STRUCTURES (EXCLUDING PREFABRICATED BUILDINGS OF HEADING 9406) AND PARTS OF STRUCTURES (FOR EXAMPLE, BRIDGES AND BRIDGE-SECTIONS, LOCK-GATES, TOWERS, LATTICE MASTS, ROOFS, ROOFING FRAME-WORKS, DOORS AND WINDOWS AND THEIR FRAMES AND THRESHOLDS FOR DOORS, SHUTTERS, BALUSTRADES, PILLARS AND COLUMNS), OF IRON OR STEEL; PLATES, RODS, ANGLES, SHAPES, SECT TUBES AND THE LIKE, PREPARED FOR USE IN STRUCTURES, OF IRON OR STEEL