Square and rectangular steel pipes are steel pipe materials with a closed cross-section that is either square or rectangular. Possessing high bending and torsional rigidity, and strong resistance to buckling, they are widely used as columns, beams, and brace materials in architectural and civil engineering structures. They are also important as mechanical structural materials where strength and lightweighting are required, such as frames for industrial machinery and booms for construction machinery. As for manufacturing methods, electric resistance welded (ERW) steel pipes, which are made by roll-forming steel strips and welding them, are common, with cold-formed types that enhance dimensional accuracy and surface quality being predominant. Other methods include hot forming, cold working from round pipes, and welding steel plates to form a box shape. Ease of processing such as cutting, drilling, and welding, and high design flexibility are also characteristic features.