Why Are Grain Silos Round?

Take a drive through the countryside or along any highway through the Midwest, where grain is most abundantly grown, and you'll see the tall, cylinder shapes that rise above the horizon. It's a sign that a grain-producing farm is near. But silos aren't built round for their monumental value. They are built as cylinders for practical reasons, ranging from cost to efficient land use.

Round silos minimize the land needed for the storage building and make more efficient use of the land. Land costs money, but the sky is free. The tall shape aids in storage and dispensing.

Most people don't think of grain in terms of weight, tension and pressure, but when you harvest a field of corn, all those kernels add up to tons and tons of weight. That weight not only exerts pressure on the bottom of the stored grain, but also on the sides of the container that hold it. The outward forces are known as hoop tension forces, which are more efficiently contained through steel reinforcement of the walls. Weight compression also creates heat, adding more pressure to the building. A square or rectangular structure would have to resist bending pressures, and their foundations would have to resist the overturning forces that can be caused by grain loads. By comparison, cylindrical structures are built more to resist the vertical pressures.

For a cylindrical silo, there are joints only along the foundation and the conical top, which should be fitted tightly for two reasons. First, it increases the rigidity of the walls. Second, it makes the silo airtight and easier to fumigate. Square or rectangular structures have joints at every corner, making sealing and fumigation more difficult. Sealing the harvest from outside weather is important to prevent spoilage. Also, harvested grain attracts rodents and insects, so buildings with fewer joints give the pests fewer spaces through which they can enter.

In a square structure, conveyors and other equipment are needed to maximize the amount of grain that can be stored inside. Machines are necessary to empty the building as well. With silos, gravity takes care of both problems. It is essentially dumped or poured from a higher elevation, and gravity does the rest. Some silos do incorporate mechanized weighing structures and an elevator to load trucks or freight cars, but in most cases that is unnecessary. Without mechanization, the weight of the truck is known before it is filled and then weighed afterward. Those two numbers let you determine how many tons (or bushels) of grain have been loaded for market. Filling the trailer utilizes gravity and a shoot. When the silo shoot is opened, the grain comes down the shoot and into the container that is being filled.

If land costs aren't a consideration, you must decide how tall you should build a silo. That depends mostly on how much grain you anticipate storing in the silo and how long you want it to last. The simple rule is that the shorter the silo, the greater the structural integrity and the greater the endurance, because the pressure of the grain will be less. To be a little more precise, the lowest cost, yet most durable silos, are those in which the wall height is about half the radius of the bin (the round foundation at the base). Therefore, shorter silos cost less and are more durable.