Game on

Australia sees the 2000 Olympics as a chance to make a difference. Heralded as the 'Green Games', the country's successful bid to host the event included an environmental commitment to which the organisers and designers have gone to great lengths to uphold.

On Friday 15 September the eyes of the world will fall on the centrepiece of this Olympic Games – a $680 000, 110 000-seater stadium at Homebush, north-west of Sydney. It is an impressive structure. Scarab-shaped, it is one of the biggest in the history of the Olympics, and easily the largest structure in Western Sydney.

While many will celebrate the glory of the design during the three-hour opening ceremony, few are likely to appreciate the environmental issues which lie behind the glitz and the glamour.

Gone is a vast area of industrially-polluted land. Dilapidated brickworks and clay pits have been replaced with water sports facilities, a smart Olympic village and cavernous exhibition halls.

The stadium's design team was challenged to forsake standard design conventions to meet the targets of the environmental agenda. Among other things, this included reducing the dependence on air conditioning and improving energy efficiency.

  British consultants were appointed to provide design advice and computer modelling skills to help the Australian architects and engineers realise their objectives. Passive engineering specialist Brian Ford teamed up with Kevin Lomas at De Montfort University, and Cambridge Architectural Research to devise, model and refine a natural ventilation and passive cooling strategy.

The stadium's two longest sides have vast transparent roofs that stretch the 165 m length of the seated stands. These 30 000 m² 'roofs' are constructed from a multi-walled translucent polycarbonate, chosen to satisfy the design condition imposed by television broadcasting to enable clear close-ups of spectators. These are supported at either end by 14 m-deep steel trusses, anchored by triangular concrete thrust blocks.

Corporate levels are interspersed with public seated areas over the six storeys in the main stands. Behind the seating areas, the usual array of food and drinks outlets span vast lounge areas ranging from 6 m to 8 m wide and 250 m long.

At each corner of the building, massive 12 m diameter concrete cores, called 'ramp cores', serve a dual purpose. Not only do they act as primary vertical circulation routes to the upper levels, but they also double as natural ventilation shafts.

When the UK consultants arrived the structural design was already in place, which limited the choice of options and technologies. Other factors in stadium design, such as the need for unobstructed spectators' views, tend to drive and constrain the form of such buildings. The design of services, whether passive or otherwise, cannot compromise the design of, say, the seating areas. Instead, the services must work around and within the set building form.

Initial studies showed the Homebush site lay on latitude 36°, with a mean annual temperature of 18°C. Being slightly inland from Sydney, summer ambient air temperatures were around 3-4°C higher. Armed with this information a chart was superimposed onto the temperature ranges for Sydney. Solar radiation was assumed to be equal for both sites.

Another factor that had to be taken into account was that the Homebush site suffers from warm winds blowing from the interior of the country.

The use of ground cooling was one of the first services strategies to be investigated. "On paper it was a very good option," stated environmental consultant Brian Ford, "but it implied a huge field of pipes under the ground." This was the main barrier to an otherwise attractive idea. In any case, the foundations were already being prepared, and to apply ground cooling technology would have meant a big change to the groundwork's package.

Brian Ford began by looking at the stadium as an entirety. At the start of the project it was proposed that certain levels could be passively ventilated – a strategy driven as much by reducing the financial costs of constructing and running the building as the environmental agenda.

The design team ranked their ideas, and natural cross-ventilation emerged as the the preferred option, along with through passive stack-driven ventilation, some mixed-mode systems, mechanical ventilation without cooling, and some full air conditioning. This design menu was necessary for various reasons. Mechanical ventilation was required for underground areas, and the banquet suites needed air conditioning as they would be required to cope with high occupation densities for prolonged periods.

For the other, less onerous levels, the design team and their UK consultants carried out computer modelling to test services strategies. The team started by trying to naturally ventilate and passively cool the whole of the building, but gradually discovered that this would not meet certain criteria.

The building is basically a huge mass of reinforced and prestressed concrete, with a volume of around 100 000 m³ of concrete. This thermal mass, much of it shielded from direct solar gain, provided a huge thermal flywheel. The surface temperatures in many areas remain relatively low, which also means low radiant temperatures. However, any services strategy would still have to deal with very large internal gains from spectators.

Simulation modelling took account of the building's intermittent occupancy. On that assumption the consultants concluded that natural night-time cooling would provide the client with acceptable temperature control.

It should be noted that 'acceptable' temperatures range up to 29°C, higher than normal as it was assumed that spectators would wear shorts and T-shirts. In any case, even if temperatures within the stadium do reach 29°C, it will actually be cooler than outside the building.

The major concern for the engineers was the introduction and movement of fresh air and removal of additional heat gains. The designers then looked at the use of the building's lift shafts as a means of transporting air through the stadium.

Dynamic thermal analysis was carried out by De Montfort University and salt bath analysis by Cambridge Architectural Research to confirm the routes of airflow through the building. After much modelling and testing, the design team developed a set of proposals. At ground level, the main point of entry for the stadium, the spectator entry and exit points, assist in a cross-ventilation strategy. Air flows through these access points, and exits above a perforated roof over the central toilet blocks.

For levels two, three, four and five, the consultants looked at ways of natural ventilation and passive cooling which could be assisted by stack and wind-driven ventilation. (The internal areas on these levels are more complex and enclosed and they were unable to be cross-ventilated).

At first floor level, air is taken into a 7 m x 7 m shaft to rise by the natural stack effect through the building, passing through carefully planned ventilation ducts along the way. On level two, the shaft rises through the building on both sides of a deep plan banquet hall and the escalator shaft is used as an air duct.

The original proposal was to eventually vent the extract air rising through these shafts at high level, immediately under the roof canopies. Modelling showed this would work, but it also proved costly. The quantity surveyor requested that the six shafts were truncated as a cost-saving exercise, which raised concerns about ventilation effectiveness and the potential for airflow stagnation.

At this point Geoff Whittle of Simulation Technology carried out an analysis to determine the effect of wind regimes from different directions. The net result of this analysis was that a positive wind blowing from behind the stadium would produce a positive pressure down the shafts. This would induce high air change rates and removal of the internal heat gains from the levels to be cooled. In this condition there is suction on the leeward side.

Under still conditions, stack buoyancy was found to continue to ventilate and move the air, but in certain wind conditions flow reversal could occur. The question then was: will this matter?

The canopies themselves were a concern as they were judged a source of huge solar gain in summer. In addition to the heat gains from the spectators, the temperatures were affected by the amount of glazing. Due to the steep sloping shape of the stands and the natural upward movement of warmer air, any major problems experienced would be at the rear of the stands, where heatstroke was a legitimate concern.

The structural opening at the rear of the stands and the height above the spectators could not be changed to enable a larger flow of air through the area. This was mainly because of possible problems of wind forces and uplift on the canopy. The solution was to make the entire back wall out of ventilation louvres to provide the required free area. Fortunately, capacity crowds are unlikely to occur during the summer months, and if they were, it will only be for short periods.

The opacity of the roof reduces as it reaches the back of the stadium – from 70% at the front to 50% at the back – to reduce additional heat gains from solar radiation.

Post completion smoke tests showed that the concrete ramp cores to the higher levels form another area of exhaust. These structures are open-topped, allowing air to naturally exhaust at each corner. These will have a continual contribution to the cooling of the stadium and removal of air by passive stack effect, reducing the risk of stagnation in back-of-house areas which are shut when no events are taking place.

The smoke tests also showed that, with wind at an oblique and shifting angle, a positive pressure can be produced, resulting in strong flow rates through the stadium. Flow reversal was found not to be a problem.

Although the final situation is the majority of the building for most of the year will be naturally ventilated, mechanical help has slipped into some areas. In addition to the banqueting halls, private suites on level three have been fitted with fan coil units at the owners' requests.

Fan coils have also been included on the level four concourse area as much higher temperatures were found to occur here. These are intended only for use in extreme summer temperatures, as the passive cooling serving the area should suffice under normal conditions, when the majority of use will be made; the sporting seasons running from autumn to spring. The level two concourse will be naturally ventilated throughout the entire year.

Sustainability counts

It was not only the services strategy that was to minimise waste and maximise sustainability on this project. The organisers were conscious of the immense construction works involved for the Olympic stadium and village.

To counter this and in keeping with the overall environmental agenda, wherever feasible existing buildings are being reused and refitted for use. Approximately 33% of all sporting competitions are being held in these venues. Most of the purpose-built venues have been designed to maximise energy efficiency, conserve water and preserve indoor air quality.

All the venues are constructed from environmentally-friendly materials, using building processes specially designed to minimise waste. Solar photovoltaic panels are incorporated in external lighting around the stadium.

A train route and station have been included to enable visitors to arrive without the use of personal cars, an important aspect as the site does lie out of town, despite Sydney sprawling ever nearer. Also, areas have been set aside for wildlife parks, where native species are protected and visitors educated.

In 2031 Stadium Australia Management's lease on the facility will run out, and ownership and management responsibility will revert to the state government. A welcome task in anyone's book, given the methods that have been chosen to build and run the structure.