Our new PROCESS series looks at particular technical aspects of our current on site projects.
This month’s feature focuses on the feature concrete staircase at the XYZ building in Spinningﬁelds, Manchester and charts the process of how this was developed from inception through to completion.
The feature stairs at the XYZ building are one of the most defining elements of the project. Conceived almost as a piece of sculpture at the heart of the building, the pre cast concrete stair is formed by alternating switchback flights crossing a central landing, creating vast volumes between the floors. The team developed a solution with TSS invisible connectors whereby holes were cast into the insitu concrete stair core and the shoot bolt connectors were cast into the sides of the flight which meant there would be no steel on view. The exposed concrete finish to the stair sits alongside the exposed insitu concrete stair core, with solid, white painted metal balustrades throughout.
Developing the design
TopFlight Precast, based in South Lanarkshire, Scotland, were appointed as specialist subcontractor for these works. We developed design intent drawings for the staircase and subsequently TopFlight issued their working drawings to the team for review. After several submittals, workshops and telephone conversations the drawings were broadly agreed aside from the winding half landings and soffits. Due to the complex geometries it was impossible to convey how the flights and soffits would ultimately look in 2 dimensions. Because the stairs were so integral to the building we needed to find a way to resolve and agree how these soffits and the interfaces would be finished.
Modelling the staircase
As a result Topflight built a small scale timber shutter (image A) to mimic the main flights and cast a plaster version of it. CP were issued photographs of the original model and we returned our initial comments on this (image B). It was critical that the transition between the landings was made as smooth as possible. By creating this scaled physical model it enabled us all to truly appreciate the complexities involved and more quickly helped us resolve and agree the detail. A final version of the model was then completed and sent to us for any final comments (image C).
With the model agreed, TopFlight poured the first flight – using timber templates the soffits were crafted and smoothed by hand. Photographs of the first flights were then issued to CP for review. Whilst the quality of the flights was clearly good and the soffits on each flight looked smooth, it did not tell us how two flights would sit aside one another and what the transition along the landing joint would look like. TopFlight were able to suspend the first two flights (weighing 4 tonnes each!), inside their factory in Scotland to demonstrate how the stair flights would look when installed (images D, E and F). Satisfied that geometries were correct with this first flight, Topflight then began casting the additional 12 flights.
In early Spring 2016, the stair flights were delivered to site and installed (images G, H and I). The development from our initial design intent to the final product was long and difficult, however the process of using physical models was invaluable to understanding how the stairs worked and it is certainly something to be encouraged for future projects.
See more on the XYZ project.