Abstract: Integral transverse flanges on duct sections made up, for example, of two L-shaped work pieces, are readily assembled into permanent rigid section-joining frames. At the frame corners, the interfitted flanged ends of corner pieces are trapped, adjacent to the integral flange webs, behind an outward-turned ridge on a surface of the duct and beneath an inward and backward turned spring margin having a progression of lanced projections. Tabs, extending across the lines of juncture of the L-shaped pieces along the outward turned ridge, are peened over to seal the joints of the work pieces positively and prevent the escape of air. Easily applied clips hold the flanges of the adjacent duct sections tightly against each other, and may be used also to retain narrow reinforcing bars likewise behind the out turned ridge and beneath the spring margins.