The Royal Welsh College of Music & Drama is planning to make a commanding appearance on the Cardiff streetscape under a sleek new roof. Stephen Kennett explains its complex geometry
As a performing arts venue, the Royal Welsh College of Music & Drama cannot be accused of hogging the limelight. Although the organisation has a high cultural profile, the seventies brick building it occupies opposite Cardiff’s grand civic centre is the sort of place that most passers-by wouldn’t give a second glance.
Jason Flanagan, director of architect BFLS, describes it as “demure”, with no real sense of presence or even a proper front door. “When we were first visiting the site we would ask the taxi driver to take us to the Royal College and they wouldn’t have a clue where it was,” he says.
Next spring, though, Cardiff’s taxi drivers should have a better idea where they are heading. The college is 18 months into a £22.5m development that will add a
450-capacity concert hall to the site, together with a 160-seat state-of-the-art theatre and studios. It will also get a prominent entrance.
It might seem strange for a building that was intended to put the college on the map, but it was designed from the inside out. Flanagan says the starting point was to get the specifications of the concert hall and theatre right, in particular the shape that would create the correct acoustic conditions. Once that was decided on, the designers addressed the external form.
The site itself was highly constrained. The concert hall has been placed at the north-west end and the new theatre and rehearsal spaces were sited alongside the old building. This could have looked rather disjointed if it weren’t for the sleek, oversailing wing-profiled roof that floated above the buildings and unified them.
The aspiration was for a sleek building with clean lines, which is always a recipe for a lot of design work, and so it proved with the complex geometry of the 140m long roof.
Structurally, making a roof appear to float was a challenge. The height of the roof edge - about 12m - was determined by the old college building; its floating appearance was achieved by separating it from the new buildings using a 1m tall glazed “shadow gap” and setting the support columns back from the building perimeter where they can’t be seen by passers-by.
In plan, the building curved considerably, and placing the support columns back from the perimeter required cantilevering the roof all around the building edge, typically by as much as 8-10m. Because of this the design team wanted to keep the roof as lightweight as possible.
If this weren’t taxing enough, there was a further complication. The concert hall had to be acoustically isolated from the rest of the building and the easiest way to achieve that was to keep the hall as a discrete structure that stopped short of the building’s full height. The solution was to use cellular beams to span the large distances over the concert hall and theatre, and triangular trusses to extend the roof beyond the perimeter. “This approach is extremely light and efficient,” says Nick Dobson of Mott MacDonald, the structural engineers on the job. “It’s using a conventional way of building but on a non-standard shape.”
As well as projecting out the buildings’ edges, the roof sticks out about 30m at one end. This projection is supported at its midpoint by a single, tapering, needle-like hollow steel column.
Achieving this was not straightforward. According to Dobson, this section of the roof tends to twist because of the wind. “The shape means it is affected by wind more than a conventional design might be because we get big accelerations over the top surface, which generates a lot of suction and uplift, especially at this end, where it tries to lift and twist at the same time,” he says. To prevent this, a diamond box truss has been used to provide torsional rigidity, preventing the roof from springing around in the wind and retaining its slender dimensions.
The original intention was to use aluminium panels to create a sleek finish to the roof, especially along the edge and the underside of the soffit and the entrance foyer. However there were concerns that because of the complex geometry, which curves in two directions, it would be difficult to create a ripple-free mirror-flat surface. Contractor Willmott Dixon hit on the idea of using grp, which gives a perfectly smooth finish and is durable. It would also avoid any exposed metal connections, which was desirable in the maritime environment
When we were first visiting the site we would ask the taxi driver ’take us to the Royal College’ and they wouldn’t have a clue where it was
Jason flanagan, BFLS
There was a downside, though: moulds would have to be made for each differently shaped panel, and in the original design there weren’t two the same, making it prohibitively expensive. BFLS ran the design through the Generative Components software package, and was able to rationalise it to five different panels for the “bullnose” detail around the roof edge and a handful of other sizes to complete the cladding on the underside elsewhere.
The silver-coloured bespoke bullnose panels, which measure roughly 1m wide by 2.7m deep, were made by Torclad and were formed with vertical plywood ribs inside. These stiffened the panels, helping to resist wind uplift, and were also the fixing point for the steel angles that were bolted to the truss around the roof edge.
This sleek design extended to the topside of the roof, which had been kept free from plant by virtue of the bottom-up servicing strategy used within the building. On the double-curved surface a single-ply roofing membrane was laid on 18mm of plywood fixed to the roof deck, which is supported off the primary roof steels. This was separated from the bullnose by a neat gutter designed to act as an expansion joint, taking up any movement in the panels.
All of the deck and membrane has been installed across the main roof and about a quarter of the grp panels have been fixed. Although the building is not due for completion until spring next year, Flanagan says the sleek, clean lines he hoped for are already emerging.