Silo and hopper design for strength

This chapter provides a brief outline of the development of understanding of pressures that develop in silos and their consequences for the safety of the silo structure. More structural failures occur in silos than in any other engineered structural form, considering the numbers of each, and these failures occur in all countries and all industries. Structural design considerations for silos are therefore a key aspect of bulk solids handling systems. 

The chapter refers extensively to the provisions of the recently developed European standards for silo pressures (EN 1991-4 2007) and for metal silo structural design (EN 1993-4-1 2007), for which the author was the chief contributor and editor. Further useful information relating to the structural design of all silos may be found in Rotter (2001a). 

The pressures that develop in a silo are very different from those developing in a tank that contains fluid. Fluid pressures depend uniquely on the head, and in most fluid storages flow velocities are so low that dynamic effects are small. By contrast, pressures in silos 

Bulk Solids Handling Equipment Selection and Operation Edited by Don McGlinchey 2008 Blackwell Publishing Ltd. ISBN 978-1-405-15825-1 

Pressures that develop in stored solids can have an important impact on their free flow from a silo if the bulk solid is prone to developing a small cohesive strength under stress 

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Handbook Design of Hoppers and Silos for Strength and Flow , various, pub. 

The anisotropic behaviour of bulk solids mentioned in connection with Fig. 5 (procedure III) is of no influence in the design of silos for flow. With help of Fig. 5 and 6 it was explained that steady state flow was achieved with Oi (at steady state flow) acting in x-direction. The unconfmed yield strength was also measured with the major principal stress acting in x-direction. During steady state flow in a hopper the major principal stress is in the hopper-axis horizontal. In a stable dome above the aperture the unconfined yield strength also acts horizontally in the hopper axis. Therefore, the Flow Function reflects reality in the hopper area. 

To design the mimmum outlet diameter for a silo hopper the unconfined yield strength Oc is needed. Lower values for the unconfined yield strength allow smaller outlet diameters, which are easier to handle, The vibrated unconfined yield strength Cc.vibr is distinctly smaller than the unvibrated one Oc, see Fig. 7 and Fig. 8. 

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