If in the erection of an iron or steel framed building, the spaces between the girders or floor beams of any floor are not filled or covered by the permanent construction of said floors before another story is added to the building, a close plank flooring shall be placed and maintained over such spaces, during the construction of each story, from the time when the beams or girders are placed in position; but openings protected by a strong hand railing not less than four (4) feet high may be left through said floors for the passage of workmen and material.