Silo collapse triggers safety warning
WorkCover Queensland has issued a safety alert after a five-metre-high grain silo collapsed on a farmer, while he was trying to position a grain auger and release the slide gate (knife plate) on the discharge cone. The safety alert highlights the risk of collapse of transportable (relocatable) silos, commonly used on farms.
The incident occurred in May 2020, when the support framework buckled on one side of the silo, causing the body of the silo to topple sideways onto the ground. The impact with the ground caused the skin of the silo to tear away from the silo base, spilling grain onto the ground. The farmer was trapped for some time before being removed; alongside the risk of serious injury, there was also a risk of the farmer suffocating in the grain as it emptied from the silo.
The cause of the failure is being investigated, and it is unclear why the support framework failed. Support frames on silos can fail due to damage, including bent support members and broken welds. Bending reduces the load-carrying capacity of a member and increases the load applied to that member because it can cause the silo to tilt, therefore moving the centre of gravity closer above the member.
Impact from mobile plant can also cause immediate collapse or can bend support members, resulting in failure. External corrosion can occur when the protective coating degrades or when parts of the support frame are buried in dirt, while internal corrosion occurs when hollow members fill with water, even with small cavities in the member and no drainage holes. Soft or sloping ground under the silo can also cause it to tilt, making the silo unstable and increasing the load on the support members. Poor design and manufacture, or poor repairs, can also cause the support frames on silos to fail.
Transportable silos are generally designed to be lightweight, to enable easy movement. When a silo is empty and not anchored to the ground or support pad, it can be blown over; its lightweight design also means that damage from being blown over is certain to occur.
Silos must be set up in accordance with the manufacturer’s instructions, and manufactured to allow for all load combinations during their lifecycle, including loads that are applied during filling, storage and discharge, and as a result of environmental factors, such as high winds. Silos should be set up on level concrete pads or on footings that can safely support the load when it is full. If a silo is set up on the ground, point loading needs to be spread so that the bearing pressure does not exceed the bearing capacity of the ground. Information about suitable dunnage or pads under the silo can be requested from the silo manufacturer or a suitably qualified engineer.
Workers must ensure that silos are only filled with product that has been approved by the silo manufacturer, as silos designed for grain may not have the same corrosion-resistant properties as those of a silo for fertiliser. As the density of fertiliser differs to that of grain, filling a silo with heavy product may lead to failure of the silo and its support frame. Silos must also be set up where they are unlikely to be damaged by mobile plant, and away from overhead power lines. They should also be periodically inspected for damage and corrosion.
Silos with damaged support frames should be isolated from workers. When repairing a damaged support frame, ensure the silo is empty, as repairs can increase the risk of failure. Do not allow anyone under a full or partially filled silo with damaged framework, unless the silo has been safely propped, as per the manufacturer’s instructions.
A safe system of work must be implemented to ensure no workers are at risk when installing the props. This includes ensuring workers are not underneath the silo when installing the props, and that the propping method does not increase the risk of collapse. Workers can get under the silo to empty it, after the safety propping has been installed. Once empty, the silo can be repaired in accordance with the manufacturer’s instructions. A damaged silo should not be refilled.
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