................also, most duct systems are "thin wall" construction (e.g., D/t greater than 100). One of the major considerations in round section industrial duct systems is keeping them "round" (they even want to "flatten" from their deadweight when they are being shipped). You should look for discussions (articles) regarding ring stiffeners and proper ways to reinforce branch connections. There are some pentstock design books that will give you an insight. Look for AISI publications on steel penstock design and look at AWWA manual A-11. Also look at the Lincoln Arcwelding Books for design of "saddle supports". Sometimes rolled angles are used (rolled into circles) as "flanged connections" and this is the ideal place to connect your hangers/supports.

Good luck, John.