## 6 Failures in Silos: Main Causes and Modes
However, at some point of the silo life, the changing of the usage purpose of the facility could be required and, to avoid any catastrophic consequences, a structural assessment should be implemented to check any possible deficiencies that would yield from these changes [116]. For ensuring long life span and safe operation of silos, it is necessary to provide regular maintenance. In this regard, it is necessary to observe the visible defects on silos, which are caused by the ordinary or overuse of the silo structure. Typical defects in steel silos could be local deformity, waviness, dents, thickness reduction, and distorted joints/bolt-holes, as mentioned in [116]. However, other common defects are typically observed for RC silos [114] (e.g., cracks, corrosion, exposed rebar, spalling concrete, and deterioration in RC walls). While ignoring the need for maintenance and overlooking the observed distortions could lead to potential collapse and structural deficiencies, improper maintenance can end with counterproductive effects. For example, changing the internal wall surface finishes by painting may lead to change in the frictional properties of the wall. Consequently, new frictional characteristics can diverge from the specifications set out in the design, resulting in a significant impact on the flow pattern and thus on the solids-exerted loads [122]. Obviously, cladding or internal lining material should be durable, and not react with stored substances inside the silo [36]. In this regard, the same effects are applied if the internal wall surface finishes change due to the corrosion (roughening) or abrasive wear (polishing or roughening) over time [71]. Furthermore, as like any other structures, silos could collapse due to failure of the soil. Considering that silos have a small floor area or diameter compared to the height (whether it is steel or concrete), large stresses can develop in the soil under it, mainly due to the massive weight of the bulk material. Generally, the silo's foundation design is more critical if compared to other standard structures. In the typical case, uniform stresses (pressure bulb) develop in the soil under foundation and problems could occur when the pressure  
Figure 8: Potential failure mode due to buckling of unsupported wall in case of arching phenomenon. Source: [116]. blub is distorted due to off-center filling/discharging [70] or when lateral loads like wind and earthquake occur [92]. When pressure blobs overlap under adjacent silos (e.g., cellular structure), the soil is overstressed, and the structure ends up with extreme settlements. A typical example of this effect is the well-known case of the Transcona Grain Elevator accident [123], Canada, where the plant was made by 65 bins covering several square miles. The silos battery was built a hundred years ago and it failed after only one month after the construction was completed, as it was loaded to 87.5% of its capacity within a month (quick loading) [123]. It was observed that foundation failures in clay occurred when the silo is quickly loaded for the first time [70]. Luckily, the foundation problems were solved, the grain elevator was righted, and it is still in use [124]. Another highly investigated cause of silos collapse is the exposure to extreme events, such as earthquakes. Seismic action can cause serious accidents as observed, for example, in Italy after the recent 2012 Emilia-Romagna earthquake event. As for buildings in [125], extreme events like earthquake can provide progressive collapses that in silos can occur also for static loads [126]. Focusing on earthquakes, in similarity with the buildings, damages to silos structures can be firstly observed and mapped [127], and after in the case of not total collapse, they can be adequate with proper retrofit techniques [128]. Additional topic of interest is the kind of hazardous event, which can cause different responses on the focused structure, on the base of some characterizing parameters (e.g., magnitude or ratio, see [129] for more information).