The Problem

A new bolted steel storing 9,000 tons of fly ash from the adjacent coal fired power generation station split apart two weeks after it was first filled to capacity. Up to this point, no ash had ever been discharged. Curiously, the collapse occurred at night when the silo was being neither filled nor emptied.

Jenike & Johanson's Expertise

During the investigation into this failure by Jenike & Johanson engineers, calculations showed that the silo was under-designed and did not identify or account for a phenomenon called “thermal ratcheting”. The walls of outdoor metal silos expand during the day and contract at night as the temperature drops. If there is no discharge taking place and the material inside the silo is free flowing, it will settle as the silo expands. However, the material cannot be pushed back up when the silo walls contract, so it resists the contraction, which causes increased tensile stresses in the wall. The effect is repeated each day that the material sits at rest. The investigation also illustrated that the silo supplier did not implement the correct bolts.

Our engineers calculated pressures exerted on the silo walls by the stored bulk solid. We analyzed two different conditions: loads applied after the initial filling of the silo, prior to any material discharge, and loads after material had started to discharge (flow loads).

The Result

There were many contributing factors to the collapse that acted together, and if any one had not been present, the collapse of the silo might have been avoided. If proper bolts had been purchased and used, the silo collapse may have been avoided. If the silo had been inspected by an independent silo expert, either during the construction or after construction was complete, perhaps the incorrect bolts would have been noticed and corrective action could have been taken.

The life of a silo can be divided into three distinct phases: design; construction; and utilization. In each of these phases there are numerous opportunities for errors that can result in structural failure. As with this silo failure, the majority of structural failures of bins and silos can be attributed to a combination of several deficiencies or errors.

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