Tread mounting system for spiral staircase

A spiral staircase comprising:

• a mounting column extending along a longitudinal axis;
• a plurality of tread assemblies each having a tread portion attached to a mounting plate, each of said tread assemblies being axially and angularly offset from an adjacent one of said tread assemblies along said mounting column;
• each mounting plate having a first face that engages a corresponding portion of an outer surface of said mounting column and a second face from which said tread portion extends;
• and
• each of said tread assemblies further having attachment hardware that secures said mounting plate of each said tread assembly to said mounting column,
• wherein, prior to attachment to the mounting column and in an unstressed state, the first face of the mounting plate has a first radius of curvature and the corresponding portion of the outer surface of the mounting column has a second radius of curvature, the first radius of curvature being less than the second radius of curvature so that when the mounting plate is secured to the mounting column with the attachment hardware and the first face of the mounting plate substantially engages the mounting column, the mounting plate is in an elastically deformed state applying tension to the attachment hardware to inhibit loosening thereof.