The circular helical staircase at Palazzo Spada

The paper deals with the unpublished survey and geometric analysis of the circular helical staircase built around 1660 in Palazzo Spada’s wing on Via del Polverone in Rome. Its role in the development of the place and the history of its design process are addressed thanks to historical drawings and the reference model proposed both in the treatise and construction between the fifteenth and seventeenth centuries. A description of the 3D digital survey procedures is followed by the reconstruction of a NURBS 3D model that describes the staircase and its geometric properties. Three helical turns of the staircase undergo an in-depth analysis of the slope angle thanks to the development of the helical lines. This leads to the recognition of the main features of this staircase: its unique central structure; the implementation of geometrical proportions descending from Andrea Palladio's treatise; the changes of the slope angle that characterize the staircase flights and the space they generate.