If in any house, building or structure in process of erection or construction, except a private barn, or a private house, the distance between the enclosed walls is more than twenty-four feet in the clear, there shall be built, kept and maintained, proper intermediate supports for the joists, which supports shall be either brick walls or iron or steel columns, beams, trusses or girders. The floors in all such houses, buildings or structures shall be capable of bearing in all their parts, in addition to the weight of the floor construction, partitions and permanent fixtures and mechanisms that may be set upon the same, a live load of fifty pounds for every square foot of floor surface.