Post-tensioned helical staircase

We are proud to be involved with the design of a unique one-off helical pre-cast concrete staircase. 
The stairs travel from ground to first floor level, involve 20 reinforced concrete treads, each one cast individually from a bespoke fibreglass mould, and then laid above each other. 

In order to support the staircase during construction, each tread was propped up with timber formwork, and as the stairs went up, three tensile steel cables were fed through each tread, all the way from base to top, and thereafter tied to steel structure at the very top & bottom and then post - tensioned (tightened up) in order to pull all the treads together. Then the formwork was removed and the stairs remain self-supporting. 

This staircase involved many consultants and specialists to ensure it worked. 

This staircase was a complicated project in its own right, independent from the extension and refurbishment works undergoing within the host building, however the structure offers the appearance of a simple floating staircase with the complexity of its concealed engineering going almost completely unnoticed.