Applies to erection of iron or steel structures including assembling or fabricating at the job site. Contemplates specialists who erect concrete structural beams or girders which have been cast by others, as well as contractors engaged in connecting the bar joists to the steel framework. Applies to contractors engaged in the erection of new exterior iron or steel balconies, fire escapes, staircases, and fireproof shutters. Applies to the erection of iron or steel radio and television towers, water towers, smokestacks, gas holders and spherical steel tanks mounted on steel supporting structures. Also contemplates replacement of old iron or steel on the exterior of buildings and the erection of iron or steel bridges. Does not apply to the placing of reinforcing steel bars in connection with any reinforced concrete bridge. Welding operations are considered incidental to the iron or steel erection work. The erection of water towers consisting mainly of iron or steel is assigned to 5040. The erection of water towers consisting mainly of other materials is assigned to 3724.