Heavy metal

Branson must have convinced the client, for his practice won the job. It also won the commission to design this highly original building, the National Centre for Popular Music, in Sheffield. Legend has it that Branson Coates was inspired by the top of a Gillette shaving foam canister. With Sheffield's claim as the City of Steel, it would be far too mean to question the veracity of the story.

Whatever the inspiration, Branson Coates' design is precisely what the client, Music Heritage, was looking for in an educational and arts centre dedicated to popular music. Located in the Cultural Industries Quarter of Sheffield city centre, the £8 million project was funded variously by the Lottery, the Arts Council, the EU Regional Development Fund and English Partnerships. Money apart, the building's existence is said to owe much to the success of Sheffield rock talents Human League, Pulp's Jarvis Cocker, Def Leppard and Heaven 17.

The client brief requested several large exhibition areas, with a shop, a cafeteria, a bar, and circulation spaces. In response to this the architect has created four large exhibition halls at each corner of the site, housed within the aforementioned drums. Concourses on the ground and first floor levels, connected by stairs, escalators and semi-scenic lifts, link all the other zones and the administrative offices.

The building's shell and core services were completed late last year. Fit-out of the ground floor spaces and the exhibition domes is close to completion in order to meet the public opening date set for early March.

Structural issues

Although they appear independent, the four drums share a common basement and ground floor. While the basement occupies a small central footprint (with services risers connecting to each drum), the ground floor extends over the full area of the site.

The drums only become separate structures at first floor level, the 20 m diameter steel cages skinned with stainless steel cladding which is supported off a first floor concrete plate. This sits on concrete columns with large shear heads – prominent features inside the ground floor foyer.

A circular internal breeze block room inside each of the drums form the exhibition halls. Being supported off a concrete floating floor, the relatively heavyweight structures satisfied a number of criteria, such as acoustic isolation for the exhibitions and the creation of a useful cavity between the blockwork and steel cages for ventilation and services distribution. In each drum a number of blockwork piers protrude 1 m into the space to break-up standing soundwaves.

Having a double-skin facade meant that the drums have a good thermal admittance. Wall U-values are said to be around 0·35, better than that required under the current Building Regulations.

The circulation space between the four drums is roofed with solar control glass, fixed to a steel space frame. The glass is coarsely fritted to reduce glare by 55% and direct solar transmittance by 60%. The high point of the space frame is also the building's epicentre from which the four drums visually cantilever at an angle of 4°. This is primarily to give the drums a dramatic perspective but has the added virtue of aiding rainwater run-off.

The structural cage of each drum meets at a ring beam, on which is poured a thin slab to create a suspended 10 m diameter floor. A second ring of steel then acts as a rail for pan-handle shaped cowls which are motorised to follow prevailing winds.

Servicing principles

A huge amount of design effort was required to make this building stack up. Beyond dimensional information, the exhibition areas were a big unknown, as were the expected cooling load and electrical and IT needs. Plantrooms with rotating walls also added a major fright factor to the m&e co-ordination drawings.

Fordham's project engineer Kin-Yoong Au-Yeong believes that without contemporary computer-aided design techniques the building could not have been constructed. It was, he said with understatement, difficult to identify where the steel-framed rotating cowls were likely to clash with the services contained beneath them. Indeed, in some places tolerances can be measured in millimetres.

It was a major benefit to the services design team (appointed on traditional duties) that the main contractor Higgs & Hill was on board six months before the start of the building contract. This brought a dimension of practical skill to the design team, and enabled the design philosophy to gain entrance to the procurement of work packages, the cost exercises and the general buildability.

A design value engineering exercise, so often a Trojan Horse for cost-cutting, was invaluable in testing the scheme to the limit. That said, it was still necessary to provide considerable baseload in most of the systems.

Basement plantrooms are home to most of the building's primary services. This including a single Hitachi chiller which feeds the air handling unit cooling coils and a ring main around the site which the client can tap into to satisfy future increases in cooling load. Six dedicated air handling units serve the ground floor cafeteria, bar, concessionary outlets and the administrative offices.

The ahu's normally operate in full fresh air mode with heat recovery from thermal wheels. During warm-up mode the ahu's run in full recirculation. There were no formal energy targets set for the building – essentially the services engineers set out to "grab as much we could".

Max Fordham & Partners specified the ABB air handling units in order to get the right kind of equipment, and also because the the ahu's were required on-site almost as soon as the tenders were returned. Although the m&e contractor did the working drawings, the consultant did a lot of the detailing.

The building's heating system relies on a connection to the Sheffield Heat and Power district heating system rather than its own gas-fired boiler plant. With gas so cheap, the energy cost benefits were said to be marginal, but the main selling points were the lack of boiler plant and associated flues. The client should also benefit from low maintenance and plant replacement costs.

The primary electrical distribution is round the perimeter of the basement which (dimensionally) overlaps the upper floors and the exhibition halls. Four risers supply power to the ground floor bar, cafeteria, shop and administrative offices, while the second floor has its own primary electrical distribution highway with local rings in each drum. Each one has up to eight local risers, colloquially known as nibs, which give the exhibition organisers local access to three-phase power.

Servicing the exhibition drums

The client's requirement for fully flexible space led to two design decisions. First, the servicing requirements of the building and the exhibition spaces had to be segregated, so that the activities of the exhibition organisers could be kept separate from the building's environmental systems. Second, future proofing and therefore a degree of redundancy in both capacity and distribution had to be provided in the shell and core services.

“Architectural frivolity can bring rewards as long as the penalty is not the gratuitous use of fossil fuels”

The design team took its cues from techniques familiar to shopfitters, who are only interested in being able to clear the decks quickly and plug in something new. Hence the exhibition spaces have been designed like the bare shells of a shop. Up to 3 m above the floor the walls are completely clear, save for call points near entrances and fire exits.

A raised floor void was considered the most cost-effective way of routing m&e services in the exhibition halls. These are suitably sectioned to reduce cavity size and maintain acoustic separation. However, while floor voids are a haven for flexibility, the downside is a tendency to degenerate into a mangle of rusting pipes and fraying cables thick with dust.

The designer's answer was to create orthogonal routes under the floor for the electrical and IT distribution. Wiring and piped services are run as a ring, with the 300 mm by 50 mm four-compartment wiring trunking from the perimeter risers fixed directly to the floor slab. Three-phase patch panels are located in four distribution boards which are arranged symmetrically around the space in the riser nibs.

A correct assessment of the trunking capacity for the communications cabling could not be overstated, said Kin-Yoong Au-Yeong. "Often the services consultant leaves it to the IT specialist to say what is required, but then you run into the problem of not getting the information soon enough. Here we ran out of space in two places, but luckily only at the bottom of risers."

The ventilation concept

The other major issue the designers had to address was the building's ventilation systems. As the exhibition spaces are 20 m in diameter and 6 m high this enabled the designers to go for a displacement system, with 100% fresh air introduced from perimeter grilles and square floor diffusers. Vitiated air is extracted at high level.

Some of the freestanding exhibition furniture has been used as a way of blending air distribution and exhibition equipment. Also, equipment cabinets were left open-topped so that heat from television screens, amplifiers and computers can then be removed above the occupied zone just by buoyancy forces, which then helps to reduce cooling loads.

The air is supplied at a lower than usual temperature of 17°C, largely because of the expected occupancy and equipment gains. The diverse nature of the cooling loads may not always justify this supply temperature, but in any case 17°C was thought tolerable given the transitory nature of the occupancy.

The means of bringing fresh air into the drums has its roots in Branson Coates' architectural concept, which included the notion of mechanised cowls with stylised intake and discharge openings. These would move to track prevailing winds, and thereby use wind power to drive the ventilation system.

Knowing well the vagaries of wind-driven natural ventilation, Max Fordham's engineers knew that the cowls would probably never be good enough to satisfy the building's loads and ventilation needs. Development of the concept was nonetheless encouraged, and amazingly it even survived the value engineering study.

Figure 1 shows the basic arrangement. Supply air enters through a coarse bird mesh in the upwind side of each cowl, where it is drawn down into an ahu and through an electrostatic filter. This was chosen to keep pressure drops to a minimum, and remove the need for maintenance engineers to lug replacement air filters up and down the roof. The filtration system is currently configured to wash down the filter every two weeks.

The cleaned air passes through a run-around coil and an axial speed-controlled fan, after which it discharges into the wall cavity created by the steel cage of the drum and breeze block walls of the exhibition halls. Downflow fan coils freely located inside the wall cavity temper the supply air to both the foyer and exhibition halls.

Extract air is drawn back through grilles in the exhibition hall ceilings, into the ahu, through the run-around coil and then ducted into an extract plenum created by the static table. From that point any negative air pressure in the cowls induces an extract flow.

All four cowls are rotated in unison. Prevailing wind direction and velocity is measured by a roof-top weather station and the information filtered and processed to give a mean direction over a set period. This is then passed to the drive mechanism, which is given instructions to rotate by the shortest route to the calculated direction. On average the cowls make an adjustment every 15 minutes.

Commentary

In many ways the building defies rational explanation. If you wanted to design an exhibition building which was easy to construct, cheap to build and flexible in operation, there must be better ways of doing it than this.

Having said that, architectural frivolity can bring rewards, so long as the penalty is not a gratuitous use of fossil fuel. If one were brutally honest, the architectural aerodynamics are a bit daft, but the building is none the worse for that. It certainly makes a change for services designers to be in on a joke rather than the butt of it. What would the building be without those rotating cowls? Probably easier to maintain of course, but the point is, it's a fun building with some fun services.

Max Fordham's Kin-Yoong Au-Yeong expects the cowls to induce a useful amount of air movement through the building, with the fans only required on windless days. Most of the time wind speeds should average 10 m/s, which should provide about 60 Pa.

Access for maintenance in the cowl plantrooms is tortuous, but the choice of electrostatic filters means there is no need to lug filters up access ladders and out onto the roof. Access to the cowls is from the outside up satin-finish ladders, and then into a hatchway in the side of the cowls.

All the fan coils serving the first floor circulation space and the exhibition domes are accessible via hatchways, although it is possible to clamber into the cavity between the skin of the domes and the breeze block inner walls. Demonstrably, the fan coils have been turned into useful toeholds.

The day of the author's visit coincided with the first controlled public viewing. Perhaps controlled is not quite the word, for the visitors were primary schoolchildren who proceeded to have a whale of a time testing all the displays, almost to destruction. Undoubtedly they are the best commissioning specialists in the world.

As for the exhibitions themselves, these are rather disappointing, lacking any form of narrative or real educational value. Visitors can play with preprogrammed sound effects, watch pop videos on endlessly duplicated tv screens, or stare dumbly at through-the-ages displays of rock-and-roll instruments. These are sadly (though probably wisely) locked like stuffed animals behind glazed cabinets.

One is left with the impression that such entertainment is more easily found at a local Virgin megastore. It's a pity, because the building is easily the most exciting part of the entire "experience". Perhaps any building which aspires to reflect pop culture is inevitably doomed to be its victim.