Helical stair geometry: arc tread depths at inner, walk-line and outer radii, clear width, rise per full turn, and outer handrail length.
A spiral stair wraps treads around a center column. Each tread is an arc whose depth grows with radius, so the usable tread depth is measured along a walking line near the outer edge. Clear width is the outer radius minus the column radius. Rise per full turn determines headroom: a person on the stair must clear the underside of the flight one turn above.