Turning to Stone
15th October, 2024

In a recent editorial, Dezeen magazine posed the question, ‘can stone be a viable modern building material?’ It is one of humankind’s oldest construction materials, and one with significant potential benefits for the contemporary world. Stone lasts a long time, it’s durable, it’s beautiful, and it has low embodied carbon. We asked structural engineers James Stevenson and Tamer Sallam to tell us about one of their recent projects, where a concrete staircase was replaced with a stone alternative. How did the numbers stack up?

We are all looking to lower the carbon footprints of our designs. We design lean. We challenge the client’s brief. And we try to specify alternatives to carbon intensive materials like steel or cement. Stone is an excellent alternative, as it is a natural material that requires little processing.

We’d like to talk about a staircase on one of our projects, where a simple change in the structural material meant lower embodied carbon because less raw material was being dug out of the ground, and programme savings as a result of fewer follow-on tradespeople being required to attend site. All this was achieved without any adverse effect on the staircase’s surrounding structure or the foundations.

Our Client wanted a more elegant stair, and we wanted to reduce embodied carbon.  Rather than replace the existing concrete stair with something similar, but designed leanly, we suggested using stone. Not only does stone have a lower embodied carbon than concrete, it also avoided the need for follow on trades required as the stone surface could be left exposed. The stone staircase would weigh less than the existing concrete version, and could therefore be swapped in without any adverse effect on the surrounding structure or the foundations.

Excuse us if we get a bit technical here. The reinforced concrete stair flight had been designed as a one-way spanning, inclined slab supported at the top and bottom on the floors. The treads and risers were mass-concrete, triangular prisms sitting on top of an inclined slab. The 'structure' is the reinforced slab that sits below the treads and risers, which makes no use of the material that is needed to form the treads and risers that we walk up - in fact the added material just adds more weight to the staircase. We thought to ourselves, ‘what if the material for the treads and risers could be incorporated as part of the structure?’ This approach would allow us to thin down the slab or ‘waist’ of the flight, leading to both a more elegant staircase design and less material being used.

Stone is the perfect material to achieve those aims. It uses the volume of each tread - necessary to form the tread and riser surfaces - to work in torsion to carry the loads to the wall and down the flight.

From the images, you can see the visual difference that stone affords, creating a finer, more elegant stair. Intuitively, the profile also reflects the reduced material consumption. The numbers support intuition, with some big savings on embodied carbon and on weight

We love seeing stone having a resurgence and we’re carefully plotting other ways we can use it in more aspects of our structural designs.

Forces and Torsions

Deflections and Rotations

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