On the structural response of flat-bottom silos under seismic excitation: Analytical developments and experimental investigations

The structural design of steel silos containing granular materials represents a challenging issue. Silos differ from many other civil engineering structures in that the weight of the silo structure is significantly lower than the one of the ensiled granular material and, in case of earthquake events, the particle-structure interaction plays an important role in the global dynamic response. The complex mechanism through which the ensiled material interacts with the silo wall has been studied since the XIX century. Nonetheless, several issues are still to be addressed and structural failures still occur during filling and, especially, discharging phases, as well as during strong ground motions. It is well known that both metal and concrete silos are characterized by a relatively high failure rate (both ground- and columns-supported silos), particularly during earthquakes. This thesis focus on different aspects related to the structural behavior of flat-bottom steel silos filled with granular material. Part I begins with a comprehensive review of the main analytical, numerical and experimental research devoted to the study of the static, dynamic and seismic behavior of filled silo systems, together with a review of the current design code provisions for the seismic design of silo systems. A comparison between the current code provisions on the seismic behavior of these particular structures and the actual body of knowledge is provided. Finally, a comprehensive review on the use of the seismic isolators in industrial facilities is reported as well. Part II presents a detailed description of experimental campaign conducted with the main aim of developing a non-standard test capable of measuring in a direct way the coefficient of friction of the stored material with the corrugated wall section of a silo structure. Part III is focused on developing a representative analytical framework of the induced horizontal forces by the granular material onto the internal silo wall during a seismic event. It starts by reviewing the historical development of the formula adopted by Eurocode EN1998-4:2006 and other current standards starting from the early research work in the 70s and 80s, passing by the main analytical model of [Silvestri et al., 2012] on which this work is based. Later on, a novel method for predicting the dynamic overpressure for other silo categories than those covered by the original development is presented. Therefore, it collects all the details of the new proposed refinements, introducing the main assumptions, the technical specifications, and the limits of validity. At the end, Part IV presents the interpretation of the results of a series of shaking-table tests on a full-scale flat-bottom manufactured steel silo filled with a granular material in fixed-base and isolated-base configurations. The isolators put between the table and the r.c. plate are Curved Surface Sliders friction pendulum devices. The results are relevant to the identification of the basic dynamic properties (frequencies and damping ratios) of the filled silo system, the experimental assessment of the static pressure (during filling phase) and seismic response (acceleration amplification values, dynamic overpressures and effective mass) of the system and the effectiveness of the isolation system at the base of the silo. Finally, the experimental results were exploited to provide a proof of concept for the proposed analytical model.