Features

By Jan-Carlos Kucharek

As an industrial mining city, St-Etienne, in the Rhône-Alpes region of France, knows a thing or two about rock. So it was spoilt for choice last month when not one but two superstars rolled into town for the inauguration of its €38m events venue, the Zenith. Cutting the ribbon was 66-year-old Johnny Hallyday, one of the country’s biggest stars, with a musical career spanning 48 years, and generally considered to be the Elvis of the French-speaking world. Also in attendance was the designer of the building, Norman Foster, 73, with an architectural career spanning the same time, and generally considered the Presley of the architectural world.

St-Etienne’s 7200-capacity Zenith results from the French government’s drive, since 1983, to decentralise the cultural domination of Paris by building high-quality mass entertainment venues in the provinces. Here, it’s also forming part of the larger regeneration of the derelict industrial area of the city. It is the 16th Zenith to be built (one by Massimiliano Fuksas at Strasbourg opened earlier this year) but it is the first venue of the type whose shape is wholly dictated by the air-conditioning strategy. The distinctive ‘stingray’ roof form in part acts as a giant wind cowl, feeding the air vents concealed under the tiered seating via the sub-floor concrete plenum.

The approach that won the firm the competition in 2004 against the likes of OMA, Odile Decq Benoit Cornette and Francis Soler was a two-pronged one. The urban site was bisected by a large trunk road, and Foster’s was keen not only to overcome this but use it to their advantage. Executive director Grant Brooker says: ‘We knew we would want to deliver people at mid-height into the auditorium, so creating the huge parvis in front of the building to gather them from the parking areas, and using it to cover the road completely and allow unrestricted access, was one of the main design drivers.’ The other was the roof. ‘Saint-Etienne Métropole had aspirations for a design to reflect the regeneration of the city,’ Brooker says. ‘We wanted a shape that would represent our environmental approach and drive the final form of the building. The way that it functions in fact created its identity.’

It’s fortuitous that the north-south orientation of the site coincides with the direction of the prevailing wind across the Massif Central, which makes St-Etienne a hot city in the summer. This generated the idea of using the roof as a ‘windcatcher’ to pull in the ambient air, condition it if necessary, and exhaust and purge the auditorium. Fundamental to this approach was the materiality of the building itself, as this was going to help with the conditioning process. ‘Given the function of the venue and the stringent acoustic requirements to contain noise, we knew that it was going to have to be of solid rather than flimsy construction,’ Brooker says. ‘The construction was never going to be anything other than concrete, so adopting the principle of using thermal mass to heat and cool the building naturally, followed on from that.’

Stingray gills

The strategy results in the roof’s characteristic twist down at the sides to form the huge air intakes, like a stingray’s gills, pulling in up to 200m3 of air an hour, to facilitate the conditioning process. The intakes are connected to equally huge concrete plenum spaces feeding directly into the substructure that forms the floor of the auditorium’s raked seating. The clue to their presence are the discreet outlet grilles, visible only if you look under your seat.

Also raking up those sides are the fans, pumps and heating systems that will condition and feed the air into the auditorium space. Piers Heath, of Piers Heath Associates environmental engineers, who developed the strategy in tandem with Foster+Partners, says that given the climate, mechanical conditioning was necessary to augment the building’s passive systems. He says the company had looked at the incorporation of geothermal heat pumps, as there are flooded mine shafts below the building which could have been used to supply the water. Geotechnical testing discounted this idea, however, for insufficient flow rates. Instead, the team opted for a combination of high-efficiency condensing boilers and chillers, and heat exchange.

‘We ostensibly had to look at three seasonal scenarios over the year. In the cold season, heat recovery from the exhaust air of the auditorium warms the fresh air coming in, with residual heating from the boilers bringing it up to the required 24-25ºC.’ explains Heath. ‘When the climate becomes more temperate, and ambient air is feeding in at 16-21ºC, we can condition the air through heat recovery alone and pump it straight into the auditorium. In summer, when the ambient temperature exceeds 26ºC, the recovery air heat exchangers kick in, delivering what cooling we can from it, and reducing the energy load of the mechanical chillers.’ The thermal mass of the concrete plenum forming the floor for the raked seating works well with the seasonal changes, he adds, gradually storing heat or coolness as the ambient temperature changes over the year.

Central to the principle of feeding the air from below your seat is what goes on above your head, as the roof acts as a reservoir for the audience’s cumulative heat gains to the space. The lightweight Kalzip roof is highly insulated anyway, to preclude the breakout of sound, but this serves a secondary purpose. With a U-value of 0.35, it’s also holding the heated air in, and allowing its embodied energy to be used for heat recovery/exchange. A double skin of baffles, one on the weather line, and then acoustic ones internally, are positioned on the wide and narrow parts of the roof, and although closed during performances, can be opened post performance to purge the auditorium and draw exhaust air out, while fresh air, forced in by the prevailing wind, passes across the roof soffit. It’s no misrepresentation to say that through this action of ‘inhalation’ and ‘exhalation’, the building is actually breathing.

As Foster+Partners’ Brooker makes clear, money was spent to ensure the air-conditioning strategy for the main auditorium worked, with lesser techniques being employed for the foyer spaces. ‘The bulk of our efforts were concentrated on developing a sophisticated, largely passive solution for the hall, and we worried less about the foyer areas, which is effectively a transient space,’ he explains.

But as much as possible, passive techniques were employed here too. The 32m overhang of the roof prow with its white louvres does as much as it can to get reflected light in without direct solar gain, and that meant savings by merely specifying low-e glass rather than solar coatings, something which would also have reduced the transparency of the glass. Under floor heating, set into the foyer’s screed and fed by the condensing boilers provides local heating when required.

Although the city council, St-Etienne Métropole, was fully supportive of the environmental strategy from the outset, Brooker admits there were times when they needed to convince the associate architects Cabinet Berger and engineers Thales that a passive conditioning system could deal with the climatic extremes of the Massif Central.

‘With projects like this, the approach would usually be to install chillers on the roof and drop cold air into the space, and when you’re coming from a typology that works, even though it’s of a cruder resolution, it takes time to buy into a new concept,’ he says. But he was impressed with their open-mindedness: ‘We’re pleased that the city and the team put their trust in us. Their investment in this project, and its success, is enormous, and they deserve their share of the credit for helping to realise it.’ A statement that you genuinely feel is not just hot air.