Formby Stair
A breath-taking two-storey French limestone staircase that is completely unsupported as it sweeps 320 degrees between floors. The stair forms the centrepiece of a new £12m Palladian-style mansion set deep in the Formby countryside.
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Client
Private
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Architect
RAL Architects
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Photography
Agnese Sanvito
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Value
£400k
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Completion
2017
Our team worked with The Stonemasonry Company to design the world first helical stair, which unites three floors and culminates in a decorative domed roof lantern which floods the centre of house with light. The shape of the stair was elegantly sculpted to maximise impact by making the edges of the treads as thin as possible, while along the central spine two post-tensioned cables hold the entire stair together.
The stair works by threading the two cables through the 22 individual treads for each flight. These cables were then tensioned hydraulically to 150kN each in order to keep the 6.6t stair stable. The assembly can be likened to a string of blocks on a string that only becomes a solid structure when the string is pulled tight enough, though add to this the helical shape of the stair and many other complex forces come into play and must be overcome.
Multiple in-depth 3D analysis models were built to calculate the flow of stresses through the stair at each stage of its construction to justify stone as the primary structural material. Coupled with this, both scale models and full-size tests were carried out to ensure the predicted analysis results matched the actual behaviour of the stair.
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