Cast Iron Spiral Stair (American Elevator)
February 1, 2007
Surprisingly, this single stair rose eight storeys (or more). Effectively it's a single column with each tread placed on top of another one and lateral bracing tiebacks occur every storey. Because this place has been abandoned for so long, some of the lateral bracing bars were no longer connected meaning this stair became fairly wobbly a few storeys up - not reassuring.