This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
26. A type of circular stairway, similar to that described on drawing plate entitled, Winding Stairs, in the section on Architectural Drawing, is shown in Fig. 22. This stair is suitable in some such position as an engine room, from a sidewalk to a subcellar, or from an attic to a dome or tower, or in any position where space is limited. The construction is of the simplest character, and consists of a central core or wrotight-iron pipe a resting on a plate b at the bottom. The treads and risers c are of cast iron in one piece, with a collar to fit over the pipe a, so that in erecting the stair the risers and treads are slipped on as shown at Fig. 23. In order to keep them in place, the upright bars of the railing d, Fig. 22, are continued down through each tread near the front edge to a lug, or projection, e on the back of the tread immediately underneath, and secured with a nut. The top riser acts as a brace to hold the pipe in place, and is fastened to the beam with a wrought-iron strap of any suitable shape. There are no strings to be bent in this stair, but the hand rail f must be bent to the proper curve. This rail is composed of a small channel iron with a 1 1/2-inch wrought-iron pipe on top. To bend any bar or pipe, the most satisfactory results are obtained by the use of rollers, preferably run by power, as shown in Fig. 24. one of these rollers being capable of adjustment, so that the pipe or bar may be bent to any required radius. In this way, not only is it possible to give the hand rail the necessary curve, but, as the amount of rise per foot is known, it can be given this upward direction at the same time by setting the gauges, or guides, a and b to the proper angle. It is presumed the stair is erected before the hand rail is bent; and to determine if the rail has been bent to the proper curve, it is laid over the top of the steps, and corrected, if need be, by hand. It might, however, be required that the hand rail should be bent and set quite independently, and assuming that the rail were a flat bar, 1 1/2 in. X 3/8 in., it would be accomplished as follows: An end view of a tread and riser would be laid out in outline, the sizes being taken at the center line of the railing, as shown in Fig. 25 at b. A part plan of the stair, showing two treads, would then be drawn, as e a and a f, with a dotted liner c d, through the points e and f where the tread lines intersect the outline of the stair plan. Then, from a as a center, and with a radius equal to b (which gives the upward rise for each step), arcs would be struck on the curve, the intersections of which, g and h, with the line c d would give points in the true curve of the rail. A circle described through the points g, a, h would give the curve to which the hand rail should be bent. The bar is then bent to this new radius and twisted evenly along the whole length of the stairs, in the manner and with the tools shown in Fig. 26. It will be found that the whole bar has now the form of a helix, and is of the required radius. Before being twisted, the positions of the balusters should be marked on the bar, and the holes for them bored or punched, as the case may be.
At (b) and (c), Fig. 22, are two plans, one showing the start at the cellar-floor level, and the other the finish of the stairs at the platform. The detail of the connection at the platform, and the method of securing the baluster through the tread, and details showing the corner of the platform and the pipe forming the rail, are shown at (d) and (e). The platform is strengthened by the ribs g cast on the under side.