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.
A plan of a circular stairway is shown in Fig. 55; all the risers except the three curved ones at the start radiate from the center b, and are equally spaced at the front and wall stringers. The first two steps are curved out, to increase the width of the stairway at the start, thus giving it a more pleasing appearance. A curtail step and hand rail at the starting of such a stairway is very appropriate and attractive. The dotted lines indicate the position of the carriage timbers. A similar method may be followed for elliptic stairways.
Instead of a cylindrical front stringer, as in this plan, a circular newel post of sufficient length is sometimes used as a central support, the treads and risers being framed into the central newel and into the wall stringer. The newel post may be cased with 7/8-inch boards of the same width as the treads at the point of connection, affording support and enclosure like mortises to the steps; the spaces for the risers may also be cut out from the edge of the boards, thereby rigidly securing them. This casing, or staving, would not be set in place until the steps were all nailed in position.