Abstract: A rectangular steel frame has a wide peripheral flange integrally connected to a narrow peripheral flange by a web, and a set of spacer bars of predetermined thickness are welded to the narrow flange around the frame. A multiple layer glass unit is positioned within the frame, and a set of trim base strips are secured by peripherally spaced screws to the wide flange and to the spacer bars around the frame for capturing a peripheral portion of the glass unit. Steel reinforcing strips are secured to either or both sets of trim base strips by the screws, and corresponding sets of aluminum trim channels snap-fit onto the base strips around the frame for enclosing the steel reinforcing strips and to provide a neat decorative appearance. The above structure is also used to form a mullion within the window unit or a window frame assembly for a stepped opening in a building wall. Glazing materials extend between each set of trim base strips and the peripheral portion of the window unit.