Scene to be believed

For the beleaguered Royal Opera House, the rebuilding of its Covent Garden premises was vital for its survival. The building has suffered from spiralling costs since the early 1980s, largely because of massive staff overheads plus the commercial inefficiency of producing opera in an 1850s building.

While popular with the public, a mammoth opera like Aida would send wage bills through the roof. To make matters worse, the monumentally cramped Opera House slowed the movement of scenery to a crawl, preventing productions from running back-to-back.

This new building should change all that. Gone are the inadequate 19th Century fly-tower, the motley collection of rehearsal rooms and the tight and tortuous routes through which scenery had to be transported. In their place, a lofty 40 m-high fly-tower, a voluminous backstage area, and new rehearsal studios enable the Opera House to put on two different shows a day while rehearsing for a third.

The massive backstage area has features which compete to boggle the imagination: the 56 tonne stage doors to the rear, 40 m-high fly-tower above, and elevating wheeled scenery wagons on a chequer board layout would not be out of place on a James Bond set.

Overall the opera house project has involved no fewer than eight linked buildings, all with differing servicing requirements. Being a mix of new build and refurbishment, the services designers and contractors had their work cut out to impose order on an extremely chaotic geometry.

As is becoming the norm with projects of this size, construction manager Schal pulled all the trade contractors together in one location, and created a seamless design and construction process. This translated primarily into centralised document control based on Schal's own system, involving scanned documentation and a disciplined method of getting approval on drawings and variations.

Project history

The planning process for the Royal Opera House has not been entirely devoid of sturm und drang. A string of planning applications were very publicly turned down in the 1980s, and it wasn't until 1994 that a much smaller scheme won approval.

At 70 000 m², this "small scheme" dwarfs the Opera House auditorium, which accounts for just 10% of the site. The remaining 90% covers the fly-tower and backstage areas, the rebuilt barrel-vaulted Floral Hall, four daylit ballet studios for The Royal Ballet, and a 420-seat studio theatre. Buildings added in the 1980s have also been extensively refurbished.

In servicing terms, the designers and contractors were prevented from routing services horizontally by an 11 m-high clear void created by many of the aforementioned spaces. Vast back-of-house stage areas at second floor level (about a quarter of the site volume) also prevented the vertical distribution of services – hence the aircraft carrier analogy.

Early attempts at services co-ordination employed 3D modelling, but this only got construction manager Schal and the services designers Ove Arup so far. The Opera House itself is a listed structure, and the services engineers had their work cut out to route plant through diaphragm walls braced by cast iron trusses.

"It is a very complex job," admitted Schal's head of building services, Flan McNamara. "Squeezing such high levels of servicing into such a confined structure was a big challenge."

As figure 1 shows, the engineers were forced to locate services plant either in the basement and tunnel up through the structure, or place plant on the roof and drill down. There is minimal horizontal distribution, especially on the second floor (stage) level and above, save for a huge 15 m-long, 16 m² flying duct running across the back of the Floral Hall. This carries supply air from a plantroom above the Russell Street building to the auditorium.

A stroke of fortune was the rediscovery of brick voids around the auditorium used originally as routes for natural ventilation. Arup quickly commandeered these for cabling. Some trunking carries up to 5000 cables – there are 1800 lighting circuits in the main auditorium alone.

Gas-fired boilers and R134a screw compressors have been located in the least acoustically sensitive space in two basement levels beneath the Russell Street building. Still, with retail units above these plantrooms, all plant is on spring mounts and the plantroom ceiling is acoustically lined.

From here the services snake out in multi-service routes to risers, again strategically positioned to avoid acoustic clashes with rehearsal rooms, studios and the like.

Acoustics

"Acoustics," said Stas Brzeski, "ran like a thread throughout the whole job." Arup's design team broke the acoustic requirements of the site into three levels: room acoustics, acoustic separation and the suppression of background services noise.

The tight site forced acoustically sensitive spaces to be surrounded by acoustically hostile spaces, such as rehearsal rooms located next to scenery workshops. "It even ran down as far as the dressing rooms, as these are used for piano and singing practice," said Brzeski. "Many are located above the public piazza, which is a very noisy place in summer."

The engineers' response was to create box-in-box construction for some of the rooms, most notably for the basement studio auditorium, and install doors with high levels of noise attenuation.

Isolating the rehearsal rooms and auditoriums from back-of-house areas has resulted in some seriously heavy doors. They are also huge: the largest is 20 m high and 26 m wide and weighs 56 tonnes. Arup Acoustics designed the doors to cope with a sound reduction of between 40dB and 65 dB –adding layers of plasterboard as appropriate. Electric motors open and close the doors, those opening vertically being equipped with counterweights. Compressed air is used to inflate rubber tubes installed in the leading edge of each door and in the closing reveal to preserve the seal.

The relationship between the new basement studio theatre and an existing rehearsal room directly above presented an interesting challenge. The rehearsal room is often home to bouncing ballet dancers, creating a real risk that tremors would shake the lighting gantry bolted to the ceiling slab of the studio theatre.

As ballet dancers don't exactly figure on the Richter scale, the design team asked a ballet troupe to leap up and down simultaneously while the floor's vibration was measured. "It was amazing how well synchronised they were," recalled Brzeski.

Another acoustic problem was posed by the intricate roof, which has multiple levels and terraces. Rainwater pipes pass through the building within the services risers, and those that pass through acoustically sensitive areas such as the fly-tower had to be encased to stop noise breakout.

Even the heating in the fly tower was selected based on low noise criteria. Electric radiant panels may not be very energy efficient, but they are quiet.

Ballet studios

The sixth floor ballet studios are extremely interesting spaces, not just for their architectural articulation but also for the comfort and acoustic criteria. The four ballet studios are strongly daylit, with one relying on bespoke lightpipes.

Arup learnt that ballet dancers are very demanding when it comes to comfort conditions, being particularly concerned about protecting muscle tone and ankle joints. With no rules in the CIBSE Guide, Arup sat down with The Royal Ballet to agree the design criteria.

The result is a high level supply and extract system to reduce air movement near the floors, and low level perimeter radiators to warm ankles rather than heat the space. Manual control of the radiators will enable ballet room users to boost flow temperatures.

Overall the ballet studios are comfort cooled with a peak-lopping heating, cooling and ventilation system designed to reduce the air temperature in the ballet studios by up to 3°C below external ambient – a design criterion applied generally for the back-of-house air conditioned areas.

Ventilation strategy

There are many different air supply strategies. While high level, fully mixed air distribution is used for the 420-seat studio theatre and ballet studios, the auditorium ventilation system employs a displacement system (see "Auditorium ventilation"). The mechanical ventilation mostly doubles up for smoke extract, although the fly-tower and basement areas have dedicated smoke extract.

"The client was very prescriptive about the acoustic performance of the main auditorium, both in terms of the construction of the box and airstream attenuation," said Arup director Stas Brzeski. "This created quite high pressure drops, but fortunately the systems are very low pressure with final branches running at between 1·5 – 2 m/s."

Noise criteria range from PNC 15 for the main auditorium to NR 35 for general spaces. The studio theatre was built to PNC 20. Some ahus have a high acoustical performance, the most critical systems being those serving the stage area behind the main auditorium, the chorus rehearsal room, the basement studio theatre, and the four ballet studios.

The need for low noise breakout from the services forced the use of extensive isolation and anti-vibration measures. Where there are lengthy air ducts serving acoustically sensitive areas, these have been treated and clad to reduce the possibility of external noise breaking out or, indeed, breaking in.

Systems serving spaces with onerous acoustic criteria demanded secondary and sometimes tertiary attenuation within the ductwork. Similarly, attenuators have been installed in ductwork where there is a risk of cross-talk between those areas.

The Floral Hall – which now acts as a public area next to the auditorium – is comfort cooled using a mixing ventilation system, with radiant heating/cooling provided using an underfloor system.

Electrical services

As might be expected, the electrical load is suitably huge – 3·8 MW. This is satisfied by two new hv supplies into the Russell Street building, each rated at 2 MW. Each feed originates from separate substations on the London Electricity network to provide security. Back-up supplies can be provided through this arrangement, negating the need for a standby generator.

Two hv switchgear rooms provide segregated distribution for the main and standby supplies, and supply four 1250 kVA transformers. The lv switchboards are protected by air circuit-breakers and fitted with surge protection. Separate cubicles containing capacitors and reactors have been installed to improve the building's power factor and filter out any harmonics created by dimmers and the motor drives.

Despite well-publicised contractural wrangling, strikes, and reports of heavy objects being dropped down the fly-tower, Schal's Flan McNamara reports that the commissioning of the project is on schedule. "With two months to go, all the m&e systems have been balanced," he claims, "but we won't finish commissioning the spaces until late October."