Problem
Within a building technology which uses compressive materials as much as possible, and excludes the use of wood, it is natural to build stairs over a vaulted void, simply to save weight and materials.
Solution
Build a curved diagonal vault in the same way that you build your Floor-Ceiling Vaults (219). Once the vault hardens, cover it with steps of lightweight concrete, trowel-formed into position.
Related Patterns
… this pattern helps complete the rough shape and location of stairs given by Staircase as a Stage (133) and by Staircase Volume (195). If you want to build a conventional stair, you can find what you need in any handbook. But how to build a stair in a way which is consistent with the compressive structure of Efficient Structure (206), without using wood or steel or concrete - Good Materials (207)?
A lightweight concrete tread, colored, waxed, and polished can be quite beautiful and soft enough to be comfortablc - see Floor Surface (233) - and will eventually take on the patina of wear called for in Soft Tile and Brick (248).
The vaulted space under the stair can be used as an Alcoves (179), a Child Caves (203), or Closets Between Rooms (198). If it is plastered, like a regular ceiling - see Floor-Ceiling Vaults (219), it makes a much more pleasant and useful space than the space under an ordinary stair.
Reference for full-text of Pattern: p. 1073medium-confidence