Tonight, while driving my daughter to a party, she mentioned that most farms don’t have silos. I’m not sure I agree with her on that, but it did make me think, as a city boy; why are silos tall and tubular? What property of the grain does this preserve exactly? I mean why not have a few sheds where grain is stored? Wouldn’t a rectangular silo be easier to construct and maintain since every other building was constructed thusly?
Silos aren’t a storage device, like a grain elevator. They are used to produce silage. They are tall and tubular to aid the anaerobic fermentation process.
In addition to the tall, tubular silos, there are also bunker silos, which are three-sided rectangular structures.
Unholy geometries are supposed to be my domain, dammit. 
Three-sided rectangular bunker silos are used exclusively for the storage of material from crop circles. I should have been clearer about that.
Regular ol’ bunker silos are rectangular, with three walls and an open side.
But why tubular? I still don’t get it. Why that instead of a normal rectangular structure?
ETA: I see maybe some aren’t a cylinder, but most are. Why?
Circles are the strongest shape for containing a particular pressure. If you take a cube and pressurize it, the sides will try to blow out like a balloon. This puts more strain on some areas of the surface than others, so you have to use more material to contain it since you’re not only supporting the contents, you’re also having to use material to preserve the shape of the container.
For a gas, a spherical shape is going to be your best bet, but with something sitting on the ground (since you don’t really need to worry that the EARTH is going to fail as a face), the only force you have to deal with is perpendicular to the surface of the Earth.
That said, you’ll see lots of cylindrical containers for gasses or placed horizontally, with flat ends made of the same material as the rest of the container. Cylinders are easier to grip and move, and generally have a more user-friendly form factor than a sphere and are strong enough and still better than a box or other shape.
http://learningstore.uwex.edu/pdf/G3660-4.pdf More than you ever wanted to know about silos. They say they are round to prevent bowing of the walls which would let air in and spoil the sileage.
I now gently bow out from this thread.
This place continues to amaze.
Thank you gonzomax.
That’s an engineering thing. The contents put a lot of pressure on the outside wall, and pipes and tubes are pretty efficient at holding pressure.
This is one of those “I always wondered but never asked” sorta things. I knew that a round silo would work better than a square silo due to the physics and such but I had never thought to ask, “What on earth do they store in those things anyway?” I was under the impression it was an early grain elevator myself. But it turns out silage is the answer. That wonderful document on the history of the silo also informed me that those long giant garbage bag lookin’ things you see in the field are really just plastic “silos” laying on the ground. Neat!
John
OK, now I’m curious. What are these giant garbage bags? I’ve seen the giant marshmallow hay bales, but have never experienced that.
They are cheaper more effective silos. They store the silage in the giant plastic bags out in the fields and keep them air tight just by closing the end with dirt or what not. When it’s time to feed the animals they just scoop some out the end.
ETA: I originally thought they were hay bales too. I guess they could be multipurpose but the article linked below says that they are used for silage as well.
Some farms also have tremendously large, low (6 or fewer feet tall) heaps of feed, covered by black plastic and weighted down with tires. Covering several thousand square feet.
Although not mentioned previously, square silos do not flow material nearly as evenly as round ones. Corners can fill with material that stays long past its prime, and build up in a clogging effect that gradually creeps inwards until you have to go in and clean it. In the coal industry this can lead to what’s known as “ratholing”.
Most farmers have gone to using a auger blower that rotates around a central pivot at the top of the silage. Think of it as a mower that you were doing a circular cut around a tree. The material is striped off the top and after a pass you lower it a little bit and make another pass. The silage can be sent down the feed line under power too. It cuts out a ton of manual hauling. It’s a modern invention that works much better with a cylinder silo too. Silos have steal banned on the outside of the structure for the purpose of handling the outward pressure. That wouldn’t work with a different shape.
Short (18 feet tall maybe) squat round metal buildings are used in IA to store corn until ready for sale. (Anecdotal in that I have seen this with my own eyes at ( 1 ) one IA farm, no clue if anyone else in the whole world does this.) They have ventilation / drying blowers attached at the bottom. How tall does it have to be to be considered a ‘silo’?
YMMV
Silage? Thanks for the info. I never realized that silos were anything more than grain storage units. Very interesting!
Silos are for silage. They don’t have dryers.
Grain elevators are for grain. They have dryers.
Corn cribs are wire walled cylinders with a cone roof or a long shed structure with spaced slats on the sides for air circulation. Field corn is stored in these on the cob with the shuck removed.
She is correct.
Silos are used for making silage, which is feed for animals (cows and pigs, mainly). Most farms now do not raise animals, only crops (i.e. ‘grain farmer’). So these farms have no use for a silo.
Some of them may still have one, left over from a time when they did raise cows. But it’s either unused, or used for temporary grain storage. (Temporary only, because it doesn’t work very well. Silos are designed to be airtight, so the contents ferment. But for grain storage, you want good air circulation, to dry the grain and prevent spoilage.)
The atmosphere inside a ‘working’ silo can be fatal to humans who enter; oxygen levels can be low and nitrogen dioxide levels can be high enough to result in near-immediate asphyxiation. My sister-in-law’s fiance suffocated in a silo accident. Sometimes there will be multiple fatalities as rescuers are overcome one after another.
The big plastic bag horizontal silos tend to minimize, although not eliminate, this risk, because they are packed full and there’s not really any reason to climb ‘inside’ one to perform maintenance. Still, a large enough pile of decomposing hay can give off enough nitrogen dioxide to damage the lungs of bystanders who are downwind, even in open air.