Silage clamp overloading

Previously in this series we have looked at clamp design and how to design a clamp to suit your storage requirements. The economics of agriculture and its related industries make it unlikely that clamps are built far bigger than are needed – so space is limited.

Add to this the chances of taking on more land or a really good growing season and you can start to run out of space very quickly. In a grain store this would be a real problem but silage is different, as long as you have the headroom, you can just pile a bit more on top….. and a bit more…. and a bit more – can’t you? Well as always – it’s not that simple, you can and you can’t stack the silage up high. 

Why shouldn’t you stack silage above the walls?

It all comes down to safety, the safety of the guys working on the top of the clamp and the safety of the structure itself. So let’s look at the guys working on the top first. Many contractors are being expected to “overload” a clamp by packing the crop way above the height of the walls. The chopped crop, however well compacted is a fairly unstable lump to drive on. There are significant risks of shear failure within the heap causing a section to slide off the clamp with or without the compacting tractor. So bodies representing silage contractors have asked for a limit to be put on what they can be asked to do. Limits of one and a half times the wall height or twice the wall height have been suggested. Doesn’t sound unreasonable and no one is suggesting the buckrake driver is lacking any bravery at these limits but is it safe for the guys throwing tyres and rolling sheets?

It’s really difficult to argue that it’s safe for a guy to stand on a slippery plastic sheet at a 40 degree angle that leads down to a three or four metre vertical drop. And the handrail is unlikely to save you if you lose your footing as it’s likely you will slip clean under the rail as you toboggan to your destiny.

So it’s not safe for the guys working on the clamp and it’s not safe for the structure either. We have all done it, and many farmers have been doing it for years, stacking the crop way up to the clouds without incident. 

So why is overloading a silage clamp an issue?

To understand this we need to look back at the history of silage clamp construction. After an increasing number of silage clamp collapses the British Standards agency introduced silage clamp loadings to part 22 of standard 5502 in 1982. This gives designers and builder the loadings that a silage clamp needs to resists. As we will see elsewhere in this series, the standards haven’t really kept pace with agricultural development so the weight of a contractors compacting vehicle will usually be in excess of the clamp design. To reduce the building costs, most designs usually limit the settled silage height to be no more than to top of the wall.

If you load silage above the top of the walls and /or use a machine that is heavier than the clamp was designed for, then you are overloading the clamp. It might not collapse immediately because there is always a safety factor in the design. If you have been doing this for years then you will have stored up all sorts of problems in the future. By loading the clamp into the safety factor you have compromised the service life – you can’t have something for nothing. An over loaded clamp will not last as the designers intended and a failure may well occur much much earlier than you think.

The load on the walls depends on the following four components:

  ·      Density of the material stores – or its weight

·      Shear angle of the material – how well it stacks up

·      The profile – level or stacked up

·      The surcharge – weight of the tractor on top

If any one of these is exceeds the design figures then the wall is overloaded.

Is it ever safe to overload a silage clamp?

Yes it is and the rules for doing it are not too complicated. The easiest way to think about this is what’s happening when you are rolling the clamp. When the tractor is right close to the wall then the loads are at their highest as the silage is being pushed away from the loaded tyre and these forces go into the wall. When the tractor is in the middle of the clamp then the forces on the walls are insignificant. The same is true of the silage stacked above the top of the wall, if it’s stacked up in the middle of the clamp then the walls will not be influenced.

The rule is fairly simple, an area as wide as the walls are high is the no go zone. Outside that you can stack silage up and use a heavier loads and you will not be compromising the walls.

In practice, let’s start by imagining a 2500t clamp that’s 3m high, 50m long and 24m wide with a 10t design limit. A strip around the walls 3m wide, should be filled level and compacted with a tractor no more than 10t in weight. Inside that area the silage can be stacked up above the walls and a larger tractor / shovel can be used. This gives you a section in the middle of the clamp that’s 18m wide and 47m long where you can store the “excess” silage. This is a safe zone where you can exceed the design limits.

·      The loads will not influence or overload the walls

·      If the pile slips there is less chance of the tractor falling off the pit

·      If someone slips on the sloping area they shouldn’t fall off the clamp

If you want to discuss how you can get a quart of silage into a pint sized clamp or to discuss any of the other aspects of silage making covered in this series – contact Jeremy Nash @ jeremynash1@btinternet.com

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