertheless, it was observed that the case of two-layer meta-foundations presents low efficiency for this tank typology. On the other hand, in case of board tanks, the base shear is reduced by up to 30% with one-layer meta-foundation and about 10-15% in case of two-layer meta-foundation. Moreover, the effectiveness of base isolation as passive control systems was presented by [PERSON] et al. [131], where the effectiveness of different isolation techniques on floating-roof steel storage tanks was investigated through numerical and experimental models of shaking table tests (\"La Casaccia\" Research Centre--Rome, Italy). Particularly, two alternative base isolation systems have been used: high damping rubber bearings devices (HDRB) and PTFE-steel sliding isolation devices with c-shaped elasto-plasticdampers (SIEPD). The test was performed on a reduced scale (1:14) physical model of a real steel tank (diameter 55 m, height 15.6 m) typically used in petrochemical plants. The results affirmed the high efficiency of both the isolation systems and, at the same time, the reliability of lumped mass model for the prediction of the seismic response of isolated above-ground tanks. In the same context, a study presented by [PERSON] and [PERSON] [35] the efficiency of traditional base isolation techniques (curved surface slider) to reduce the seismic vulnerability of elevated steel silo group, taking a real case study located in Livorno (Italy) as a reference. A three-dimensional model was developed, by using the simplified approach proposed by [PERSON] et al. [97], where the silo content is simulated with static pressures and lumped-distributed masses.