Transforming the centre of Stockport with considered engineering
The Interchange project is completely transforming Stockport’s city centre. A huge part of that transformation is the work being done at the bus station. Coined, ‘The Oculus’, the new structure will accommodate a beautiful two-acre park atop the bus terminus; itself housed inside a grand 150m by 75m concrete structure.
Far from being a simple upgrade to the transport infrastructure of the town, the development will become the centrepiece of a new, pedestrian-led Stockport. With green open space for all to enjoy, the refreshed location will attract greater footfall to this evolving Northern town centre.
The challenge of building the Oculus
We became involved with the project at RIBA stage 3. Our role was to develop the design, working closely with Willmott Dixon as the main contractor and Mayo Civils as the concrete frame contractor. The engineering proposals that were in place for the Interchange project were workable but over-engineered. By utilising our ability to rationalise complexity and drive through an efficient design, we were able to develop proposals for this unique project that removed excess material, simplifying the design and construction.
Removing concrete from the structure
The initial thing that was apparent from the plans we inherited was that the beam arrangement was overcomplicated and inefficient. By rationalising this arrangement, we were able to reduce the number of beam types per structured bay, hence reducing the volume of material used and significantly reducing embodied carbon.
The rationalised podium slab reduced the weights of the structure and therefore the loads on the supporting columns and foundations.
Collectively, these design changes allowed us to remove significant amounts of concrete from the entire development.
Installing movement joints
Given the scale of the Oculus and given that it’s essentially an external structure, it’ll be subject to movement due to temperature variations in summer and winter. This is because materials shrink and expand when subjected to temperature differentials.
To accommodate these movements, we rationalised the proposals which would have had a detrimental effect on the architectural detailing and adopted four movement joints that would accommodate the relative movements.
Because of the sheer scale of the structure and the associated forces passing through these movement joints, we worked with the supply chain to source suitable connecting dowels that could transfer the forces across the joints. This simple strategy saved close to £0.5m for the project.
The advantages offered by ground granulated blast furnace slag (GGBS)
GGBS is a by-product of quenching molten iron slag in water or steam. The process creates a glassy, granular product that is dried and ground into a fine powder for use in the construction industry.
Surprisingly for a waste product, GGBS has many applications in engineering. Not only is it a viable material to use in structural components, but it also has many advantages over cement. When replacing up to 65% of the cement in a concrete mix, it vastly reduces the risk of thermal cracking during curing. What’s more, it dries slower and shrinks more slowly, reducing the amount of relative movement generally: helpful on a project like the Oculus when the environmental conditions naturally result in shrinkage and expansion.
Finally, the use of GGBS means less cement is used. Being that GGBS is a waste product, it’s more a sustainable substitute to cement and reduces the embodied carbon in the structure
For all these reasons, renaissance looked to maximise the use of GGBS in the concrete mixes, using 60% GGBS cement replacement.
A focus on beautiful architecture
The Oculus project isn’t purely functional. It will be a visual spectacle both in terms of the green space it provides and the attention to detail in the architectural engineering delivery. As an example, the supporting columns that prop up the two-acre park have nuanced details.
At the top of each column, a collar detail will provide a shadow gap and the impression that the roof is floating over the columns. This is achieved by a small indentation that runs around the top 150mm of the column. The columns are 750mm in diameter right up until this last 150mm where they reduce to 650mm. Aside from the aesthetic value of this feature, it also improves the overall efficiency of the structure.
Our design of the above elements enabled us to remove considerable amounts of embodied carbon from the Oculus project. With less concrete used and more use of sustainable concrete mixes, we were able to lower the embodied carbon count of the project by almost one-fifth.