FCB Studios’ support services hub for Cardiff University mixes advanced concrete design with classically inspired features
The Cathays Park conservation area in Cardiff is known for the classical grandeur of buildings such as City Hall, the National Museum and Gallery of Wales and Cardiff University’s main building. So it’s entirely fitting for the university’s new Centre for Student Life to have a 140m-long colonnade, featuring in-situ concrete columns that soar to over 13m high.
Designed by Feilden Clegg Bradley Studios, the Centre for Student Life is a five-storey building that brings together a number of support services previously housed in disparate locations across the university estate. It includes an advice bar and counselling rooms, informal study and common areas, staff offices and a 550-seat lecture theatre. The colonnade is its public face, veiling the largely glazed facade and quietly echoing the Portland stone neoclassicism of the university’s main building, which stands opposite across the busy thoroughfare of Park Place.
While such classical allusions tend to exude calm and order, this belies an array of tricky engineering and logistical challenges faced by the project team. The site has an awkward footprint, hemmed in between the road and a railway line, and with a sewer running through the site and below the building footprint, which needed to remain accessible. As a result, the building is irregular in plan, splaying out towards the southern end as the road curves round. The building extends to the back edge of the site, on piles surgically threaded between the rail tracks and the sewer, while a transfer structure carries the building over the sewer’s exclusion zone at first-floor level.
Partly due to these constraints, and partly because of the bespoke nature of the centre’s wide range of facilities and functions, every single floorplate is unique. This lent itself to an in-situ reinforced concrete structure: cores are positioned at the north and south ends and approximately in the middle, with typically 350mm-deep flat slabs spanning about 8.5m. These are supported on a slightly irregular grid of exposed-concrete cylindrical columns, the diameters of which vary between 450mm and 750mm.
The complexity of the structure is most obvious in the asymmetrical atrium at the heart of the building. Here, the exposed-concrete columns become an impressive visual element, with some soaring through the space vacated by the irregular voids to span multiple levels. “It has almost a forest feel,” says Owain Morgan, senior structural engineer at Arup. “The main challenge was finding the optimum column positions to support the varying floor shapes while maximising the open space within the atrium.” The solution was to stiffen the slab by using the perimeter upstands as structural beams, transferring the loads to the columns below via 750mm-deep inverted shear heads hidden in the raised floor.
At first floor level, the void extends to the front of the building, creating an airy double-height entrance space. This is part of an architectural concept to create a building that steps down from back to front, with the concrete structure becoming progressively less intrusive as the offices and meeting rooms on the railway side of the building give way to the more open spaces on the main entrance facade. “We became interested in the steps you find cut into quarries,” explains Andy Couling, partner at FCBS. “We liked the idea of a carved-out void with things happening at different levels, continuing that landscape of steps through the building.”
Continuing this language of voids and steps, the northern end of the building is a largely external space, incorporating a two-level covered concourse, with a broad staircase from the street and a first-floor terrace. This forges an elegant route between the main building and the adjacent Students’ Union, a 1970s brick building that oversails the railway line at the back of the new development. It is also intended as a stage for outdoor events or exhibitions, so it was important to keep it as open as possible, with a minimum number of columns interrupting the space. To achieve this, a deeper transfer slab was used at third-floor level between the north and central cores.
Such features added complexity to the concrete design. The transfer slabs were of various depths, the amount of reinforcement and cover changed throughout the irregular structure, and some concrete transitioned between external and internal exposure conditions. “From a concrete contractor’s perspective, it becomes difficult to understand the structure without the architecture,” says Morgan. “We created a detailed set of drawings to convey the concrete information for each element and area; we used a reference matrix to show the mix, finishes and cover for each face.” So, for example, where the soffits are exposed beneath the external transfer slab, the contractor knew to pay particular attention to the joints in the plywood shuttering so that they align with the columns.
Many of the other big construction challenges revolved around maintaining a 24-hour access from the Students’ Union onto the main road which cuts through the site. The main contractor, BAM Construct UK, decided to build the centre in three phases, casting the structure for the north end and southern ends first. The northern phase needed to be braced with a temporary steel frame; once this part of the structure had been completed an airport-style tunnel was erected across the first floor. This provided a new temporary access through the site to the Students’ Union. The site team could then continue with the demolition of the original Students’ Union access stair and construct the central and final phase of the building.
One potential consequence of this staged construction approach was that the concrete could have been visibly different across the three phases. This was particularly concerning as the colonnade – the most prominent design element of the whole project – was being cast in situ with the rest of the structure, largely due to the logistical complexity of transporting heavy precast elements to a tight, inner-city site. Any changes in tone between columns cast six months apart would be as visible as tide marks on a beach.
BAM worked closely with concrete frame contractor Thames Valley Construction and the concrete supplier Hanson was able to stockpile enough constituent material for the visual areas of the project upfront – some 350 tonnes, with all the aggregate sourced from a single quarry. The team were able to benchmark the tone and quality against another project that structural engineer Arup had worked on just down the road – One Central Square. This had a similar 40% GGBS mix and a light, Portland colour, which samples showed was a close match for the facade of the main building. A key factor of the concrete for the floor plates was the shrinkage characteristics of the mix, Morgan adds. “The building is nearly 140m long and there are no movement joints within the structure. We undertook a detailed analysis in terms of shrinkage forces and the proportion of cement replacement to minimise the impact of that.”
Extra care was also taken to ensure consistency of the mix for the colonnade – particularly as the ground-level columns were 13.1m high. With this in mind, a finer, 10mm grade of aggregate was specified to improve compaction. Steel formers were used, leaving a smooth as-struck finish that is visually consistent with the interior columns. “We only used two sets of steel shutters on the whole colonnade” says Kingsley Davies, design manager at BAM. “We were able to cast them in pairs and reuse them. And the pour heights were standardised so we could also use them on the levels above.”
There are a number of thoughtful details in the casting of the colonnade. The seams from the column formers all correspond with the grid lines, and the recess that disguises the join between the two pours is of the same height and depth as the mortar in the stonework on the main building. The third-floor deck meanwhile, which divides the colonnade in two, is finished with a continuous 250mm-high precast-concrete lip.
A final flourish is provided by the precast-concrete cladding, which again neatly matches the tone of the in-situ structural elements. These one-storey panels, which clad the concourse and side elevations, are smooth at ground level but have a fluted motif on the upper storeys, like a Doric column. Combined with the sweeping colonnade, it suggests that the building will more than hold its own among its classical neighbours.