A duct structure (50), includes lengths of square/rectangular duct (52) manufactured by preforming spiral duct in round cross-section (51) and then transforming the round cross-section into square and rectangular cross-sections utilizing expandable die structures (58). The duct (52) may be connected together end to end by flanged connectors (54) formed from strip stock (220), that is notched at locations that correspond to the corners of the formed flange connector. The cross-sectional profile of the flanged connector is then formed by roll forming and/or bending and/or other techniques. Thereafter, the strip stock is bent at its notched locations to define the corners (230) of the flange connector (54) and then the free ends of the strip stock are fixed together.