Stairway to HeavenPosted on March 13th 2017
One of our proudest achievements recently has been the completion of a one-off house we have delivered in St. George’s Hill, Surrey, for Werner Capital.
At the very heart of this, literally and figuratively, is the grand helical staircase that elegantly descends through the central atrium. Acting as a focal point to all of the principal spaces of the house, the impossibly slender stair – which is constructed of solid stone – defies gravity.
Let’s take a closer look at this world-first for Lees Associates:
The layout of the house, designed by our founding partner John Lees, follows a disciplined classical approach and one gets an immediate sense of the strong axial arrangement of the plan, with the elliptical stair hall at its core, visible from every principal room. Flooded with natural light from above, and with a glimpse of the spectacular view beyond, the hall frames the centrepiece of the house: the unique helical solid stone floating stair, which winds down from the first floor gallery, through the ground floor hall to the basement wine bar.
This beautiful and stunning feat of engineering at its most pure is the project’s defining architectural gesture, regularly leaving first- time visitors open-mouthed in awe.
Lees Associates developed the design of the stairs in collaboration with engineers, Webb Yates, and stonemasons, The Stonemasonry Company. With its beautifully slender waist the stair is no different to any traditional stone “cantilevered” stair, with one notable exception – there’s no wall to support it! Instead, the engineer has taken post-tensioned technology to another level, employing it for the first time ever on an elliptical stair that turns in excess of 180º.
Engineer, Steve Webb of Webb Yates explains the science behind the magic:
The stair is formed with a number of stone blocks, each one forming a tread. Each of these incorporate drilled holes through which pass steel cable tendons. The tendons are stressed by a hydraulic jack, pulling the stone blocks into compression.
Normal structural sections work by the one face (the bottom face in a simple beam) taking tension and the other, compression. The reason that stone (or concrete) needs to be reinforced is because the tension capacity of these brittle materials is not very high. When ordinary reinforced sections are used, the reinforcement bar stretches and the concrete cracks, significantly reducing the stiffness of the beam. Pretensioning a section means that all of the material is forced to stay in compression even as the beam flexes. There is a simple gradient between very low compression at the bottom face and high tension at the top face. This, therefore, never cracks and is far stiffer in use.
Post-tensioned concrete is a common and very effective structural form, however concrete suffers from long term creep deformations. This means that under a given load the concrete continues to deflect for a long time. This long term movement is caused by water, trapped in the concrete when it is cast, slowly working its way out of the concrete when it is compressed. This causes progressive movements in concrete structures that can only be countered by making concrete sections deeper. Stone doesn’t suffer from this effect. Stone is about twice as stiff as concrete in respect of long term movements because of this.
These measures, using stone and post-tensioning, mean that stone sections formed in this way can be approximately half the depth of their equivalent in concrete. It is also true that the carbon footprint of stone is about 1 / 10th that of concrete, so an environmental benefit as well.
The analysis of the stair is complex. The image above shows the solid FE model used to assess the performance of the stone. This analysis is used to determine both the compressions, tensions and movements in the stone, and to calculate the amount of prestressing that the cable requires to hold the stones together.
The technology employed here was developed on straight flights of stairs, then on a 90º turn, and then 180º. This is the first time a free-floating stone stair has ever been attempted in excess of 180º. It is incredible what can be achieved with an ambitious client with a strong belief in their designers and contractors: Camp End Manor’s stairs are truly unique and uplifting.