The round barn at the Hancock Shaker Village is a fine example, although not entirely timber framed (they invented the circular saw blade in the 1830's so why not use it.)
Chimneys can not typically support wood framing members so research this aspect carefully. I have designed reinforced masonry "shells" around chimneys seperated by an airspace to get around this code issue, kind of like a chimney within a chimney and never the two shall meet. Back to the Shakers though. To reduce the number of rafters joining at the peak of their circular barn, they saw-kerfed them up the middle maybe about half of their length or more, like an old wooden pitch fork. The two halfs of the lower part of the rafter were then spread apart to reduce the span of the roof sheathing. The rafters are thru-bolted at the top of the kerf to keep them from splitting further up. It's really something to see. Good Luck. Chris
