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EN 1991-4

If you are planning a new silo or you want to store a different bulk material than originally intended in an existing silo, you must have the silo statically verified.

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For the strength design in accordance with EN 1991-4, the structural engineer requires the following parameters, which can be determined by measurement: 

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  • Maximum bulk density under load

  • Angle of repose 

  • Angle of internal friction 

  • Lateral pressure ratio K (DIN 1055 part 6 (1987): λ)

  • Wall friction coefficient resp. wall friction angle.​​​​​​​

Maximum bulk density under load: It is therefore not the generally            known bulk density, but the density under load. It is also not the                vibration or tapped density. The maximum density under load is                determined on the basis of the maximum vertical stress occurring in          the planned silo. To do this, the maximum vertical stress in the silo is          first calculated. The "compactability" is then measured. The result is          a curve of the density increase as a function of the vertical stress.                Density increases of > 50 % in relation to the loose bulk density                  measured are not uncommon. The picture shows the compactability          of limestone powder with a density increase of 76 % at a compaction        stress of only 32 kPa.

Angle of repose: The angle of repose is determined in accordance with DIN 53916, whereby a cone is heaped          up from a low drop height of the bulk material.

Angle of internal friction: The angle of internal friction is determined using a ring shear tester. The effective              friction angle and the internal friction angle for stationary flow, which corresponds to the internal friction angle        in accordance with EN 1991-4, are calculated from the measured flow locations.

Lateral pressure ratio K (DIN 1055 part 6 (1987): λ): The lateral pressure ratio describes the ratio of the stresses          of a bulk material sample that is loaded with a vertical stress. The resulting horizontal stress is then set in                  relation to the applied vertical stress. The most accurate way to measure such stress ratios is with a                            lambdameter. It was also developed by the founders of the company together 

with Prof. Kwade in the 1990s and quickly found its way into the world of standards. In chapter "C8 Horizontal load ratio" of Eurocode 1 (EN 1991-4)" Action on structures" - Part 4: "Action from silos and liquid containers", the horizontal load ratio is implemented for the static dimensioning of silos.

Wall friction coefficient or wall friction angle: The wall friction angle is also determined using a shear tester. To          do this, the bulk material sample is measured precisely against the wall material of the planned silo.

The wall sample is positioned under the bulk material (a measurement with a wall sample resting on the bulk material can lead to incorrect measurement results, as the relevant fine fraction sinks to the bottom and therefore does not participate in the measurement). The force is then determined to allow the bulk sample to "slide" over the wall sample depending on the vertical tension. Our laboratory can carry out wall friction tests against more than 400 wall samples.

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