These have become almost universal in modern fireproof buildings of all types, except where unusual monumental effect is desired, and the design of steel staircases is largely standardized. They consist of a steel string on each side, which takes the form of a plate or channel of sufficient rigidity to carry the weight.
Iron spiral staircases are usually formed of cast iron members, each of which forms a tread and riser, and has at its inner end, a collar arranged to slip over a vertical pipe or cylindrical bar form ing the central newel. Such spiral staircases are rigid independ ently of any wall, and are frequently used in open spaces, such as those existing in power houses, gymnasiums and the like. Rein forced concrete, sometimes finished as artificial stone, is used for stairs and permits of infinite variety of form.
In many places where continuous trans it between floors is necessary, as in large shops, underground railway stations and the like, moving stairs, sometimes known as escalators, are much used. These are of two types. In one, an endless belt carries on its surface links or blocks of sufficient roughness to act as treads. This endless band, at top and bottom, rotates around wheels or pulleys, motor-driven. At the bend the band disappears through the floor into a slot armed with steel arms, so designed as to occur one in each groove between the raised portions of the treads. Another continuous band on each side, geared to the same speed, serves as a hand-rail. The other type has a similar mechanical basis, but the belt, instead of carry ing directly upon its upper surface blocks to form the treads, is designed to motivate actual continuous horizontal treads with risers radially shaped, so that at top and bottom the treads slide up on the risers into a continuous moving plane. Moving hand
rails are furnished as in the other type, and in addition, a moving band angularly across the top and bottom, to remove auto matically any dropped objects. Moving stairs of the second type are always entered and left at the side, those of the other type directly at top and bottom.
As stairs form one of the most necessary methods of exit from a building, their design has formed one of the most important portions of the building legislation of modern municipalities, especially in the case of public, commercial, in dustrial and educational buildings. Although there is much varia tion in the detailed requirements of different cities, the methods of regulation are almost the same and state : (I) where fireproof stairs are required; (2) the number of staircases; (3) size; (4) design. Thus, in addition to the universal requirements for struc tural solidity, the attempt is generally made to furnish at least two stair exits to any large, enclosed, upper floor area ; and where fire risk and the danger of panic are unusually great, as in the case of theatres, to demand sufficient stairs to allow the building to be emptied in a very short time, without disorder. Widths of stairs are frequently determined in accordance with the number of people using them. In New York, for instance, each staircase must be designed to contain all the people of the floor area it serves, on the basis of one person to each 21 in. of width and each one and one half treads. Design is controlled by specifying the rela tionships of tread and riser, the distance apart and size of landings, whether or not curved stairs are allowed, etc., and often, because of the smoke danger, requiring exit stairs to be enclosed in a fireproof tower, with fireproof, self-closing doors at all entrances.