Steel and aluminium silos

1 Bolted and welded construction A first distinction must be made according to the form of joint that is used in metal silo constmction. Many smaller steel silos have bolted joints, and where these are present, every stress developing in the wall, at every point, must be transmitted through a joint. The joints are lines of weakness, so they should be made stronger than is strictly necessary. Careful attention should be paid to edge distances, and it is most desirable that the weakest failure mode of the joint should be by bearing rather than bolt shear, since the latter is not very ductile and lack of fit in the joints may cause 

2 Bursting of the vertical wall Bursting failures are very uncommon and are almost all found in bolted silos where a joint detail has failed. A careful analysis of the loads and strengths in different modes shows that this failure mode is only critical near the surface, or in squat silos. 

3 Axial compression buckling of the vertical wall Buckling of the vertical wall is by far the commonest failure mode in metal silos. The buckles can be huge or quite local, but all buckles should be treated as very serious because this mode of failure is often dramatically catastrophic. 

Under high internal pressures, a different form of axial compression buckle occurs, termed the elephant s foot because of its smooth flat squashed shape. Also, where a buckle occurs adjacent to a support, a buckle may develop in the local high stress fleld, needing a more careful evaluation (the force being transmitted may not be easily determined). 

The evaluation of the buckling strength under different conditions is quite complicated and can be found in Rotter (2001a), EN 1993-4-1 (2007) or EN 1993-1-6 (2007). 

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