On song

Cardiff Bay has been undergoing something of a renaissance. The docks and wharves which grew up around the coal and steel industries might have faded but the arrival of the Cardiff Bay barrage in the late 90s heralded the capital’s rebirth as a waterfront city, and the stream of developments that followed have injected new life into the area.

The latest, and probably most significant addition to these is the Wales Millennium Centre, which opened its inaugural season at the end of November. Acting as a showcase for opera, ballet, musicals and dance, the Centre will be home to an impressively long list of cultural organisations. Topping the role-call is the world-renowned Welsh National Opera, who will be joined by the Diversions Dance Company, Academi (the Welsh National Literature Promotion Agency and Society for Writers, Ty Cerdd (a Welsh music information centre and recording studio), Urdd Gobaith Cymru (the Welsh youth organisation), Hijinx Theatre and Touch Trust, a charity for educational touch therapy.

Plans to build an opera house in Wales were hatched over a decade ago. An architectural competition run in 1994 was won by Zaha Hadid but the scheme, seen by some as too radical, ran into difficulties and was eventually scrapped in acrimonious circumstances. Despite this setback the aspiration lived on and the project, now without its opera house tag, was resurrected. This time local practice Percy Thomas – who had been executive architects on the Hadid scheme – was appointed by the WMC to carry out the design with Arup providing the bulk of the engineering design including structural and m&e through its Cardiff office. This team took the scheme to RIBA Stage D before it was tendered as a design and build. Sir Robert McAlpine successfully bid for the project and subsequently re-employed the original design team, who were colocated in site offices along with the main contractor’s team and sub contractors when work began on site at the end of February 2002.

The brief to the architects was that the building had to be “unmistakably Welsh and internationally outstanding”. It’s certainly arresting. On the approach along Lloyd George Avenue the building dominates the skyline with its expansive stainless steel clad shell roof. This steel has been pre-patinated with a light bronze coloured oxide and changes colour with the light, from a stormy black through to champagne. Flanking this at the main entrance are rugged slate walls interspersed with 150 mm thick bands of glass, while a mix of wood and brick clad the sides and rear of the structure.

Stepping inside

The voluminous steel framed building contains a plethora of facilities for the various organisations it will house, ranging from rehearsal rooms, offices as well as a 150 bed hostel for children staying with Urdd Gobaith Cymru. The sweeping foyer leads into a three-storey concourse area surrounded by cafes, bars and shops along with several settings for open performances. From the open balconies of the concourse visitors can gain direct access to the studio theatre and residents’ areas as well as the vast Donald Gordan Theatre.

Sitting beneath the curvaceous roof this theatre is the centrepiece of the building. The front and rear stalls along with two circle levels and side balconies provide total seating for 1852 people. Of the four key services zones within the building the lyric theatre is probably the most demanding, requiring a ventilation design able to maintain strict noise levels below the Preferred Noise Criteria of 15 (PNC15) set out in the client brief. Darian Jones, Arup’s mechanical engineer for the scheme explains: “In order to avoid regenerated noise within the ductwork we had to have a very low velocity system, but as you can imagine we might have 1900 people in there and that is a lot of air that has to be moved.”

A displacement ventilation system has been employed. Beneath the stalls a large supply air plenum has been created with air introduced into the auditorium through individual displacement diffusers positioned beneath each seat at a delivery temperature of 19-20°C to maintain the space at 21°C. The position of the diffusers, tested by Krantz in an anechoic chamber in Cologne to ensure they complied with the noise criteria, was fixed early in the scheme as the outlets are cast into the concrete slab and had to avoid clashes with the structure and seat supports. A similar underfloor set up is used for the circle levels.

During construction pressure tests were carried out on the plenums. “We were worried that if the plenums were leaky there was a danger that we wouldn’t get the air supply that we needed for ventilation and also for cooling,” says Jones. A fan connected up to the end of the installed ductwork was used to pressurise the plenums up to 50 Pa. Jones says this was an eye opener. “We had guidelines on what was an acceptable air loss for an underfloor plenum. But the first time we did it there were holes everywhere, there was dust coming out of places where there weren’t grilles.”

Arup ran a cfd study on the auditorium to determine how best to deal with air extract. “In particular the main concern was that at the back of the stalls and the back of each circle there might be a micro climate with warm air stagnating below the bottom of the balcony above,” says Jones. The cfd analysis showed that it would be necessary to pull air out from the underside of each balcony. Both tier floor sections are divided in two with the upper half acting as the supply plenum and the lower section as high level exhaust (see figure 1). A further exhaust point is located at the very top of the auditorium.

The system uses variable speed fans to accommodate for partial occupancy with control via temperature sensors within the space and CO2 sensor in the return air duct.

The fly tower, which extends right up to roof level (a height of over 27 m) sits virtually at the centre of the building between the auditorium and the main air handling plantroom towards the rear of the fifth floor. The ductwork from the plantroom runs in the shell roof void skirting around either side of the fly tower before dropping down risers at the back of the auditorium and branching off to supply each plenum.

The ductwork is large, measuring some 2 m by 2 m in order to achieve the 1·5 m/s ductwork velocities required. Its size combined with the complexity of the roof structure created a co-ordination nightmare. To overcome any potential clashes a 3d model (figure 2) was built early on in the design stage to ensure the fundamentals of the structure and the services could work together. “You had to give thought to 2 m square ductwork in the really early days,” says Jones. “You couldn’t introduce that at a later date.”

Heating and cooling

The decision to opt for roof-mounted chillers working alongside ice storage was largely determined by the occupancy pattern of the WMC. With audiences of up to 1900 for a performance in the main auditorium and heavy-duty production lighting, substantial internal gains are inevitable. However, the building’s peak cooling load of around 2 MW extends for a relatively short period of the day. The design team was keen to avoid installing three or four high capacity chillers to deal with this peak load and assessed a number of alternative cooling options including boreholes, pumping water from the bay as well as more conventional cooling towers. They concluded however that a roof-mounted chiller in conjunction with ice thermal storage suited the load pattern and importantly was the most cost-effective, both in terms of capital and running costs.

During the daytime the chiller picks up the building’s 1·4 MW base load while at night, with the lower electricity tariff, it switches over to supply the labyrinth of pipes circulating glycol within the two heavily heavily insulated 70 000 litre ice tanks on the building’s ground floor. The ice bank is capable of supplying up to 1·8 MW of additional ‘top-up’ cooling when the auditorium is in use. In addition to halving the size of the mechanical chillers the system significantly reduces the peak electrical load from the chilled water system.

Three 1·5 MW gas-fired boilers pick up the Centre’s heating needs. These serve the ahu heating coils as well as perimeter radiators and fan coil units. Jones explains that there is a high load on the air systems: “We have to allow for heating all of the air for the auditorium when it is at full capacity, which might mean taking it from an outside temperature of -5°C up to 20°C.”

Although the stage area relies on the conditioned air from the auditorium to keep it at the required temperatures, radiant panels are installed in the fly galleries for performers – particularly the dancers – to stand beneath.

Acoustic treatment

The careful consideration given to the acoustics throughout the project can be broadly broken down into three main areas: room acoustics, acoustic separation and suppression of background and services noise. The glass reinforced gypsum blocks lining the walls of the auditorium are used to create the basic acoustic qualities within the space – there are 24 variations each individually glued to the walls. However the acoustics can be fine tuned for the diverse range of performances that will be held there using variable acoustic panels. Mounted at high level around the edge of the space these drop down allowing the reverberation time to be optimised depending on the performance. Heavy curtains that come around the back of the space can also be used to supplement these.

Throughout construction Arup’s acoustic team carried out tests to validate their calculations, the first being carried out when the scaffolding inside the auditorium came down, with further checks when the seating went in, after the variable acoustics were installed and finally a ‘hard hat test’ with an audience of 1800.

Variable acoustics are also employed in the rehearsal spaces, this time taking the form of wall-mounted panels that can be slid back to reveal absorbent material as well as drapes and deflector panels mounted to the ceiling. Together these allow the conductor to arrange the room out to their liking, with the orchestra set out shallow and wide or tight together.

The rehearsal spaces also had to meet the the PNC 15 requirements. The acoustic engineers response was to create box-in-box construction for these spaces with the base of the floor sitting on resilient mounts to prevent any vibrations being transmitted.

Structural isolation joints are also used to separate the main building from the plant room and wherever this is crossed with ductwork, cable trays or pipework a flexible joint is used to make sure non of the plant sends vibrations into the auditorium.

In acoustically sensitive areas a sample of each luminaire type was tested for issues such as transformer buzz and noise on ‘cooling down’ and approved by the acoustic designers prior to placement of the order. Some in fact failed and alternatives had to be found.

As well as the services, the location of the building, which sits above a road tunnel, caused some concerns. It was thought that traffic rumble might be transmitted into the auditorium through the piles. Arup Acoustic’s ran tests by driving a diesel lorry back and forth through the tunnel late at night while testing each pile cap – the loudest recorded sound was the hum caused by the 50 Hz ac current from an hv cable running through the site.

Fire protection

The entire complex is protected by an analogue addressable fire detection system to provide coverage to L1 standards. In the main auditorium beam detectors are installed to give coverage at the uppermost levels while detectors in the extract systems measure for the presence of smoke. In the fly tower a VESDA system is installed to cope with the height of the space.

In the main auditorium during a performance there is no automatic fire alarm to avoid panicking the audience. Instead a staged evacuation will be carried out using the public address voice alarm. Arup Fire carried out smoke modelling exercises to reduce the extent of the smoke extract system. Three

25 m3/s smoke extract fans installed within the fly tower deal with the main auditorium. So as not to spoil the clean lines of the shell roof these exit into large walk-in gutters as do those for the concourse and foyer areas.

Beam detectors were originally also going to be used in the concourse, but concerns that these might become obstructed led to the installation of flame detectors, which sense the ultraviolet and infrared characteristics of any flame. Sprinklers are installed throughout the back of house areas, the foyer bar area, the entrance foyer and toilets, which work on a two stage alarm.

Electrics and lighting

An 11 kV incoming supply serves the Centre’s hv distribution via separate routes within the building to three packaged sub stations incorporating ring main units, cast resin transformers and lv switch boards. The low voltage distribution throughout the building is via rising busbar trunking systems and steel wired armoured cables routed through vertical and horizontal service zones. Each main and sub-main distribution board is connected to an addressable site configured metering system. A meter is radially connected to the metering local area network circuit and measures the energy used by each main distribution panel.

Emergency standby power is provided via an automatic 750 kVA diesel generator set housed externally in an acoustic enclosure. This supports the statutory life safety/life critical systems. The emergency lighting is provided via two distinct systems. The auditorium, inner foyers, concourse and foyer bars are served by a centralised static battery/inverter set up. While all other areas are self-contained

“The idea for the lighting scheme was to try and build up a theatrical atmosphere when you come into the entrance foyer,” says Graham Phillips, Arup’s electrical engineer on the project. From the foyer through into the lower concourse areas a ‘random starry night’ effect is created by the scattering of low voltage recessed halogen downlights, which are fully dimmable for different scene setting. These are supplemented with continuous light slots, running along the ceilings and down the walls that are created using fluorescent batten fittings concealed behind diffusers. On the upper concourse levels there are also custom-made ‘fossil columns’, manufactured by lighting specialists Box Products. These provide uplighting and incorporate four 32 W compact fluorescents.

Within the main lyric theatre the main lighting for the circle areas is provided by recessed downlights with 240 V, 35 W and 50 W dichroic lamps. These were selected after considerable acoustic testing was carried out on alternatives. The lights are controlled from a central house lighting system with the maximum voltage to the lights limited to extend lamp life.

Over the stalls area lighting is provided by floodlights mounted on the high level lighting bridges, while side wall and ceiling lighting is lit via perimeter projector lights to enhance the entire auditorium environment.

Emergency lighting within the auditorium typically uses the recessed downlights which are fed from a central static battery/inverter and complies with the requirements of BS 5266: A modern approach to emergency lighting in theatres, issued by the Association of British Theatre Technicians.

For visitors approaching at night the most prominent lighting will probably come from within the building, blazing through the glazed inscriptions cut into the walls of the foyer. Sitting side by side the Welsh lines declare: Creating truth like glass from inspiration’s furnace, while the English lines read: In these stones horizons sing. Sentiments that, along with the building, should add to any performance.

Wales Millennium Centre, Bute Place, Cardiff

Client Wales Millennium Centre
Project manager Clarus PCM
Architect Percy Thomas Partnership
Main contractor Sir Robert McAlpine
M&E, structural, civil, consulting engineer Arup
Quantity surveyor Clarus PCM
M&E contractor N G Bailey
Acoustic consultant Arup Acoustics
Fire consultant Arup Fire
Theatre consultant Carr & Angier
Controls Arup C&C