Webb Yates Engineers were responsible for the design of this solid stone feature stair and the steel transfer structure it sits on, installed in a listed third floor duplex apartment in Central London.
What you don't see is just as important as what you do see as a stair made from 9 tonnes of Italian travertine limestone is suspended 10mm above the existing slab beneath.
A loading restriction on the existing concrete slab of this third floor Grade II listed duplex apartment required an innovative steel transfer grillage. With a structural depth of just 70mm, the grillage was designed to transfer the load of the stair to a downstand beam and core wall nearly 4m apart. The grillage was precambered and preloaded prior to stair installation to carefully control movements during construction enabling the grillage to be hidden within the floor finishes while maintaining a clear 10mm gap beneath to ensure the slab under was not loaded.
The stair itself is constructed of 13 individual blocks of stone and is a true loadbearing structure with a series of intricate notches and joints used to transfer load and maintain stability with only thin mortar joints between them. Even the solid stone balustrades are made from a series of unconnected slabs that use careful positioning of grooves and notches to lock them in place. The size of each block was limited to 1.2 tonnes to allow them to be lifted into the building via a hoist. The shape of the stair was optimised and any stone not required structurally was hollowed out to reduce weight, leading to an efficient and elegant structural form with the stair arching from one floor to the next.
What first appears as simply an impressive sculptural staircase, when considered further, reveals itself to be a careful display of structural cunning to make the unexpected a reality.
- Structural Awards 2015, Small Projects, shortlisted, Jul 2015