Webb Yates Engineers designed this breath-taking two-storey stone staircase that springs from one landing and is completely unsupported as it sweeps through 320 degrees to the next. 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 6.6 ton 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.
In order to allow the stone to be the primary structure of the stair, multiple in-depth 3D analysis models were built to calculate the flow of stresses through the stair at each stage of its construction. Coupled with this both scale models and full-size tests were done to ensure the predicted analysis results matched the actual behaviour of the stair.