Eccentric loading of silos
If a silo is filled or discharged off-centre, the loads are no longer distributed evenly around the circumference. If the outlet is eccentric, a flow channel forms at the wall: the pressure drops in the channel and rises at its edges – the wall is subjected to bending.
From your measured parameters we calculate the patch loads and, for large eccentricities, the flow channel wall pressures to EN 1991-4.
How eccentric loads arise
Causes
- Eccentric outlet or one-sided draw-off by the feeder
- Several outlets that are not operated at the same time
- Eccentric filling with a skewed top pile
- Segregation with a preferred flow channel
- Funnel flow with an eccentric flow channel
Consequences
- Uneven horizontal loads around the circumference
- Bending stress in the silo wall
- Buckling of thin-walled steel silos, cracks in concrete silos
- In extreme cases failure of the silo structure
Load cases for eccentric loading
The standard distinguishes two levels: patch loads for small eccentricities, which are superimposed on every design, and separate load cases for large eccentricities during filling and discharge.
If the outlet eccentricity eo exceeds the critical value eo,cr = 0.25dc, silos in Action Assessment Classes 2 and 3 should be checked for eccentric discharge in the form of a flow channel above the outlet, using the procedures given for the load distribution.
Clause 5.2.4.1 (1)
These loads should be determined using the lower characteristic value of wall friction and the upper characteristic value of the angle of internal friction φi.
Clause 5.2.4.1 (4)
Patch loads
Patch loads are unsymmetrical additional loads for small eccentricities and irregularities. They are superimposed on the symmetrical filling and discharge load. Their magnitude follows from the coefficients Cpf (filling) and Cpe (discharge), which depend on the measured conversion factors through the patch load solid reference factor
Cop = 3.5 · aφ + 2.5 · aK − 6.2 (Eq. 4.8)
The more the parameters scatter, the larger the patch load. In Action Assessment Class 1 patch loads may be omitted.
Large eccentricities
If the outlet eccentricity eo exceeds 0.25 · dc, a flow channel forms at the wall. During filling the same limit applies to the top pile ef, for slender silos from hc/dc > 4.0. The wall pressures in the flow channel and next to it are calculated as separate, independent load cases.
For Action Assessment Class 2 a simplified procedure is permitted; for Action Assessment Class 3 the detailed procedure must be used. The calculation tool selects the procedure according to the Action Assessment Class of your silo.
Measurements for eccentric load cases
The eccentric load cases use upper and lower characteristic values – for this, the conversion factors are needed in addition to the mean values.
| Measurement | Parameters | Use |
|---|---|---|
| Yield locus | angle of internal friction φi, lateral pressure ratio K, conversion factors aφ, aK | patch load solid reference factor Cop, upper value of φi |
| Wall friction | wall friction coefficient μ, conversion factor aμ | lower value of wall friction |
| Bulk and compression density | unit weight γ, conversion factor aγ | loads in the vertical section |
Measures against eccentric loading
- OutletA concentric outlet or a feeder that draws off the whole cross-section evenly.
- Several outletsOperate them at the same time or design the wall for one-sided operation.
- FillingFill concentrically and keep the top pile as central as possible.
- Mass flowNo eccentric flow channel in the stationary bulk solid.
- DesignDesign the silo wall for the eccentric load cases to EN 1991-4 if the eccentricity cannot be avoided.
Frequently asked questions about eccentric loading
- When is an eccentricity considered large?
- When the eccentricity of the outlet eo is greater than 0.25 · dc. During filling the same limit applies to the eccentricity of the top pile ef, for slender silos from hc/dc > 4.0. For silos in Action Assessment Classes 2 and 3, additional, separate load cases must then be examined.
- What are patch loads?
- Unsymmetrical additional loads superimposed on the symmetrical filling and discharge load. They represent small eccentricities and irregularities. Their magnitude depends on the measured conversion factors aφ and aK through the patch load solid reference factor Cop; in Action Assessment Class 1 they may be omitted.
- Which parameters does the standard require for large eccentricities?
- Under 5.2.4.1 (4), the lower characteristic value of wall friction and the upper characteristic value of the angle of internal friction φi. Both follow from the measured mean values and the conversion factors aμ and aφ.
- Which measurements are needed?
- Yield loci at a minimum of three consolidation stresses for φi, K and the conversion factors aφ and aK, wall friction on the wall material for μ and aμ, and bulk and compression density for the unit weight γ.
Have the eccentric load cases calculated
Describe your bulk solid and your silo, including the positions of inlet and outlet – we put together the right measurements and prepare a non-binding quote for you.
- Mean values and conversion factors to EN 1991-4
- Patch loads during filling and discharge
- Flow channel wall pressures for large eccentricities