Spiral stairs

A stairway, staircase, stairwell, flight of stairs, or simply stairs is a construction designed to bridge a large vertical distance by dividing it into smaller vertical distances, called steps. Stairs may be straight, round, or may consist of two or more straight pieces connected at angles. Special types of stairs include escalators and ladders. Some alternatives to stairs are lifts (US:elevators), stairlifts and inclined moving walkways as well as stationary inclined pavements (US:sidewalks ). Spiral stairs wind around a newel (also the central pole). They typically have a handrail on the outer side only, and on the inner side just the central pole. A squared spiral stair assumes a square stairwell and expands the steps and railing to a square, resulting in unequal steps (larger where they extend into a corner of the square). A pure spiral assumes a circular stairwell and the steps and handrail are equal and positioned screw-symmetrically. A tight spiral stair with a central pole is very space efficient in the use of floor area. Spiral stairs have the disadvantage of being very steep. Unless the central column is very large, the circumference of the circle at the walk line will be small enough that it will be impossible to maintain a normal tread depth and a normal rise height without compromising headroom before reaching the upper floor. To maintain headroom most spiral stairs have very high rises and a very short going. Most building codes limit the use of spiral stairs to small areas or secondary usage.  Source