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These frames built with 20x20 extrusion are very rigid, especially if the lengths are cut with adequate precision and squareness. However, at that overall height, 0.75 meters or more, I'd worry about twisting moments. For that, centered side spans may not help much and you might be better off with some diagonal elements. I know that Jetguy on his large bots has ended up needing to use diagonals. He's posted about them to the 3dprintertipstricksreviews @ googlegroups.com list starting with his first for the NYC MakerFaire in 2013. (P.S. Jetguy lives in NC and does frequent some of the makerspaces down that way.)