Carver County Minnesota has the distinction of having several round or nearly round barns still standing. This is one I’ve photographed. I have a line on the others and need to plan a trip to capture them. A true round barn is much more wind resistant than one that is nearly round. The flat sides do catch more wind and make for a structurally weaker building than a true round barn.
I’ve run across a list of round barns in Minnesota if anyone is interested.