We’ve recently completed a co-working office development project in Stratford, London for our client us&co which features what we think to be a unique one-off helical pre-cast concrete staircase. The stairs travel from ground to first floor level, involves 20 reinforced concrete treads, each one cast individually from a bespoke fibreglass mould, and then laid above each other.
In order to support the staircase during construction, each tread was propped up with timber formwork, and as the stairs went up, three tensile steel cables were fed through each tread, all the way from base to top, and thereafter tied to steel structure at the very top & bottom and then post - tensioned (tightened up) in order to pull all the treads together. Then the formwork was removed and the stairs remain self-supporting.
It was quite a bit more complicated than that, but that is the essence of the structural methodology. This staircase involved many consultants and specialists to ensure it worked and we are proud of the outcome.
The building itself underwent complete renovation as well as introducing a new two-storey roof extension to provide co-working space for our client.
Below is a description of the staircase design process:
The stair design followed our initial concept plans for a reception focal point, a feature where office users could gather around and from which other spaces could form, such as the curved facade of the rear garden and the curved partitions forming surrounding meeting rooms and seating areas. The stair acts as a transition piece from ground to first floor only, connecting reception and break-out space with open plan offices above.
The building already offers a lot of exposed structural steel and we wanted to contrast against this with a concrete stair, but much like the surrounding steelwork the concrete stair would expose its junctions and reveal that it is in fact made up of individual pre-cast tread components. The only way this would be self-supporting is by post-tensioning the whole staircase - this means threading three steel cables through each of the twenty treads and fixing them to a steel bracket located at the top and bottom of the stairs and fully concealed, thereafter the cables are carefully tensioned and tightened until the stair is gradually pulled together without exceeding specific forces which could force the treads to individually shatter.
We appointed Fluid Structures (now MLM Group) who had teams with good experience working on bespoke projects - and this post-tensioned pre-cast helical staircase was one that was quite a rarity. Fluid’s structural engineering team worked with us to resolve structural forces and moment junctions with the staircase. Whilst we designed the appearance of each tread and essentially the whole helical staircase, the structural integrity and integration of each stair as a part of the whole was very much down to them. The post-tensioning exercise was down to Praeter Engineering who specialise with post-tensioning structures.
Casting each tread off site meant that we had good control of the final finished appearance, rather than rely on a single on site pour. We engaged Milbank Concrete to carry out the tread manufacturing process, who in turn sub-contracted out the fibreglass mould formation to replicate our 3D CAD model designs. The first mould turned out just fine, thereafter the steel reinforcement bars were offered inside the tread mould which was a difficult task as they had to avoid the many steel sockets carefully positioned around the perimeter of the tread in order to allow the steel balustrades to be post-fixed. We tested several concrete mould pours to ensure we had the right mix texture and appearance. This took several weeks to perfect, simply because with each concrete pour failure the mould had to be thoroughly cleaned, new steel reinforcement bars bent to the correct shapes and added before it could be re-poured again.
The installation process was another tricky task, and we had to carefully work out with the main contractor (John Cirmaci Builders) how to support each individual tread in the correct position before the next could be offered up. Before this the steel bottom bracket (to which the steel cable would be tied) had to be connected to its own concrete foundation footing. We decided to proceed with a bespoke a timber formwork frame which took the exact helical form of the staircase, within which the stair would be gradually offered into place and connected. Since each of the 20 pre-cats treads was identical, the stair took its helical form as it went up, and all three internal cable routes within each tread lined up perfectly with the next. Each tread was carefully grouted at junction points to help it ‘glue’ together.
Once the treads were up, the post-tensioning process began. This process was a gradual one, being tensioned in stages to allow the stair to move into place before the next tightening process began. Once completed, the surrounding timber formwork was removed and the stair appeared in it’s self-supporting naked form - this is where the stair looked astounding without handrails, and without any visible fixings. The stair was gradually completed by fixing the stair balustrade components to each of the concrete steps to suit the exact tread curvature.
This staircase was a complicated project in its own right, independent from the extension and refurbishment works undergoing within the host building, however the structure offers the appearance of a simple floating staircase with the complexity of its concealed engineering going almost completely unnoticed.
What was the brief?
To create a focal point visible from the main entrance and to use this as a feature from which to design the surrounding spaces. Holistically, we were also asked to create a new building to act as a show-piece for our clients business.