In-depth: Grain load assessments for silos
Dr. Ing. Carlos Gonzalez Montellano, Technical Department, Bentall Rowlands Storage Systems
rain storage at an industrial level is usually synonymous with steel circular silos made of corrugated wall sheets with vertical stiffeners all around the silo. Bentall Rowlands, UK based silo manufacturer has been dedicated to this task for the last 50 years. One of the first steps required in silo design is the estimation of the pressures exerted by the stored material. This is one of the key factors in silo design, as it will definitively impact the amount of steel required to build the silo and, eventually, the silo cost. There are mainly two alternative design procedures generally accepted in the market. The first one is based on the North American code ANSI/ASAE EP 433 “Loads Exerted by FreeFlowing Grain on Bins.” This code has traditionally been the most common method used for decades and generally widely accepted, even outside the US. The second method has only been on stage for the last two decades and is based on the European code “EN 1991-4: Eurocode 1: Actions on structures - Part 4: Silos and tanks”, an evolution of the former German code DIN 1055. There is still a lot of expectation and interest on why pressures based on EN 1991-4 seem to be bigger than those based on ANSI/ASAE EP 433 (See Figure 1), even when the stored
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material and the silo dimensions are supposed to be the same. To provide an answer to this question, this article analyses the main differences between these two procedures.
Both codes classify the silos according to their slenderness λ, and different design considerations are used depending on that classification. In the case of ANSI/ASAE EP 433, silos are divided into Funnel Flow (λ<2) and Plug Flow silos (λ≥2). In the case of EN 1991-1-4, silos are divided into Slender Silos (λ≥2),
Figure 1. Discharge normal pressure (ph) (left) and vertical frictional force (Fw) (right) for a silo of 32 m diameter storing wheat.