Keywords: masonry, stairs, helical staircase, stress conditions, equilibrium problems, bearing capacity, torsional solution.

In this paper a simplified interpretation of the equilibrium mechanism for helical stairs is investigated, derived from a qualitative elaboration of some numerical results available in the literature, suggesting a path for approximate structural assessment based on a suitable one-dimensional model of the stairs flights.

The masonry is not effective in sustaining tension stresses and bending, and the stairs should work as a cantilever, or however as an incomplete arch-vault which lacks the counter-thrust from the missing part of the arch. This contradiction cannot be simply solved by means of plane analysis, but by exploring three-dimensional equilibrium paths accounting for the space articulation of such structural systems, searching stress fields in equilibrium and compatible with the resistant abilities of the masonry material.

The present paper illustrates a course of action for the analysis and verification of the helical staircase, based on the classic "beam model". The beam model can be traced, after a procedure of understanding of the resistant mechanisms is set up, in accordance with the modern numerical techniques. The basic internal force distributions through which the stairs can equilibrate their own weight and live loads, and the correlation that was intended by the original builders between statics and masonry tissue are recognized.

In professional practice this simplified approach can help the interpretation of the structural behaviour of helical stairs and suggest procedures for decisions and the design of refurbishment interventions.

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