Stave Silo Collapse
Agricultural silos come in a variety of shapes and sizes and materials. As with all structures, bad things can happen that compromise the structural integrity of the shell and cause a collapse of the silo. Steel reinforced poured concrete silos tend to be the most resilient structures against fires, impacts, and age related distress. It’s common to see an empty field with an old concrete silo sticking up as a memory of a farm long since passed. Metal silos are cheap, durable and easy to erect, but they require extra attention to maintenance and proper operation. Stave silos are silos that are built with interconnecting blocks that are held together in a circular shape using steel rods around the perimeter. While these silos often endure the ages, they have a specific weakness that can cause sudden catastrophic collapses. Corrosion of the steel rings.
When a silo is full, there is a large internal pressure pushing out on the shell. The pressure increases as the silage or grain level gets higher and higher. On a stave silo, this pressure is held almost entirely by the steel rings. The spacing of the rings gets tighter near the bottom of the silo because of this increased pressure. If a ring fails for any reason, the stave blocks can easily push outward from the internal pressure. With the loss of a few blocks near the bottom, the weight of the silo above is no longer fully supported in that area and the silo can begin to tip. The tipping adds further stress to the remaining rings and blocks and the entire silo fails in a rapid collapse.
The rings are more closely spaced near the bottom of the silo since the bottom sees the highest internal pressure. The rings are spaced further and further apart as you go higher towards the top. Access doors at the bottom of the silo are generally larger than the spacing of the rings, and therefore require bridging for the rings. The rings that cross over the doors connect to straight metal bridge sections that carry the load from the rings above and below the door. These bridge sections are critical to the entire silo. Should one of these bridge sections fail, the ring tension is lost, stave blocks push out, and the silo collapses. The rings and bridges are exposed to the elements and are subject to long term corrosion which is a slow but steady process. The moment when the strength of the rings becomes ‘just’ below the internal pressure of the silo is impossible to predict.
Stave silos are an efficient design for constructing silos. They have been around for many years and have stood the test of time. However, they require extra attention to corrosion issues. The steel rings and access door bridges are a critical element for the entire silo structure. Failure of the ring assemblies is sudden and unpredictable. Even if the silo does not completely collapse, the shell is generally sufficiently compromised that it cannot be repaired.
Caskanette Udall Consulting Engineers has investigated several silo collapses over the years for a variety of different types of silos. If you have a silo claim, contact our team of experts to assist.
