Advanced seismic modeling and analysis of flat-bottom cylindrical steel silos interacting with stored granular-like materials

The increasing size and complexity of the industrial sites in developing and developed countries would translate to high exposure of these sites to the different natural hazards. Seismic events have proven to be among the most destructive natu-ral hazards making detrimental impacts on the plants’ components. Within industrial plants, silos and storage tanks are indispensable assets for several industrial activi-ties, such as agriculture, petrochemicals, food processing and pharmaceuticals, where proper management of raw materials and finished products is essential. As steel shell structure (in the most cases) with massive loading and masses, silos are vulnerable structures whose collapse can trigger Natech accidents, which involve the release of hazardous substances, fires, or explosions. In turn, malfunctioning of these structures can lead to production delays, equipment downtime, increased repair costs, and possibly, human life loss. Latest seismic events have demonstrated an adverse impact on silos structural integrity, imposing dynamic severe conditions and provoking various damage pattern. The work presented in this thesis aims to investi-gate the seismic behavior of a specific typology of steel silo, that is, flat-bottom cy-lindrical, particularly focusing on the interaction with the stored granular-like material if subjected to earthquake events. The first part of the thesis presents a literature compendium about silos, their struc-tural configurations, behavior, failure, and different hazard sources., The second part of the dissertation provides an evaluation of seismic fragility of cylindrical ground-supported steel silos (the silos under consideration are of circular plan, flat smooth walls, unstiffened and constant wall thickness.). The main goal of the first part of dis-sertation is to propose a numerical procedure aimed to assess the seismic fragility of different cylindrical steel silos, accounting for varying geometries and service conditions (i.e., filling level of granular-like material), and observing different failure modes. Consequently, a set of smooth steel silos was selected for this part of work, considering different geometrical configurations (i.e., varying from squattest to slen-derest structures). In addition, different service conditions were simulated, with the aim to observe the behavior of empty and filled silos (30%, 60%, and 90% of filling degree with respect to the maximum capacity). Thus, for each configuration, a de-tailed numerical model was developed under proper boundary conditions, adequately simulating the shell structure, the solid material inside, and the interactions between them. After validating the numerical models against existing literature data, three dif-ferent failure modes were identified and assessed, accounting for the most recurrent post-elastic buckling type (i.e., elephant foot) and considering the possible occur-rence of the elastic ones (i.e., diamond or similar shape failures at the middle and top of the structures). In the framework of the proposed procedure, static and dynamic analyses were performed to identify the most probable failure modes and evaluate the probability of exceeding each one. As the output of this proposed approach, the seismic performance of each silo under a specific limit state was provided in the form of fragility curves. The results highlight some novel aspects, starting from the role that service conditions assume in the silos seismic performance up to the pos-sible differences in terms of failure modes for different silos geometrical structural configurations. In the third part of the thesis, an evaluation of the dynamic overpressure induced by earthquakes in flat bottom steel silos is presented. Aiming to properly understand and predict the dynamic conditions to which silos are subjected, especially under seismic excitations, this part of the dissertation presents detailed numerical analyses to estimate the dynamic overpressure experienced by silos wall under seismic exci-tation. Consequently, nonlinear finite element, FE, models were created for two ge-ometries of silos, i.e., slender and squat and nonlinear time history analyses were carried out. The detailed models accounted for geometrical and material nonlinearity of steel silos and of stored granular-like solid material. This latter was simulated by employing hypoplasticity as constitutive model. The output of the analyses allowed to quantify the additional dynamic pressure, which was compared to the one provid-ed by the European standards (i.e., equivalent static approach). Additional improvements might help in fine-tuning the outcomes of the proposed procedure. Nevertheless, this work contributes a reliable technique that allows pro-fessionals and assessors to comprehensively understand the seismic behavior of the ground-supported steel silos and predict their probability of failure.