Structural members widely adaptable for use to build various composite structures and assemblies having horizontal and vertical supports, such as columns, beams and rails, are shown and described. The composite structural members include a longitudinal channel having a web and first and second legs. Each leg includes a plane that extends from a side of the web and is inclined relative to the web, and a cylinder is located at an edge of the leg that is spaced from the web. Various fittings can be used to engage at least one of the cylinders of the channel to fix the channel in position relative to a second, third and/or fourth channel to form the column, beam or rail. Further, select brackets can be used to attach together columns, beams and rails of the assembly.